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LIBRARY  CATALOGtIE  SLIPS. 

United  States.     Deparlment  of  the  interior.     (  U.  S.  ijeoluffical  nurvey.) 

Departuieut  of  the  interior  |  —  |  Monograplis  |  of  the  |  United 
States  geological  .survey  |  Volume  XIX  |  [Seal  of  the  depart- 
uieut] I  Washington  |  government  printing  office  |  1892 

Second  title:  Uuite<l  States  geological  survey  |  J.  W.  Powell 
director  |  —  |  The  \  Penokee  iron-bearing  series  |  of  |  Michigan 
and  Wisconsin  |  by  |  Roland  Duer  Irving  |  and  |  Charles  Richard 
Van  Hiae  |  [Vignette]  | 

Washington  |  goveruuient  printing  office  |  1892 

4°.     XIX,  534  pp.  37  pi. 


Irving  (Roland  Duer)  and  Charles  Richard  Van  Hise. 

United  States  geological  snrvey  |  J.  W.  Powell  director  |  —  | 
The  I  Penokee  iron-beariug  series  |  of  |  Michigan  and  Wiscon- 
sin I  by  I  Roland  Duer  Irving  |  and  |  Charles  Richard  Van  Hiee 
I  [Vignette]  | 
Washington  |  government  printing  office  |  1892 
4°.    XIX,  534  pp.  37  pi. 

[TfNiTED  States.    Department  of   the  interior.    {U.   A',   yeolotjical  survey). 
Monogr.ipli  XIX.] 


United  States  geidogieal  survey  |  J.  W.  Powell  director  |  —  | 
The  I  Penokee  iron-bearing  series  |  of  |  Michigan   and  Wiscon- 
sin I  by  I  Roland  Duer  Irving  |  and  |  Charles  Richard  Van   Hise 
I  [Vignette]  | 

Washington  |  governnn'iit  printing  oflice  |  1892  , 

4°.     XIX,  534  pp.  37  pi. 

[United  St.vi'ES.     Department   uf  the  interior.      {U.   S.   geuloijical  tiirvey. 
Monogi'aph  XXX.] 


[Monograph  XIX.] 


The  publications  of  tho  United  States  Geological  Survey  arc  issued  in  accordance  with  the  statute 
approved  March  S,  1879,  which  declares  tliat — 

"The  publications  of  the  Geological  Survey  shall  consist  of  the  annual  report  of  operations,  geo- 
logical and  economic  maps  illustrating  the  resources  and  classification  of  the  lands,  and  reports  upon 
general  and  economic  geology  and  paleontology.  The  annual  report  of  operations  of  the  Geological 
Survey  shall  accompany  the  annual  report  of  tlie  Secretary  of  the  Interior.  All  special  memoirs  and 
reports  of  said  Survey  shall  be  issued  in  uniform  quarto  series  if  deemed  necessary  by  the  Director,  but 
otherwise  in  ordinary  octavos.  Three  thousand  copies  of  each  shall  be  published  for  scientific  exchanges 
and  for  sale  at  tlio  price  of  publication ;  and  all  literary  and  cartographic  materials  received  in  exchange 
shall  be  the  property  of  the  United  States  and  form  a  part  of  the  library  of  the  organization :  And  the 
money  resulting  from  the  sale  of  such  publications  shall  bo  covered  into  the  Treasury  of  the  United 
States." 

The  following  joint  resolution,  referring  to  all  government  publications,  was  passed  by  Congress 
July  7,  1882: 

"That  whenever  any  document  or  report  shall  he  ordered  printed  by  Congress,  there  shall  be 
printed,  in  addition  to  the  number  in  each  case  stated,  the  '  usual  number '  (1,900)  of  copies  for  binding 
and  distribution  among  those  entitled  to  receive  them." 

Except  in  those  cases  in  which  an  extra  number  of  any  publication  has  been  supplied  to  the  Sur- 
vey by  special  resolution  of  Congress  or  has  been  ordered  by  the  Secretary  of  the  Interior,  this  office 
has  no  copies  for  gratuitous  distribution. 

ANNUAL  REPORTS. 

I.  First  Annual  Report  of  the  United  States  Geological  Survey,  by  Clarence  King.  1880.  8°.  79 
pp.     1  map. — A  preliminary  report  describing  plan  of  organization  and  publications. 

II.  Second  Annual  Report  of  the  United  States  Geological  Survey,  1880-'81,  by  .J.  W.  Powell, 

1882.  8°.     1  V,  588  pp.     62  pi.     1  map. 

III.  Third  Annual  Report  of  the  United  States  Geological  Survey,  1881-'82,  by  J.  W.  Powell. 

1883.  8°.     xviii,  564  pp.     67  pi.  and  maps. 

IV.  Fourth  Annual  Report  of  the  United  States  Geological  Survey,  1882-'83,  by  J.  W.  Powell. 

1884.  8°.     xxxii,  473  pp.     85  pi.  and  maps. 

V.  Fifth  Annual  Report  of  the  United   States   Geological  Survey,  1883-'84,  by  J.  W.  Powell. 

1885.  8°.     xxxvi,  469  pp.     58  pi.  and  maps. 

VI.  Sixth  Annual  Report  of  the  United  States  Geological  Survey,  1884-'85,  by  J.  W.  Powell. 
1885.     8°.     xxix,  570  pp.     85- pi.  and  maps. 

VII.  Seventli  Annual  Keport  of  the  United  States  Geological  Survey,  1885-86,  by  J.  W.  Powell. 

1888.  8°.     XX,  656  pp.     71  pi.  and  maps. 

VIII.  Eighth  Annual  Report  of  the  United  States  Geological  Survey,  1886-87,  by  J.  W.  Powell. 

1889.  8°.     2v.     xix,  474,  xii  lip.     53  pi.  and  mops;  1  p.  1.     475-1063  pp.     54-76  pi.  and  maps. 

IX.  Ninth  Annual  Report  of  the  United  States  Geological  Survey,  1887-88,  by  J.  W.  Powell. 

1889.  8<^.     xiil,  717  pp.     88  pi.  and  maps. 

X.  Tenth  Annual  Report  of  the  United  States  Geological  Survey,  1888-89,  by  J.  W.  Powell. 

1890.  8°.     2  V.     XV,  774  pp.     98  pi.  and  maps ;  viii,  123  pp. 

XI.  Eleventh  Annual  Report  of  the  United  States  Geological  Survey,  1889-'90,  by  J.  W.  Powell. 

1891.  8'^.     2  v.     XV,  757  pp.     66  pi.  and  maps ;  ix,  351  pp.     30  pi.  and  maps. 

XII.  Twelfth  Annual  Report  of  the  United  States  Geological  Survey,  1890-'91,  by  J.  W.  Powell. 
1891.     8°.    2v.     xiii,  675  pp.     53  pi.  and  maps ;  xviii,  .576  pp.     146  pi.  and  maps. 

XIII.  Thirteenth  Annual  Report  of  the  United  States  Geological  Survey,  1891-'92,  by  J.  W. 
Powell,  1893.    8°.    3  v. 


II  .  ADVERTISEMENT. 

MONOGRAPHS. 

I.  Lake  Bonneville,  liy  Urove  Karl  flilbert.     1890.     4°.     xx,  438  pp.     .51  pi.     1  map.     Price  $1..50. 

II.  Tertiary  Hi.-ftory  of  the  CTranil  ( 'afiou  District,  with  atlas,  by  Clarence  E.  Dutton,  Capt.,  U.  S.  A. 
1881'.     4".     xiv,  264  pp.     42  pi.  ami  atlas  of  24  sheet.s  folio.     Price  $10.00. 

III.  Geology  of  tlio  Comstock  Loile  aud  the  Washoe  District,  with  atlas,  by  George  F.  Hecker. 
1882.     4'-'.     XV,  422pii.     7  pi.  and  atlas  of  21  sheets  folio.     Price  $11.00. 

IV.  Comstock  Mining  and  MineV.s,  by  Eliot  Lord.     1883.     4°.     xiv,  451  pp.     3  pi.     Price  $1.50. 

V.  Tlie  Copper-Bearing  Rocks  of  Lake  Snperior,  by  Roland  Diier  Irving.  1883.  4".  xvi,  464 
l)p.     151.     21)  pi.  and  maps.     Price  $1.85. 

VI.  Contribntions  to  the  Knowledge  of  the  Older  Mesozoic  Flora  of  Virginia,  by  William  Jlorria 
Fontaine.     1883.     4'^.     xi,  144  pp.     54 1.     54  pi.     Price  $1.05. 

VII.  Silver-Lead  Deposits  of  Eureka,  Nevada,  by  Joseph  Story  Curtis.  1884.  4^.  xiii,  200  pp. 
16  pi.     Price  $1.20. 

VIII.  Paleontology  of  the  Eureka  District,  by  Charles  Doolittle  Waleott.  1884.  4>^.  xiii,  298 
Pli.     24  1.     24  pi.     Price'$1.10. 

IX.  Brachiopoda  and  Laaiellibranchiata  of  the  Raritan  Clays  and  (Jreeuaaud  Marls  of  New 
Jersey,  by  Robert  P.  Whitfield.     1885.     4°.     xx,  338  pp.     35  pi.     1  map.     Price  $1.15. 

X.  Dinocerata.  A  Mono^rapli  of  an  Extinct  Order  of  Gigantic  Mammals,  by  Othuiel  Charles 
Mar.sh.     1886.     4^.     xviii,  243  i)p.     56 1.     56  pi.     Price  $2.70. 

XI.  Geoloivieal  History  of  Lake  Lahontan,  a  Quaternary  Late  of  Northwestern  Nevada,  by 
Israel  Cook  Russell.     1885.   '4^.     xiv.  288  pp.     46  pi.  and  maps.     Price  $1.75. 

XII.  Geology  and  Mining  Industry  of  Leartville,  Colorado,  ^Yith  atlas,  by  Samuel  Franklin  Em- 
luons.     1886.     4°.  '  xxix,  770  lip.     45  pi.  and  atlas  of  35  sheets  folio.     Price  $8.40. 

XIII.  Geology  of  the  Quicksilver  Deposits  ol  the  Pacific  Slope,  with  atlas,  by  George  F.  Becker. 
1888.     4".     xix,  486  pp.     7  pi.  and  atlas  of  14  sheets  folio.     Price  $2.00. 

XIV.  Fossil  Fishes  and  Fossil  Plants  of  the  Triassic  Rocks  of  New  Jersey  and  the  Connecticut 
Valley,  by  .John  S.  Newberry.     1888.     4°.     xiv,  152  pp.     26  pi.     Price  $1.00. 

XV'.  The  Potomac  or  Younger  Mesozoic  Flora,  by  William  Morris  Fontaine.  1889.  4'^.  xiv, 
377  p]i.     ISO  pi.     Text  and  plates  bound  separately.     Price  $2.50. 

XVI.  The  Paleozoic  Fishes  of  North  America,  by  John  Strong  Newberry.  1889.  4>^.  340  pp. 
53  pi.     Price  $1.00. 

XVII.  The  ]''lora.  of  the  Dakota  Grouj),  a  posthumous  work,  by  Leo  Lesquereux.  Edited  by  F. 
H.  Knowlton.     1801.     4'^.     400  pp.     66  pi.     Price  $1.10. 

XVIII.  (Gasteropoda  and  Cephahipoda  of  the  Raritan  Clays  and  Greensand  Marls  of  New  Jersey, 
by  Robert  P.  Whitfield.     1891.     i°.    402  pp.     50  pi.     Price  $1.00. 

XIX.  The  Penokee  Iron-Bearing  Series  of  Northern  Wisconsin  and  Michigan,  by  Roland  D. 
Irving  and  C.  R.  Van  Hisc.     1892.     4'^.     xix,  534  pp.     Price  $1.70. 

XX.  Geologyof  the  Eureka  District,  Nevada,  with  an  atlas,  by  Arnold  Hague.  1892.  4"^,  xvii, 
419  pp.     8  pi.     Price  $5.25. 

XXI.  The  Tertiary  Rhynchophorous  Coleoptera  of  North  America,  by  S.  H.  Scudder. 

XXII.  A  Manual  of  To'pographii'  Methods,  by  Henry  Gannett,  chief  topographer. 

In  press : 

XXIII.  Geology  of  the  (ireen  Mountains  in  Massachusetts,  by  Messrs.  Pumpelly,  Wolff,  and  Dale. 

In  preparation : 

— Molhxsca  and  Crustacea  of  the  Miocene  Formations  of  New  Jersey,  by  R.  P.  Whitfield. 

— Sauropoda,  by  O.  C.  Marsh. 

— Stegosauria,  by  O.  C.  Marsh. 

— Briratotheridai,  by  O.  C.  Marsh. 

— Report  on  the  Denver  Coal  Basin,  by  S.  F.  Emmons. 

—Report  on  Silver  Cliff  .and  Ten-Mile  Mining  Districts,  Colorado,  by  S.  F.  Emmons. 

— The  Glacial  Lake  Agassiz,  by  Warren  Upham. 

BULLETINS. 

1.  On  Hvpersthene-Andesite  and  on  Triclinic  Pyroxene  in  Augitic  Rocks,  by  Whitman  Cross, 
with  a  (ieolog'ical  Sketch  of  Buffalo  Peaks,  Colorado,  by  S.  F.  Emmons.  1883.  8°.  42  pp.  2  pi. 
Price  10  cents.  ^ 

2.  Gold  and  Silver  Crtnversion  Tables,  giving  the  coining  values  of  troy  ounces  of  line  metal,  etc., 
computed  by  Albert  Williams,_jj.     1883.     8^.     8  pp.     Price  5  cents. 

3.  On  the  FossU  Faunas  of  the  Upper  Devonian,  along  the  meridian  of  76°  30',  from  Tompkins 
County,  N.  Y.,  to  Bradford  County,  Pa.,  by  Henry  S.  Williams.     1884.     S°.     36  pp.     Price  5  cents. 

4.  On  Mesozoic  Fossils,  by  Charles  A.  White.     1884.     8".     36  pp.     9  pi.     Price  5  cents. 

5.  A  Dictionary  of  Altitudes  in  the  Tnited  States,  compiled  by  Henry  Gannett.  1884.  8°.  325 
pp.     Pri<'e  20  cents. 

6.  Elevations  in  the  Dominion  of  Canada,  by  .1.  W.  Spencer.     1884.     8°.     43  pp.     Price  5  cents. 

7.  Mapoteea  Geologica  Americana.  A  Catalogue  of  Gecdogical  Maps  of  America  (North  and 
South),  1752-1881,  in  geographic  and  chronologic  order,  bv  .lules  Marcou  and  John  Belknap  Marcou. 
1884.     8°.     184  jip.     Price  10  cents. 


ADVERTISEMENT.  Ill 

s.  On  Spciiiuliir.v  F,iilni-};i'tncM(s  nl'  Miiirral  V'rafjiiiuiilH  in  CiTtain  Rocks,  by  K".  I),  livint;  iiiiil  C. 
K.  \aii  llisr.      ISSI.      S.     .'iC  j))).      (>  ]il.      I'rici'  10  centB. 

i).  A  Ki'piirt  ot'woik  (Icmc  in  (he  WasliiMgluu  Lalioratiiry  iliiring  th(^  liscal  yv;n-  IHHS-'Hl.  ]•".  W. 
Clarke,  I'hii'l' I'licniist;  'i'.  M.Chataril,  assistant  flieniiNt.      1KH4.      ><°.     4(1  pp.      Trice  ,5  conts. 

10.  On  the  Cambrian  Faunas  (irXmlh  AiniTicii.  I 'icliniinary  studies,  by  Charles  Doolittlo  Wal- 
ciitt.      b><Sl.      S'^.      71  pp.      10  111.      Price  .")  cents. 

11.  Oi\  the  (^naternary  and  Kecciil  .Mnllusca  of  the  (iroat  liasin;  with  Descriptions  of  New 
Kornis,  l)y  l{.  KUswortb  Call.  Introiliiced  by  a  sketch  of  tlu^  Quaternary  Lakes  of  the  Great  Rasin, 
by  (;.  K.Oilberl.       1SK4.      H- .      (iG  pp.     ti  jil.  '    I'riee  :">  cents. 

12.  A  Crystallographic  Study  of  the  Thiuolite  of  Lake  Lahontan,  by  Edward  S.  Daua.  1884.  8". 
34  pp.     S  pi.     Pri<e  5  cents. 

IM.  Hoiindarics  of  the  Cnited  .Stales  ,-uh1  of  the  several  States  and  Territories,  with  a  Historical 
.•sketch  of  the  Territoriiil  ChauKos,  by  Henry  (iannett.     1X85.     8^.     135  pp.     Price  10  cents. 

14.  The  Klectrical  and  Magnetic  Properties  of  the  Iron-Carburets,  by  Carl  Barns  and  Vincent 
.strouhal.     1885.     8"^.     258  pp.     Price  15  cents. 

15.  On  the  >[esozoic  and  Cenozoic^  Paleontology  of  California,  by  Charles  A.  Wliite.  1885.  8°. 
33  pp.     Price  5  cents. 

16.  On  the  Higher  Devonian  Faunas  of  Ontario  Coitnty,  New  York,  by  .John  M.  Clarke.  1885.  8°. 
86  pp.     3  pi.     Price  5  cents. 

17.  On  th(^  Development  of  Crystallization  in  the  Igneous  Rocks  of  Washoe,  Nevada,  with  Notes 
on  the  Geology  of  the  District,  by  Arnold  Hague  and  Joseph  P.  Iddings.  1885.  8°.  44  pp.  Price  5 
cents. 

18.  On  Marine  Eocene,  Fresh-water  Miocene,  and  other  Fossil  Mollusca  of  Western  North  America, 
by  Charles  A.  White.     1885.    8^\     2g  pp.     3  pi.     Price  5  cents. 

19.  Notes  on  the  Stratigraphy  of  California,  by  George  F.  Becker.   1885.   8^.   28  pp.   Price  5  cents. 

20.  Contributions  to  the  Mineralogy  of  the  Rocky  Mountains,  hy  Whitman  Cross  and  W.  F.  Hille- 
braud.     1885.     8^.     114  ]ip.     1  pi.     Price' 10  cents. 

21.  The  Lignites  of  the  (Jreat  Sioux  Reservation.  A  Rejiort  on  the  Region  betweeu  the  Grand 
and  Moreau  Rivers,  Dakota,  by  Bailey  Willis.     1885.     8^.     16  pp.     5  pi.     Price  5  cents. 

22.  On  New  Cretaceous  Fossils  from  California,  by  Charles  A.  White.  1885.  8°.  25  pp.  5  pi. 
Price  5  cents. 

23.  Observations  on  the  .Junction  between  the  Eastern  Sandstone  and  the  Keweenaw  Series  on 
Keweenaw  Point,  Lake  .Superior,  by  R.  D.  Irving  and  T.  C.  Chamberlin.  1885.  8^^.  124  pp.  17  pi. 
Price  15  cents. 

24.  List  of  Marine  Mollusca,  comprising  the  Quaternary  fossils  and  recent  forms  from  American 
Localities  between  Cape  Hatteras  and  Cape  Roqne,  including  the  Bermudas,  by  William  Healey  Dall. 
1885.     8°.     336  pp.     Price  25  cents. 

25.  The  Present  Technical  Condition  of  the  Steel  Industry  of  the  United  States,  by  P.hineas 
Barnes.     1885.     8°.     85  pp.     Price  10  cents. 

26.  Copper  Smelting,  by  Henry  M.  Howe.     1885.     8°.     107  pp.     Price  10  cents. 

27.  Report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  tisoal  year 
1884-'85.     1886.     8°.     80  pp.     Price  10  cents. 

28.  The  Gabbros  and  Associated  Hornblende  Rocks  occurring  in  the  Neighborhood  of  Baltiniore, 
Md.,  by  George  Huntington  Williams.     1886.     8'-\     78  pp.     4  pi.    Price  10  cents. 

29.  On  the  Fresh-water  Invertebrates  of  the  NorthAmerioan  .Jurassic,  by  Charles  A.  White.  1886. 
8-.    41  pp.     4  pi.     Price  5  cents. 

30.  Second  Contribution  to  the  Studies  on  the  Cambrian  Faunas  of  North  America,  by  Charles 
Doolittle  Walcott.     1886.     8".     369  pp.     33  pi.     Price  25  cents. 

31.  Systematic  Review^  of  our  Present  Knowledge  of  Fossil  Insects,  inchnling  Myriapods  and 
Arachnids,  by  Samuel  Hubbard  Scudder.     1886.     8°.     128  pp.     Price  15  cents. 

32.  Lists  and  Analyses  of  the  Mineral  Springs  of  the  United  States;  a  Preliminary  Study,  by 
Albert  C.  Peale.     1886.     8°.     235  pp.     Price  20  cents. 

33.  Notes  on  the  Geology  of  Northern  California,  by  .J.  S.  Diller.     1886.    8°.    23  pp.    Price  5  cents. 

34.  On  the  relation  of  the  Laramie  Molluscau  I'auna  to  that  of  the  succeeding  Fresh-water  Eocene 
and  other  groups,  by  Charles  A.  White.     1886.     8*=.     54  pp.     5  pi.     Price  10  cents. 

35.  Physical  Properties  of  the  Iron-Carburets,  by  Carl  Barns  and  Vincent  Strouhal.  1886.  8^. 
62  pp.     Price  10  cents. 

36.  Subsidenceof FineSolidParticlesiuLi(iuid8,l3yCarlBarus.    1886.    8°.    .58pp.    PricelOceuts. 

37.  Types  of  the  Laramie  Flora,  by  Lester  F.Ward.     1887.     8°.     354  pp.     57  pi.     Price  25  cents. 

38.  PeridotiteofElliottCouuty,  Kentucky,  by, J.  S.  Diller.     1887.     8-^.    31pp.    Ipl.    Price5oeuts. 

39.  The  Upper  Beaches  and  Deltas  of  the  Glacial  Lake  Agassiz,  by  Warren  Upham.  1887.  8°. 
84  pp.     1  pi.     Price  10  cents. 

40.  Changes  in  River  Courses  in  Washington  Territory  due  to  Glaciation,  by  Bailey  Willis.  1887. 
8°.     10  pp.     4  pi.     Price  5  cents. 

41.  On  the  Fossil  Faunas  of  the  Upper  Devonian — the  Genesee  Section,  New  York,  by  Henry  S. 
Williams.     1887.     8°.     121  pp.     4  pi.     Price  15  cents. 

42.  Report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal  year 
1885-86.     F.  W.  Clarke,  chief  chemist.     1887.     8^'.     1.52  pp.     1  pi.     Price  15  cents. 

43.  Tertiary  and  Cretaceous  Strata  of  the  Tuscaloosa,  Tombigbee,  and  Alaliama  Rivers,  by  Eugene 
A.  Smith  and  Lawrence  C,  Johnson,     1887.     8^ .     189  pp,     21  pi.     Price  15  cents, 


IV  ADVERTISEMENT. 

44.  Bibliography  of  North  American  Geology  for  1886,  hy  Nelson  H.  Darton.     1887.     8°.     35  pp. 

45.  The  Present  Condition  of  Knowledge  of  the  Geology  of  Texas,,  by  KobertT.  Hill.  1887.  8°. 
94  pp.     Price  10  cents. 

46.  Nature  and  Origin  of  Deposits  of  Phosphate  of  Lime,  by  E.  A.  F.  Penrose,  jr.,  with  an  Intro- 
duction by  N.  S.  inhaler.     1888.     8°.     143  pp.     Price  15  cents. 

47.  Analyses  of  Waters  of  the  Yellowstone  National  Park,  with  an  Account  of  the  Metho<ls  of 
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48.  On  the  Form  and  Position  of  the  Sea  Level,  by  Kobert  Simpson  Woodward.  1888.  8°.  88 
pp.     Price  10  cents. 

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50.  Formulas  and  Tables  to  Facilitate  the  Construction  and  Use  of  Maps,  by  Robert  Simpson 
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51.  On  Invertebrate  Fossils  from  the  Pacific  Coast,  by  Charles  Abiathar  White.  1889.  8'^.  102 
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.52.  Subaerial  Decay  of  Rocks  and  Oirigin  of  the  Red  Color  of  Certain  Formations,  by  Israel 
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60  Report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal 
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61.  Contributions  to  the  Mineralogy  of  the  Pacific  Coast,  by  William  Harlow  Melville  and  Wal- 
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62.  The  Greenstone  Schist  Areas  of  the  Menominee  and  Marquette  Regions  of  Michigan,  a  con- 
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with  an  introduction  by  Roland  Duer  Irving.     1890.     8°.     241  pp.     16  pi.     Price  30  cents. 

63.  A  Bibliography  of  Paleozoic  Crustacea  from  1698  to  1889,  including  a  list  of  North  Amer- 
ican species  and  a  systematic  arrangement  of  genera,  by  Anthony  W.  A^ogdes.  1890.  8°.  177  pp. 
Price  15  cents.  ..,,., 

64.  A  Report  of  W(u-k  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal 
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65.  Stratigraphy  of  the  Bituminous  Coal  Field  of  Pennsylvania,  Ohio,  and  West  Virginia,  by 
Israel  C.  White.     1891.     8°.     212  pp.     11  pi.     Price  20  cents. 

66.  On  a  Group  of  Volcanic  Rocks  from  the  Tewan  Mountains,  New  Mexico,  and  on  the  occur- 
rence of  Primary  Quartz  in  certain  Basalts,  by  Joseph  Paxson  Iddings.     1890.     8°.     34  pp.     Price  5 

67.  The  relations  of  the  Traps  of  the  Newark  System  in  the  New  Jersey  Region,  by  Nelson 
Horatio  Dartou.     1890.     8<=.     82  pp.     Price  10  cents. 

68.  Earthquakes  in  California  in  1889,  by  James  Edward  Keeler.  1890.  8°.  25  pp.  Price  5 
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69.  A  Classed  and  Annotated  Biography  of  Fossil  Insects,  by  Samuel  Howard  Scudder.  1890. 
8°.     101pp.     Price  15  cents. 

70.  A  Report  on  Astronomical  Work  of  1889  and  1890,  by  Robert  Simpson  Woodward.  1890.  8'=. 
79  pp.     Price  10  cents. 

71.  Index  to  the  Known  Fossil  Insects  of  the  World,  including  Myriapods  and  Arachnids,  by 
Samuel  Hubbard  Scudder.     1891.     8°.     744  pp.     Price  50  cents. 

72.  Altitudes  between  Lake  Superior  ^nd  the  Rocky  Mountains,  by  Warren  Upham.  1891.  8'^. 
229  pp.     Price  20  cents.  ^, 

73.  The  Viscosity  of  Solids,~1:)V  Carl  Barns.     1891.     8'^.     xii,  139  pp.     6  pi.     Price  15  cents. 

74.  The  Minerals  of  North  Carolina,  by  Frederick  Augustus  Genth.  1891.  8'=.  119  pp.  Price 
15  cents. 

75.  Record  of  North  American  Geology  for  1887  to  1889,  inclusive,  by  Nelson  Horatio  Darton. 
1891.     8°.     173  pp.     Price  15  cents. 

76.  A  Dictionary  of  Altitudes  in  the  United  States  (second  edition),  compiled  by  Henry  Gannett, 
chief  topographer.     1891.     8^.     393  pp.     Price  25  cents. 


ADVERTISEMENT.  V 

77.  'I'lio  Toxan  Pi^riiiiiiii  :iiiil  ifs  Mcso/dic  typcM  nl'  I''ii,N,silH,  liy  Clmi'lcs  A.  Wliiti'.  1H!(1.  K'  .  ."jl 
jip.      I  )>l.     I 'lice  10  (■(lilts. 

78.  .V  report  of  work  done  in  tho  Division  of  ClicniiHtrv  and  Physics,  niaiiilv  (lining  tlic  liscal 
year  188!t-'9U.      !•'.  W.  Clarke,  chief  chcniist.     1891.     8'.     131  pp.     Price  15  i-cntB.    ' 

79.  A  Late  \'oh'anic  Kriijilion  in  Xortliern  California  and  its  peculiar  lava,  liy  .1.  S.  Diller. 

80.  Correlation  jiapers — Devonian  and  Carlioniferons,  hy  Henry  Slialer  Williams.  1891.  8°. 
279  pp.     Price  20  cents. 

81.  Correlation  i>apcrs — Cambrinn,  liy  Cliinles  Doolittle  VValcott.  1891.  8'^'.  517  p]i.  '^  pi. 
Price  25  cents. 

82.  Correlation  |iapers— Cretaceous,  )iy  Charles  A.  Wliite.  1891.  8^.  273  pp.  3  pi.  Price  20 
cents. 

83.  ('orrelation  jiapers — Eocene,  by  William  I'.nllock  Clark.  1891.  8^\  173  pp.  2  pi.  Price 
15  cents. 

84.  Correlation  papers— Neocene,  l)y  W.  11.  Dall  and  (;.  D.  Harris.  1892.  8'^.  349  pp.  3  pi. 
Price  25  cents. 

85.  Correlation  papers — The  Newark  System,  by  Israel  Cook  Eussell.  1892.  8".  344  pp.  13  pi. 
Price  25  cents. 

86.  Correlation  papers — Archean  and  Algonkiaii,  hy  C.  E.  Van  Hise.  1892.  8^.  549  pp.  12  pi. 
Price  25  cents. 

90.  A  report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal 
year  1890-'91.     F.  W.  Clarke,  chief  chemist.     1892.     8°.     77  pp.     Price  10  cents. 

91.  Record  of  North  American  Geology  for  1890,  by  Nelson  Horatio  Darton.  1891.  8°.  88  pp. 
Price  10  cents. 

92.  The  Compressibility  of  Li(]uids,  liy  Carl  Barns.     1892.     8°.     96  pp.    29  pi.     Price  10  cents. 

93.  .Some  Insects  of  special  intei  ist  from  Florissant,  Colorado,  and  other  points  in  the  Tertiaries 
of  Colorado  and  Utah,  by  Samuel  Hubuard  Scudder.     1892.     8°.     35  pp.     3  pi.     Price  5  cents. 

94.  The  Mechanism  of  Solid  Viscosity,  by  Carl  Barns.     1892.     8°.     138  pp.     Price  15  cents. 

95.  Earthtjuakes  in  California  iu  1890  and  1891,  by  Edward  Singleton  Holdeu.  1892.  8°.  31pp. 
Price  5  cents. 

96.  The  Volume  Thermodynamics  of  Liquids,  by  Carl  Barns.     1892.     8^.     100  pp.     Price  10  cents. 

97.  The  Mesozoio  Echinodermata  of  the  United  States,  by  W.B.Clark.  1893.  8°.  207  pp.  .50pl. 
Price  20  cents. 

98.  Flora  of  the  Outlying  Carboniferous  Basins  of  Southwestern  Missouri,  by  David  White. 
1893.    8".     139  pp.     5  pi.     Price  15  cents. 

99.  Record  of  North  American  Geology  for  1891,  hy  Nelson  Horatio  Darton.  1892.  8°.  73  pp. 
Price  10  cents. 

100.  Bibliography  and  Index  of  the  publications  of  the  U.  S.  Geological  Survey,  1879-1892,  by 
Philip  Creveling  Warman.     1893.     8°. 

101.  Insect  fauna  of  the  Rhode  Island  Coal  Field,  by  Samuel  Hubbard  Scudder.  1893.  8°. 
27  pp.     2  pi.     Price  5  cents. 

In  press : 

102.  A  Catalogue  and  Bibliography  of  North  American  Mesozoic  Invertehrata,  by  C.  B.  Boyle. 

103.  High  Temperature  Work  in  Igneous  Fusion  and  Ebullition,  chiefly  in  relation  to  pressure, 
by  Carl  Barus. 

104.  Glaciation  of  the  Yellowstone  Valley  nortli  of  the  Park,  hy  W.  H.  Weed. 

105.  The  Laramie  and  the  overlying  Livingstone  Formation  in  Montana,  by  W.  H.  Weed,  with 
Report  on  Flora,  by  F.  H.  Knowltou. 

106.  The  Colorado  Formation  and  its  Invertebrate  Fauna,  by  T.  W.  Stanton. 

107.  The  Trap  Dikes  of  Lake  Champlain  Valley  and  the  Eastern  Adirondacks,  by  ,J.  F.  Kemp. 

108.  A  Geological  Reconnoissauce  in  Central  Washington,  by  Israel  C.  Russell. 

109.  The  Eruptive  and  Sedimentary  Rocks  on  Pigeon  Point,  Minnesota,  and  their  contact  phe- 
nomena, by  W.  S.  Bayley. 

110.  The  Paleozoic  Section  in  the  vicinity  of  Three  Forks,  Montan.i,  hy  A.  C.  Peale. 

In  preparation : 

—  Correlation  papers — Pleistocene,  by  T.  C.  Chamherlin. 

—  The  Moraines  of  the  Missouri  Coteau  and  their  attendant  deposits,  by  James  Edward  Todd. 
— •  On  the  Structure  of  the  Ridge  between  the  Tacouic  and  the  Gieen  Mountain  Ranges  in  Ver- 
mont; and  On  the  Structure  of  Monument  Mountain  in  Great  Barrington,  Mass.,  by  T.  Nelson  Dale. 

—  A  Bibliography  of  Paleobotany,  by  David  White. 

STATISTICAL  PAPERS. 

Mineral  Resources  of  the  United  States  [1882],  hy  Albert  Williams,  jr.  1883.  8'=.  xvii,  813  pp. 
Price  50  cents. 

Mineral  Resources  of  the  United  States,  1883  and  1884,  by  Albert  Williams,  jr.  1885.  8°.  xiv, 
1016  pp.     Price  60  cents. 

Mineral  Resources  of  the  United  States,  1885.  Division  of  Mining  Statistics  and  Technology. 
1886.    8^.    vii,  576  pp.     Price  40  cents. 


VI  ADVERTISEMENT. 

Mineral  Resources  of  tlie  United  States,  1886,  by  David  T.  Day.  1887.  8°.  viii,  813  pp.  Price 
60  cents. 

Mineral  Resources  of  the  United  States,  1887,  by  David  T.  Day.  1888.  8".  vii,  832  pp.  Price 
50  cents. 

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50  cents. 

Mineral  Resources  of  the  United  States,  1889  and  1890,  by  David  T.  Day.  1892.  8°.  viii,  671  pp. 
Price  50  cents. 

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50  cents. 

The  money  received  from  the  sale  of  these  publications  is  deposited  in  the  Treasury,  and  the 
Secretary  of  that  Department  declines  to  receive  l)ank  checlcs,  drafts,  or  postage-stam^js ;  all  remit- 
tances, therefore,  mu.st  be  by  i>()ST\i,  note  or  money  order,  made  jinyalile  to  the  Librarian  of  the 
U.  S.  Geological  Survey,  or  in  chhuency  for  the  exact  amount.  Correspondence  relating  to  the  pub- 
lications of  the  Survey  should  be  addressed 

To  the  Dire<_:tor  of  the 

United  States  Geological  Survey, 

Washington,  D.  C. 
Washington,  D.  C,  July,  1893. 


DEPARTMENT   OF   THE    INTERIOR 


MONOGRAPHS 


OP  THE 


United  States  Geological  Survey 


VOLUME    XIX 


WASHINGTON 

GOVERNMENT    PRINTING     OFFICE 
1892 


QEp 

vol./? 


UNITED  STATES  GEOLOGICAL   SURVEY 

.1.    W.    I'OWKI,!,    lilKWri'OK 


THE 


PENOKEE  IRON-BEARmG  SERIES 


01' 


MICHIGAN  AND  WISCONSIN 

BY 

KOLAND  DUER  IKVING 

AND 

CHAELES  KlOHAliJD  VAF  HISE 


WASHINGTON  ' 

GOVERNMENT    PRINTING    OFFICE 

1893 


CONTENTS. 


Letter  op  Transmittal -^'n 

Outline  of  this  I'aphu xvu 

lNTROnnCTI()X 1 

CllAPTEU   I. — (iEOI.OCUAL   KXI'LUKATIOXS   AXI>   I^ITEUATUliE O 

Geoldf^iciil  oxploratiipii  in  thr  district,  p.  5.  Barnes  (1817),  |).  5.  Whitney  and  Barnes 
(1817),  p.  6.  Kandall  (181-8),  p.  7.  Whittlesey  (1849  and  1860),  p.  7.  Liiphani  (1858), 
p.  7.  BrooksaudPnnipelly(1871),p.  8.  Irving  (1873, 1876. 1877, 1885),  p.  8.  Wight, 
Sweet  and  Wriglit  (1875),  p.  9.  Wriglit  (1876),  p.  9.  Chaniberliu  (1877),  p.  9.  Con- 
over  (1878),  p.  10.  Rominger  (1882),  p.  10.  Van  Ilise  (1884-1887),  p.  10.  Object  of 
the  work  nnder  the  IT.  S.  Geological  Snrvey,  p.  10.  Annotated  list  of  the  literatnre 
of  the  snlijeet,  p.  13.  * 

Chapter  II. — The  Soitiieux  Complex 103 

General,  p.  103.  Geographical  distribution,  p.  104.  The  W^eateru  granite,  p.  106.  The 
Western  green  schist,  p.  107.  The  Central  granite,  p.  111.  The  Eastern  green  schist, 
p.  116.     The  Eastern  granite,  p.  122.     Wnmuiary,  p.  122. 

Chapter  III.— The  Cherty  Limestone 127 

Relation  of  the  limestone  and  chert,  p.  127.  Geographical  distribntiou,  p.  128.  Pos- 
sible former  greater  continnity,  p.  129.  Thickness,  p.  130.  Petrographlcal  character 
of  the  limestone,  p.  130.  Petrographical  character  of  the  chert,  p.  132.  Change  to 
the  overlying  Quartz-slate,  p.  134.  Tabulation  of  petrographical  observations,  p. 
134.  Prominent  exposures,'  p.  138.  Origin  of  the  limestone  and  chert,  p.  140. 
Sumnuvry,  i>.  141. 

(Chapter  IV. — The  Quartz-Slate  Member 143 

Applicability  of  the  name,  p.  143.  Geographical  extent,  p.  144.  Topographical  fea- 
tures, p.  145.  Thickness,  p.  146.  General  petrographical  character  and  stratigraphy, 
p.  146.  Microscopical  character  of  the  feldspathic  (piartz-slates,  p.  149.  Micro- 
scopical character  of  the  biotitic  and  chloritic  quartz-slates,  ji.  151.  Microscopical 
character  of  the  vitreous  quartzite,  p.  153.  Microscopical  character  of  the  sand- 
stone, novaculite,  and  argillaceous  slates,  p.  154.  Tabulation  of  petrographical 
observations,  p.  155.  Contacts  with  the  Cherty  limestone  member,  p.  171.  Con- 
tacts with  the  Southern  Complex,  p.  172.  Change  to  the  Iron-bearing  member,  p. 
175.  Prominent  exposures,  p.  175.  Mode  of  deposition  and  source  of  material, 
p.  179.     Summary,  p.  180. 

Chapter  V.— The  Iron-bearing  Member 182 

Section  I. — Details 182 

Applicability  of  the  name,  p.  182.  Abruptness  of  transition  from  the  underlying 
Quartz-slate  member,  p.  184.  Geographical  extent,  p.  185.  Topographi(tal  features, 
p.  188.  ■  Thickness,  p.  189.  General  petrographical  character,  p.  190.  Distribu- 
tion of  the  three  types  of  rock,  p.  198.  Microscopical  character  of  the  cherty  iron 
carbonates,  p.  200.  Microscopical  character  of  the  ferruginous  slates  and  ferruginous 
cherts,  p.  202.  Microscopical  character  of  the  actinolitic  slates,  p.  210.  Tabulation 
of  petrographical  observations,  p.  215.  v 


VI  CONTENTS. 

Chapter  V — ContiTiiied. 

Section  II. — Origiu  of  the  rocks  of  the  Iron-bearing  member 245 

The  original  rock,  p.  246.  The  ferruginons  shites,  p.  253.  The  ferruginona  cherts,  p. 
254.     The  aotinolitic  slates,  p:  257. 

Section  III. — The  Animikie  iron-hearing  series 260 

The  olierty  iron  carbonates,  p.  262.  Tlie  ferruginons  slates,  p.  264.  The  ferruginous 
cherts,  p.  264.     Tlie  actinolitic  slates,  p.  266.     General,  p.  267. 

Section  IV.— The  iroil  ores 268 

Position  of  the  ores  in  the  Iron-bearing  member,  p.  268.  Dikes  in  Iron-bearing  member, 
p.  271.  Position  of  ore  in  reference  to  the  dikes,  p.  274.  Rock  above  the  ore,  p.  275. 
Practical  deductions  to  be  applied  in  prospecting  and  mining,  p.  276.  Nature  of  the 
rocks  of  the  Iron-bearing  member  adjacent  to  the  ore  bodies,  p.  279.  The  character 
of  the  ore,  p.  280.  A  particular  occurrence  of  iron  ore,  p.  283.  Chemistry  of  the 
process  of  concentration,  p.  283.  Time  at  which  concentration  of  the  main  ore  bodies 
occurred,  p.  284.  Process  of  conceuti-ation,  p.  285.  Exceptional  localities,  p.  290. 
Probable  extent  in  depth  of  ore  bodies,  p.  292.  Emmons  on  ore  deposits,  p.  293.  Iron 
ores  in  other  parts  of  Lake  Superior  country,  p.  293.  Summary  of  more  important 
conclusious,  p.  294. 

Chapter  VI. — The  Upper  Si.ate  Member 296 

Section  I.— Details -' -  296 

Name  and  basis  of  separation,  p.  296.  Transition  from  Iron-bearing  to  Upper-slate 
member,  p.  297.  Geographical  distribution,  p.  298  TopograiJhical  features,  ji.  301. 
General  petrographieal  character,  p.  302.  Petrographical  characters  of  the  four 
types  of  rock,  p.  304.     Tabulation  of  petrographical  observations,  p.  309. 

Section  II. — Origiu  of  the  upper  slate  rocks 332 

(1)  Quartzose  graywacke,  p.  333.  (2)  Muscovitic  and  biotitic  graywacke,  p.  336. 
(3)  Biotitic  graywacke,  p.  337.  (4)  Muscovitic  biotite-slate,  p.  338.  (5)  Nearly 
crystalline  muscovitic  biotite-schist,  p.  339.  (6)  Crystalline  muscovitic  biotite- 
schist,  p.  340.  Black  mica-slates,  p.  341.  Source  of  material,  p.  343.  Summary, 
p.  344. 

Chapter  VII. — The  Erxiptives 346 

Structural  relations,  p.  346.  General  character  of  the  rock,  p.  348.  Comparison  of 
Pouokee  greenstones  with  greenstones  of  the  Southern  Complex  and  Keweenaw 
series,  p.  349.  Microscopical  characters  of  the  diabases,  ji.  350.  Eruptives  in  the 
Iron-bearing  member,  p.  355.     Summary,  p.  3.58. 

Chapter  VIII. — The  Eastern  Area ^ 360 

Introduction 360 

Section  I. — The  Irou-bearing  member 361 

Distribution,  p.  361.  Petrographical  character,  p.  362.  Mingled  fragmental  and 
uonfragmental  sedimentation,  p.  362.  Probability  of  ore  deposits  in  the  Eastern 
area,  p.  365.     'i'^abnlation  of  petrographical  observations,  p.  366. 

Section  II. — FragmentarTocks  soutli  of  the  Greenstone-conglomerates 368 

Geographical  distribution,  p.  368.  Petrographical  character,  p.  369.  Tabulation  of 
petrographical  observations,  p.  371. 

Section  III. — The  Greenstone-conglomerates 374 

Distribution,  p.  374.     General  characteristics,  p.  374.     Origiu  of  the  Greenstone-con- 
glomerates, p,  377.     Tabulati(ui  of  petrographical  observations,  381. 
Section  IV. — Fragmental  and  ferruginous  rocks  north  and  east  of  the  Greenstone-conglom- 
erates    387 


CONTENTS.  vii 


I'age. 


CHAI'TIM!  Vlll^i'onliiiiioil. 
Section  l\' — (Idiiliimcd. 

Geof^i'iiiihiciil  (list  li  1)11  (ion,  1 1.  :iS7.  SunDiiiiiliiif;  rocks,  ji.  ;i8S.  C()iitiini:itii)ii  of  thi!  liolt 
eiisiaiiil  \vcst,)i.  :WS.  Slnicf.iirc  nl'  Mir,  hiOl,  p.  liKlt.  (inicnil  |>(',triif;r;iiilii(:il  i-liara(H(!r, 
]).  :i90.  .Mini;li'(l  l'nif;iii<'iil;il  and  iKiiid'anniciitnl  Nrdiiiicnts,  ]).  ;i!U.  (Jiiarsely  I'nit;- 
lll(^IItaI  rocks,  ]).  3itl.      Tabulation   of   iic'li-of;ra|iliical   oliscrvalions,  ]i.  lilMi. 

Seotion  V.— Tlic  (irci'iistonns 410 

Tlio  main  area,  \>.  110.  Tlic  area  in  seotion.s  20,  29,  anil  39,  townsliij)  47  N.,  raiif^e  43 
W.,  Micliiyan,  p.  411.  The  area  in  sections  13,  24,  14,  au<l  15,  township  47  N., 
raiif>e  44  W.,  Miohifjaii,  \t.  415. 

Section  VI.  -Stratigraphy ., 419 

Litliologieal  evidence  as  to  equivalence  witli  the  main  Peuokee  .area,  p.  419.  Strati- 
graphical  evidence  as  to  equivalence  with  the  main  Penokee  area,  p.  420.  Belations 
of  the  belts  of  the  Eastern  area  to  one  another,  ]>.  423.  Great  width  of  parts  of  the 
Eastern  area,  p.  423.  The  southern  dips,  p.  426.  Sequence  of  events,  p.  428.  Min- 
gled fragmental  and  uoufragmentai  sediments,  p.  432.  .  Sunnnary,  p.  433. 

C'HAl'TEK   IX. — GeNKRAL  GeOLOGY   OP   TlIK    DISTRICT 437 

Section  I. — Flexures  and  faults 437 

Curving  of  the  layers,  p.  437.     Fault  at  Bad  river,  p.  438.     Fault  at  Potato  river,  p. 
440.     Fault  in  the  Eastern  area,  p.  441. 
Section  II. — Structure 441 

The  Southern  Complex,  p.  441.  The  Cherty  limestone  and  Quartz-slate  members,  p. 
443.  Unconformity  between  the  Southern  complex  and  the  overlying  Cherty  Ijime- 
.stoue  and  Quartz-slate,  p.  444.  Unconformity  between  the  Chei'ty  lime.stone  and  the 
Penokee  series  proper,  p.  454.  The  Iron-bearing  and  Upper  slate  members,  p.  455. 
The  unconformity  at  the  base  of  the  Keweenaw  series,  p.  456.  The  Eastern  sand- 
stone aud  the  unconformity  at  its  base,  p.  461.  Rfisum^  of  geological  history,  p. 
463.  Why  the  district  is  given  a  sep.arate  memoir,  p.  466.  Depth  and  metamor- 
phism,  p.  467. 

Section  III. — Correlation ,.... 468 

Equivalency  of  Penokee  series  proper  with  Animikie  series,  p.  468.  Equivalency  of 
Penokee  and  Marquette  aeries,  p.  470.  Comparison  with  other  series,  p.  472.  Table 
showing  relations  of  Penokee  succession  to  that  of  other  Lake  Superior  districts, 
p.  473. 


ILLUSTRATIONS. 


Page 

Plate  I.  Preliminary  geological  map  of  the  Northwest , xx 

II.  General  geological  map  of  tlie  Penokee  district - 2 

III.  liejjrotluctiou  of  Whittlesey's  cross  section  of  Penokee  range '  .  18 

IV.  Reproduction  of  Whittlesey's  geidogical  map  of  the  Penokee  range 20 

V.  Detailed  geology,  Sheet  1 * ' 128 

VI.  Detailed  geology,  Sheet  2 130 

VII.  Topography,  Sheet  3 - 132 

VIII.  Detailed  geology.  Sheets 134 

IX.  Topography,  Sheet  4 136 

X.  Detailed  geology,  Sheet  4 ■ 138 

XI.  Topography,  Sheet  5 -  -  -  1^0 

XII.  Detailed  geology,  Sheet  5 142 

XIII.  Detailed  geology.  Sheet  6 144 

XIV.  Thin  sections  from  the  SoutUeru  Complex -'-  -  - 476 

Fig.  1.  Biotite-grauite. 
Fig.  2.  Biotitic  granitoid  gneiss. 
Fig.  3.  Hornhlende-schist. 
Fig.  4.  Hornblende-schist. 

XV.  Thin  sections  from  the  Southern  Complex 478 

Fig.  1.  Hornblende-granite. 
Fig.  2.  Hornblende-biotite-syenite. 
Fig.  3.  Biotite-gnelss. 
Fig.  4.  Hornblende-gneiss. 

XVI.  Thin  sections  from  the  Cherty  limestone  member 480 

Fig.  1.  Tremolitic  dolomite. 
Fig.  2.  Cherty  limestone. 
Fig.  3.  Concretionary  chert. 
Fig.  4.  Chert. 

XVII.  Thin  sections  from  the  base  of  the  Quartz-slate  member 482 

Fig.  1.  Chert  containing  fragmental  quartz. 

Fig.  2.  Quartzose  chert. 

Fig.  3.  Chert-conglomerate. 

Fig.  4.  Green  schist  and  conglomerate. 

XVIII.  Thin  sections  from  the  Quartz-slate  member. - - 484 

Fig.  1.  Graywacke-slate. 

Fig.  2.  The  same,  in  polarized  light. 

Fig.  3.  Cherty  slate. 

Fig.  4.  Sericitic  and  chloritic  slate. 

rs 


X  ILLUSTRATIONS. 

Page. 

Plate  XIX.  Thin  section-^  from  the  Quartz-slate  member 486 

Fig.  1.  Biotitic  clilorlte-slate. 

Fig.  2.  Biotite-slate. 

Fig.  3.  Sandstone. 

Fig.  4.  Argill.iceons  sliale. 

XX.  Thin  sections  from  the  upper  horizon  of  the  Quartz-slate  member 488 

Fig.  1.  Quartzite. 

Fig.  2.  The  same,  in  polarized  light. 
Fig.  3.  Ferruginous  quartzite. 
Fig.  4.  The  same,  in  polarized  light. 
XXI.  Thin  sections  of  sideritic  rocks  from  the  Iron-bearing  member  and  from  Law- 
rence county,  Ohio  — :.  — 4:90 

Fig.  1.  Sideritic  chert. 

Fig.  2.  The  same,  in  polarized  light. 

Fig.  3.  Sideritic  slate. 

Fig.  4.  Sideritic  and  ferruginous  chert. 

XXII.  Thin  sections  of  ferruginous  cherts  from  the  Iron-bearing  member 492 

Fig.  1.  Concretionary  clie.rt. 

Fig.  2.  The  same,  in  polarized  light. 

Fig.  3.  Brecciated  chert. 

Fig.  4.  Ferruginous  and  brecciated  chert. 

XXIII.  Thin  sections  of  ferruginous  cherts  and  actinolitic  slates  from  the  Iron-hearing 

member ^"* 

Fig.,  1.  Ferrnginous  chert. 

Fig.  2.  The  .same,  iu  polarized  light. 

Fig.  3.  Actinolitic  schist. 

Fig.  4.  The  same,  in  polarized  light. 

XXIV.  Thin  sections  of  the  actinolitic  .slates  from  the   Iron-bearing  member  of    the 

Penokee  series,  and  cherty  iron  carbonates  from  the  Vermillion  series 496 

Fig.  1.  Actinolite-magnetite-sohist. 
Fig.  2.  Actinolitic  slate. 
Fig.  3.  Cherty  iron  carbonate. 
Fig.  4.  The  same,  in  polarized  light. 

XXV.  Thin  sections  of  sideritic  slates  from  the  Animikie  series 498 

Fig.  1.  Cherty  iron  carbonate. 
Fig.  2.  Sideritic  chert. 
Fig.  3.  Actinolite-siderite-slate. 
Fig.  4.  The  same,  in  polarized  light. 
XXVI.  Thin   sections  of  ferruginous  clierts  and   iron   carbonates   from  the  Animikie 

300 

Fig.  1.  Concretionary  chert. 
Fig.  2.  The  samej  in  polarized  light. 
Fig.  3.  Ferruginous  chert. 
Fig.  4.  Sideritic  chert. 

XXVII.  Thiu  sections  .showing  formatiou  of  «oucretions  of  Iron-bearing  member 502 

Fig.  1.  Sideritic  chert. 

Fig.  2.  Sideritic  chert. 

Fig.  3.  Another  part  of  the  same  section. 

Fig.  4.  Ferruginous  chert. 


series  . 


ILLCSTlfATIONS.  XI 

f'l.A'lK   XXVI II.   'I'liiii  srrlioiisdl'  iiKiniictilic  Miuhii'l  iiHilil  ic  si. -lies  rroiii  Ihi^  I  r<>n-lie;iriiin-  ini'rn 

l"i 504 

l'i;j;.  1.  Act.iiiolitic  slalo. 
riji;.  2.  MuKiiotitic  roiiciotioniiiy  c.hi'rt. 
I'Mfj.  S.   MiukNmI  lUiifiiiotilic  jiisper. 
I'M;;'.  1.  Ai'l'iiKilil.ii',  8l;it:i\ 
XXIX.  Thin  .scctiDiiH  fniiu  tlio  ii'dii  I'Dnnatioii  of  the  AuiiiiikiH  sp.rie.s,  mid  IVnin  Law- 

rriic<'  coniity,  Ohio 506 

Kij;.  1.  Concrctiduary  (•hert.* 

Fig.  2.  Actiiiolitic  slati^ 

Fig.  3.  Actiiiolitic  ami  sideritic  slate. 

Fig.  4.  Clierty  iron  carboiiatc. 

XXX.  Ore  deposits : 508 

Fig.  1.  Longitudiual  sectiou  of  south  dbpoNit,  Moutieal  iiiinc. 

Fig.  2.  Longitudinal  sectiou  of  north  deposit,  Montreal  mine. 

Fig.  3.  Cross  section  of  south  deposit,  Moutreal  mine. 

Fig.  4.  Cross  sectiou  of  south  and  ^rth  depo.sits,  Moutreal  mine. 

Fig.  ~y.  Longitiulinal  sectiou  of  Pence  mine. 

Figs.  6,  7,  and  8.  Cross  sections  of  Pence  mine. 

XXXI.  Ore  deposits 510 

Fig.  1.  Longitudinal  sectiou  of  south  dcposih,  Colliy  miue. 
Fig.  2.  Longitudin.al  sectiou  of  north  deposit,  Colby  mine. 
Fig;  3.  Cross  section  of  north  ami  south  deposits,  Colby  miue. 
Fig.  4.  Longitudin.at  section  of  Trimble  mine. 
Fig.  5.  Cross  section  of  Trimble  mine. 
Fig.  6.  Cross  section  of  Minnewawa  mine. 

Fig.  7.  Theoretical'sectiou  to  show  variation  from  unaltered  carbonate 
to  ferruginous  chert  and  ore  bodies. 

XXXII.  Thin  sections  of  gray  waokes  from  Upper-slate  member 512 

Fig.  1.  Micaceous  graywaeke. 

Fig.  2.  Biotitic  and  mviscovitic  gr,aywacke. 

Fig.  3.  Biotitic  graywacke. 

Fig.  4.  Biotitic  and  chloritic  graywacke. 

XXXIII.  Thin  sections  showing  development  of  mica-slates 514 

Fig.  1.  Biotitic  and  muscovitic  graywacke. 
Pig.  2.  Biotite-slate. 
Fig.  3.  Biotite-slate. 
Fig.  4.  Biotite-slate. 

XXXIV..  Thin  sections  showiug  development  of  mica-schists  and  mica-slates 516 

Fig.  1.  Muscovitic  biotite-schist. 
Fig.  2.  Biotite-schist. 
Fig  3.  Biotite-slate. 
Fig  4    Biotite-slate. 

XXXV.  Thin  section.,  from  the  Eastern  area 518 

Fig.  1.  Ferruginou."  chert  and  quartzite. 
Fig.  2.  Greeustoue-conglomerate. 
Fig.  3.  Greenstone-conglomerate. 
Fig.  4.  The  same,  in  polarized  light. 


XII  ILLUSTRATIONS. 

Page. 

Plate  XXXVI.  Detailed  geology  in  tlie  vicinity  of  Penokee  gap 520 

XXXVII.  Geologicnl  map  of  Gimflint  lake  anil  vicinity,  Animikle  series 522 

Fiu.    1.  Reproduction  of  Barnes  and  Whitney's  geological  map  of  region  between  Agogebic 

lake  .and  Montreal  river - - 13 

2.  Reiirodnction  of  a  portion  of  Brooks  .and  Pumpelly's  geological  map  of  the  npper 

peninsnla  of  Michigan -  - 31 

3.  Schist  cut  by  massive  granite,  NW.  i  Sec.  4,  T.  46N.,  E.  2  E.,  Wisconsin 117 

4.  .Schist  cat  by  granite,  NE.  corner  Sec.  28,.T.  47  N.,  R.  45  W.,  Michigan 117 

5.  Map  of  exposures  at  Pot.ato  river - - 172 

6.  Junction  of  quartz-slate  and  green  schists  at  Potato  river 173 

7.  Large-scale  drawing  of  junction  of  quartz-slate  and  green  schists  at  Potato  river  ...  174 

8.  Map  .and  section  showing  position  of  rock  exposures  at  Tyler's  fork 177 

9.  Map  of  exposures  at  West  branch  of  Montreal  river -- - 178 

10.  Hornblende  enlargement  of  augite  in  diabase -  -  -  .112 

11.  Hornblende  enlargement  of  augite  in  di.abase - - -  -  -  413 

12.  Basal  conglomerate  in  contact  with  granite  in  Sec.  28,  T.  47  N.,  E.  42  W.,  Michigan..  450 


LETT-HR    OF    TRANSMITTAL 


Departmen'I'  ok  thk  Interior, 
U.  S.  Geological  Survey,   Lake  Superior  Division, 

.Madison,    Wis.,  May  1,   1890.. 

Sir:  I  transmit  lierewith  the  nianuscrii)t  of  a  memoir  ujiou  tlie  Peiio- 
kee  iron-bearing  series  of  Michigan  and  Wisconsin,  by  tlie  late  Prof 
Rohmd  D.  Irving-  and  myself. 

Prof.  Irving's  death  occurred  so  early  in  life  that,  while  his  contribu- 
tions to  geology  are  large,  his  later  works  show  that  he  had  but  fairly 
entered  the  period  which  would  have  been  the  most  fruitful  in  scientific 
results.  The  report  submitted  was  designed  by  Prof  Irving  to  be  the  first 
of  a  series  which  should  treat  each  of  the  important  iron-producing  districts 
adjacent  to  Lake  Superior.  To  him  who  built  up  the  division  and  who 
plainied  this  investigation  is  very  largely  due  whatever  excellence  this 
volume  may  have.  I  must  necessarily  assume,  for  the  most  part,  the 
responsibility  for  the  present  form  of  the  memoir. 

The  field  survey  for  the  present  report  began-  five  '  years  ago.  The 
seasons  of  1884,  1885,  and  the  larger  part  of  that  of  the  following  year  I 
gave  to  this  work.  For  a  time  in  1885  and  in  1886  Prof  Irving  accom- 
panied the  party  in  person.  When  the  work  began  the  district  was  one 
which  explorers  had  but  fairly  entered  and  no  railroad  reached  any  part  of 
the  range  except  the  Wisconsin  Central,  which  crosses  it  at  Penokee  gap. 
The  district  has  since  developed  into  one  of  the  most  important  iron-pro- 
ducing areas  of  tlie  country,  its  product  in  the  lake  Superior  region  being 
exceeded  by  that  of  the  Marquette  district  only. 

Before  the  beginning  of  this  investigation  Prof  Irving  had  done  a 
large  amount  of  field  work"  upon  a  portion  of  the  range  for  the  Wisconsin 


XIV  LETTER  OF  TRANSMITTAL. 

Geological  Survey  and  had  prepared  a  systematic  report  upon  this  part 
of  it.  He  was  thus  able  to  direct  the  more  detailed  examination  of  the 
whole  area  so  that  no  loss  of  time  should  occur.  The  plan  of  the  work 
and  of  the  present  book  is  very  largely  his. 

The  descriptions  of  the  formations  and  thin  sections  are,  for  special 
reasons,  given  in  greater  detail  than  is  intended  with  "any  subsequent  area. 
This  is  the  first  of  the  iron-producing  districts  of  lake  Superior  in  which  the 
geology  has  been  worked  out  in  detail,  and  the  fundamental  conclusions 
reached  are  in  opposition  to  those  expressed  by  some  geologists ;  so  that  it 
was  thoiight  necessary  to  make  the  facts  fully  accessible  to  those  who 
desire  to  have  them.  The  succession  of  formations  is  so  clear  also  that  it 
is  believed  that  the  results  contained  in  this  report  will  serve  as  a  key  to 
unlock,  in  a  measure,  the  more  complicated  geological  structure  of  the 
adfaceut  iron-producing  districts.  In  order  to  enable  the  general  reader  to 
avoid  details,  the  descriptions  of  particular  rock  exposures  and  their  thin 
sections  for  each  of  the  formations  are  placed  together  in  small  type. 

To  Mr.  J.  Parke  Channing  our  especial  thanks  are  due  for  a  large 
amount  of  gratuitous  work,  and  in  particular  for  his  detailed  examination 
and  drawings  of  many  of  the  mines,  and  for  the  facts  contained  as  to  the 
structural  relations  between  the  dike  rocks  and  ore  bodies.  As  mining 
inspector  of  Gogebic  county,  he  resided  in  the  district  and  has  from  time 
to  time  given  us  the  results  of  the  latest  developments.  Mr.  J.  M..  Long- 
year,  of  Marquette,  Michigan,  has  also  given  us  nuich  assistance,  including 
access  to  his  very  large  collection  of  specimens  of  the  Gogebic  end  of  the 
range,  made  as  the  agent  of  the  Lake  Superior  and  Portage  Ship  Canal 
Company.  To  Mr.  B.  N.  White  and  Mr.  Charles  Oley,  woodsmen,  we  are 
indebted  for  all  the  assistance  which  could  be  given  by  skilled  men  some- 
what familiar  with  the  district.  To  numerous  miners  and  mine  superin- 
tendents we  are  indebted  for  many  courtesies.  With  a  few  exceptions, 
information  with  referertce  to  mining  properties  has  been  freely  granted  to 
us  by  all. 

The  original  design  was  to  publish  the  whole  book  as  a  joint  produc- 
tion, but  the  death  of  Prof  Irving  occurred  in  the  midst  of  the  preparation 
of  the  volume,  so  that  umch  of  it  has  wholly  devolved  upon  roe;  conse- 


LKTTEIJ  OF  THANRl\riTTAL.  XV 

quently,  I  luivc  not  I'cll  \v:ni;mlcil  in  iisiuj^-  his  lumui  in  this  general  way. 
Chapter  i  lie  prcpiircd,  wilh  the  cxceijtiiin  of  a  lew  pages,  and  this  is 
credited  to  him;  cliaptci-s  in,  iv,  and  v  wt^-o  jointly  pre[)ared;  the  remain- 
ing ehai)ters  have  fallen  upon  me.  In  order  to  lUJike  it  pert'eetly  clear  to 
what  extent  the  woi'k  can  have  the  shelter  of  Prof.  Irving's  authority,  each 
chapter  is  headed  by  the  name  of  its  author. 

Very  respectfully,  yoiu-  obedient  servant, 

C.  R.  Van  Hise, 
Geologist  in  charge. 
Hon.  J.  W.  Powell, 

Director  U.  8.  Geological  Survey,  Washington,  D.  C. 


OUTLINE    OF    THIS    PAPER. 

The  Peuokcc  scries  jjiopei-  is  ii  .siiccessiou  of  formations  extendiag,  with  some 
bvealcs,  from  Like  Gogebic,  Michigau,  to  lake  Numakagon,  in  Wisconsin,  a  distance 
of  abont  80  miles.  It  is  a  mouoclinal  series,  its  dips  being  universally  to  the  north. 
Tlie  three  formations  making  nj)  the  Penokee  succession  are  the  Quartz-slate  member, 
the  Iron-bearing  member,  and  the  IJpj)er-slate  member,  and  below  these  is  the 
Cherty  limestone  formation.  The  series  is  sharply  separated  geographically  from  a 
crystalline  complex  to  the  south,  called  the  Southern  Complex.  It  is  separated  with 
equal  shar^mess  from  the  Keweenaw  series  to  the  north. 

Cha])ter  I  gives  a  history  of  tlie  geological  explorations  iu  the  Penokee  district 
and  a  full  summary  of  previous  literature. 

Chapter  II  treats  of  the  Southern  Complex.  This  consists  of  two  main  types  of 
rocks — light  colored,  coarse  grained  granites  and  granitegueisses,  and  dark  colored, 
flue  grained,  iiuely  laminated  schists.  In  passing  from  west  to  east  arc  found  in 
order  the  Western  granite,  the  Western  green  schist,  the  Central  granite,  the 
Eastern  gi-een  schist,  and  the  Eastern  granite  areas.  The  rocks  of  the  Southern 
Complex  are  always  comi^letely  crystalline.  If  any  of  them  are  of  fragmeutal  origin 
their  present  constitution  gives  no  evidence  of  this.  The  contacts  between  the  granite 
and  the  schists  are  eruptive,  the  granite  being  the  intrusive  rock.  The  schists  arc 
consequently  older.  Certain  of  the  most  laminated  schists  grade  iuto  rocks  which 
are  of  distinctly  eruptive  ty])es,  and  hence  the  only  rocks  in  the  Southern  Complex  the 
origin  of  which  is  known  are  igneous. 

Chapter  III  treats  of  the  Cherty  limestone  below  the  Penokee  series  proper. 
This  formation,  instead  of  being  continuous,  is  fouUd  only  at  intervals,  and  varies  in 
thickness  uii  to  300  feet.  It  consists  of  cherty  dolomitic  limestone  alternating  with 
layers  of  chert.  The  Cherty  limestone  is  a  water-deposited  sediment,  but  wluithei'  of 
chemical  or  organic  origin,  or  of  both,  is  uncertain. 

Chapter  IV  treats  of  the  Quartz-slate  member.  This  member  rests  directly  upon 
the  Cherty  limestone  or  upon  the  Southern  Complex.  It  is  a  persistent,  well  charac- 
terized horizon,  having  an,  average  thickness  of  about  500  feet.  The  Quartz-slate  is 
always  plainly  clastic,  and  quartz  is  its  prominent  constituent,  although  other 
minerals,  and  especially  feldspar,  are  not  unimportant,  Its  uppermost  horizon  is  a 
layer  of  pure  vitreous  quartzite, 

MOH^  XIX — -II  ^VH 


xviir  OUTLINE  OF  THIS  PAPER. 

Chapter  V  treats  of  the  Iron-bearing  member.  This  persistent  formation,  aver- 
a"-iug  about  800  feet  in  thickness,  rests  upon  the  vitreous  qnartzite  of  the  Upper-slate 
member.  It  now  consists  of  three  main  types  of  rock,  cherty  iron  carbonates,  ferru- 
ginous slates  and  cherts,  and  actinolitic  and  maguetitic  slates.  The  first  of  these  is 
the  original  type  of  rock  and  from  it,  by  means  of  chemical  changes,  the  second  and 
third  types,  as  well  as  the  ore  bodies,  have  been  produced.  Theore  bodies  are  found 
in  the  loAvest  horizons  of  the  formations  and  are  secondary  concentrations.  They 
occur  in  V-shaped  troughs,  one  side  of  the  V's  being  the  upper  quartzite  of  the  Quartz- 
slate,  and  the  other  diabase  dikes.  In  the  Animikie  series,  on  the  opposite  side  of 
lake  Superior,  is  an  iron-bearing  formation  which  has'the  same  types  of  rock,  derived 
from  the  same  original  form  as  in  the  Penokee  series. 

Chapter  VI  treats  of  the  Upper-slate  member.  This  formation  rests  upon  the 
Iron-bearing  member.  It  has  a  maximum  thickness  of  12,000  feet,  and  has  an  extent 
east  and  west  for  many  miles,  although  it  is  not  so  extensive  as  the  Iron-bearing  and 
Quartz-slate  members.  The  formation  is  of  clastic  origin  and  consists  mainly  of  gray- 
wackes  and  graywacke-slate.  It  is  now  locally  altered  by  metasomatic  changes  so 
that  it  has  become  a  crystalline  mica-schist. 

Chapter  VII  treats  of  the  eruptives.  The  Penokee  eruptives  are  diabases,  which 
structurally  are  of  two  classes,  dikes  cutting  the  formations,  and  interbedded  sheets 
which  are  probably  intrusives  of  the  same  age  as  the  dikes.  The  eruptives  are  fresh 
in  the  slate  members,  but  are  much  or  completely  altered  in  the  Iron-bearing  member, 
showing  that  environment,  not  age,  is  the  important  factor  in  the  preservation  of 
these  rocks. 

Chapter  VIII  treats  of  the  Eastern  area.  In  the  eastern  part  of  the  district,  as  a 
result  of  contemporaneous  volcanic  action,  the  Penokee  succession  is  disturbed,  and 
associated  with  the  ordinary  detrital  rocks  are  surface  basic  volcanic  flows,  and  also 
greenstone-conglomerates  and  agglomerates.  Consequent  upon  this  volcanic  distirrb- 
ance  the  regular  alternation  of  clastic  and  nonclastic  members  of  the  Penokee  vsuc- 
cession  is  much  modified,  so  that  the  number  and  order  of  the  formations  here  found 
differ  from  those  in  the  remainder  of  the  district. , 

Chapter  IX  treats  of  the  general  geology  of  the  region.  While  the  oiitcrop  of  the 
members  of  the  Penokee  series  as  a  whole  are  gently  curved,  sharp  flexures  and  faults 
are  not  common.  One  fault  occurs  at  Bad  river,  another  at  Potato  river,  and  per- 
haps one  in  the  Eastern  area.  The  base  of  the  Quartz-slate  member  contains  frag- 
ments derived  from  the  "Cherty  limestone  member,  showing  that  between  these  forma- 
tions there  was  an  erosioiiiuterval.  How  great  the  time  gap  represented  by  this  is 
there  is  no  means  of  judging,  except  that  the  chert  of  the  limestone  was  certainly  in 
its  present  condition  at  the  time  of  the  deposition  of  the  Quartz-slate.  The  Quartz- 
slate,  Iron-bearing  and  Upper-slate  members  form  a  conformable  succession.  Between 
tl*B  Penokee  series  proper  and  the  Southern  Complex  there  is  a  very  great  uncon^ 


UUTLINE  OK  THIS  PAPER.  xix 

fonnity.  Before  flic  dcpositicin  nl'  (he  Peiiokec  series  the  .SouIIhtm  (Jomplex  Lad 
reaclictl  its  |)rcs('iit  coiiiplclcly  cryslalliiic  condition  and  was  reduced  nearly 'to  a  ))a,se 
level.  Between  the  J'lMioUee  series  and  the  oveilyinfi'  Kcweeiiawau  is  a  second  very 
considerable  uneoiitt)rniity,  siilliciijnt  to  ha-ve  removeil  in  places  tlui  entire  Penokee 
succession.  After  the  erosion  and  depositiun  of  the  Keweenaw  series  the  whole  was 
tilte<l  toward  the  north,  so  as  to  <iive  the  jyresent,  monoclinal  stj'ucture,  after  which  the 
Uastern  san<lstone  was  deposited  njjon  the  upturned  series  in  its  inesent  horizontal 
attitude.  The  Penokee  series  is  limited  on  the  east  by  the  overla|)i)ing-  Eastern  sand- 
stow;  on  the  west  it  was  cut  away  by  erosion  before  Keweenawan  time,  so  that  the 
Copper-bearing-  series  rests  directly  upon  the  Southern  Complex.  The  Penokee  series 
proper  is  the  efpiivalent  of  the  Animikie,  the  Upper  Original  Huronian,  and  constitutes 
a  part  of  the  Upper  Hiu-onian  of  the  lake  Superior  region.  The  Clierty  limestone  is 
the  equivalent  of  some  part  of  the  Lower  Hujonian.  These  two  together  are  a  part  ot 
the  Algonkiau,  and  the  Southern  Complex  is  Archean, 


U.  S.  GEOLOGICAL   SURVEY. 


MONOGRAPH    XIX.  PL.I. 


I'l.KIMOCKNK  CHKTACKOHS 


KMi 


m 


CAIfnONII'KliOllS  IIKVONIAN 

Conl  Mpcisiii-ps         Siib  Ciirhoiiilcrinis 


Cc 


SILUHIAN 


I'ppL'i  Sihinaii  Lower  SiIuikhi 

;     Su     I  SI       I 


l'lfi;i,IMlNAHY     GKOI.OGIC'Al. 


Cmnpilrd  lr\'  H  fJ  li-vimjiowrcoiiipuriylfeiiiili    ffto^rt-ssut  tlii'  Wv  ['imilni.iii  Kiiiiniliuiis 


.■  D^/'f  ojj 


ALOtJNKIAN 


(drantte  Gnruss 


I    Ah  T/iff/rit/'Hfif  flui-oiiifii 

Ah!  /*/(,.  MniYiiiilte  -  Mi'iiU'ii'/Kf  livii    /li-nnni/  .Srhislt. 

AhJ  /■/<(■  H'lvrvi/is./.  IViHf I- .SV/./,-fl 

Ah  4  fh.- Pfiuiltef  {n»i  flrar.iiif.SclnikUi. 

I    AhS  Tl.r  Si  l..»,x.H  Shite^ 

1    Ane  r/if  ainif'ivii  I'l'l/i-f  QiiaHxitcs 

A  he  Vlw  »tua''ooVuiiitxiirs 

Ans  rtie  .tioiiiO-Kirtyitiv, 


\\M^    OK     TiU':    xoHrii\vi;sT 


THE     PENOKEE    IRON -BEARING    SERIES    OF 
MICHIGAN   AND    WISCONSIN. 


By  R.  D.  Irving  and  C.  R.  Van  Hise. 


INTRODUCTION. 

In  a  previous  publication  of  tlie  Survey^  one  of  the  writers  of  this 
memoir  has  provisionally  mapped  under  a  common  color  all  of  the  various 
areas  in  the  lake  Superior  region,  the  rocks  of  which  are  now  regarded  by 
any  authority  as  the  equivalent  of  the  Huronian  series  of  the  Canadian 
geologists.  The  provisional  mapping  was  preliminary  to  a  thorough  study 
of  each  of  these  areas  in  the  United  States,  with  the  object  of  determining 
not  only  its  own  structure  and  stratigraphy  and  the  genesis  of  its  rocks, 
but  also  the  general  stratigraphy  of  the  pre-Cambrian  formations  in  the 
northwestern  states.  This  map,  with  corrections  iip  to  date,  is  repro- 
duced in  this  volume  (PI.  i).     The  areas  there  included  are: 

The  Original  Huronian  (H). 

The  Marqviette-Menorainee  Iron-bearing  schists  (Hj). 

The  Wisconsin  valley  slates  (Kg). 

The  Penokee  Iron-bearing  series  (H4). 

The  St.  Louis  slates  (Kg). 

The  Chippewa  valley  quartzites  (Hg). 

The  Black  river  Iron-bearing  schists  (H,). 

The  Baraboo  quartzites  (Hg). 

'  Preliminary  Paper  on  an  Investigation  of  the  Archean  Rooks  of  the  Northwestern  States,  by 
R.  D.  Irving.     U.  S.  Geol.  Survey,  Fifth  Annual  Report,  pp.  181-241,  PI.  xxil. 

MON  XIX 1  1 


2  THE  PENOKEE  IRON-BE AEING  SERIES. 

The  Sioux  quartzites  (H9). 

The  Animikie  series,  which  inckides  the  Mesabi  range  (Hjo). 

Folded  schists  of  Canada,  inckiding  VermiHon  lake  series  (Hu). 

This  classification  has  reference  not  only  to  a  certain  geographical 
separateness  of  the  various  areas,  but  also  to  certain  peculiar  geological 
characteristics  which  each  area  or  group  of  areas  displays. 

In  the  six  and  a  half  years  that  have  elapsed  since  the  beginning  of 
our  present  study  of  the  older  formations  of  the  Northwest,  work  has  been 
done  in  a  number  of  these  areas,  much  new  material  has  been  collected, 
and  many  interesting  new  conckxsions  have  been  reached.  In  the  case 
of  the  Penokee  area,  this  study  has  been  completed.  An  account 
of  this  region  is  particularly  called  for,  because  of  the  absence  of  folds 
and  of  the  accompanying  changes  due  to  pressure — which  so  greatly 
increase  the  difficulties  of  study  in  the  Marquette-Menominee  region,  for 
instance — and  because  of  the  clear  and  mnnistakable  nature  of  its  relations 
to  adjoining  formations.  Our  design,  then,  is  to  publish  at  the  present  time 
a  full  account  of  the  area,  showing  in  detail  the  processes  by  which  the 
principal  results  are  reached,  and  to  use  these  results  in  future  in  the  study 
of  other  areas  of  the  Northwest. 

The  general  geographical  position  of  the  Penokee  belt  will  be  best 
understood  by  reference  to  PI.  i.  The  larger  scale  map  of  PI.  11  will  serve 
to  show  more  definitely  the  extent  and  position  of  the  area  the  geology  of 
which  is  to  be  discussed.  This  belt  stretches  from  lake  Gogebic,  Michigan, 
to  lake  Numakagon,  in  Wisconsin,  a  distance  of  about  80  miles.  Its 
course  from  lake  Gogebic  to  the  Montreal  river  is  about,  west,  and  from  the 
Montreal  to  lake  Numakagon  about  20°  south  of  west.  Our  more  detailed 
investigations  have  extended  over  an  average  width  of  5  miles.  Of  this 
width,  the  iron-bearing  series,  which  particularly  forms  the  subject  of  the 
present  volume,  occupies  from  a  quarter  of  a  mile  to  about  3  miles,  the 
remainder  being  occupied  to  the  south  by  granites,  gneisses,  and  schists, 
and  to  the  north  by  the  interbedded  eruptives  and  fragmentals  of  the 
Keweenaw  series. 

Whatever  the  relations  of  the  schist,  gneiss  and  granite  mapped  in  PI.  11 
to  one  another  and  to  the  rocks  adjacent,  they  are  sharply  separated  in  surface 


U   5  SEOLOGICAL  SURVEY- 


MONOGRAPH    XIX  PL  II 


"i~>r 


f\ 


Cam  BRIAN 

Eastern  Sandstone       Keweenawan 

r^  \jl]  r*^ 


GENERAL     GEOLOGICAL    MAP    OF     THE 

BY    R    D     IRVINO     AND  C   R    VAN 
Scale  of  Map  I  in  -  3  miles 

Algonkian 


PENOKEE     DISTRICT 

HISE 


Penokee    Series 


ARCH  EAN 

SOOTHERN    COMPLEJI 


BEAniNG   UEMBEff  qUARTZ- SLATE  MEMBER       CHERTr    LIMESTONE 

^m         \jK]         IB 


;"STONE  4)NG10MERATC5      FEftnuGINOUS  SLATES  ERUPTIVCS  ^  6»»H(TE  (  OOmrrOiO  CNI'SS    SCmiSTIum  CMixEOeNtlSS 

CMB         [23        ■■        Qn        cm 


INTRODUCTION.  3 

(listribution  tVom  the  series  of  parallel  belts  northward.  These  belts,  four 
iu  uuiubi'r,  follow  in  reg'ular  succession,  with  the  exception  of  a  few  miles 
at  the  east  end  of  the  area,  'l^hey  are  separated  from  each  other  upon  the 
principle  of  frag'mental  and  uonfragmental  character.  This  memoir  treats 
of  the  rocks  between  the  gneiss-granite  areas  and  the  Keweenaw  series,  and 
to  theiu  is  applied  the  name  Penokee  series. 

The  Wisconsin  geologists,  as  will  be  seen  by  the  literature,  (tailed  the 
iron-bearing  rocks  and  associated  slates  the  Penokee  series.  On  the  Michi- 
gan side  of  the  boundary  the  course  of  travel  was  largely  by  the  way  of 
Agogebic,  or  as  it  is  now  contracted,  Gogebic  lake,  and  the  geologists, 
explorers,  and  miners  gave  to  that  part  of  the  area  the  name  Gogebic  district. 
As  the  areas  are  parts  of  continuous  geological  series,  one  or  the  other  of  these 
two  names  must  be  accepted  for  the  whole,  or  else  a  new  name  be  coined 
by  their  combination.  The  latter  course  would  be  perhaps  the  less  objec- 
tionable if  the  resultant  name  Penokee-Gogebic  were  not  so  awkward.  The 
only  systematic  geological  treatment  of  any  part  of  the  district  is  that  by 
Irving  and  Wright  in  the  Geology  of  Wisconsin;  hence,  under  the  law  of 
priority,  the  term  Penokee  series  is  in  this  volume  used  to  cover  the  whole 
area. 

The  southernmost  of  the  belts  of  the  Penokee  series  is  called  the  Cherty 
limestone.  This  name  sufficiently  indicates  its  character.  Whether  it  is  of 
direct  chemical  or  of  organic  clastic  origin,  it  now  gives  no  evidence  of 
having  been  fragmental.  The  next  belt  to  the  northward  is  called  the 
Quartz-slate  member,  because  quartz  is  the  preponderating  constituent  and 
a  slaty  structure  the  normal  one.  It  is  sharply  separated  from  the  under- 
lying Cherty  limestone  member  l^y  the  fact  that  it  everywhere  reveals  in 
thin  sections  its  essential  fragmental  character,  and  also  by  the  fact  that  it 
bears  debris  from  the  Cherty  limestone.  Next  to  the  north  is  the  Iron- 
bearing  member,  so  called  because  all  the  known  ore-bodies  occur  within 
it.  Whatever  its  origin,  it,  like  the  Cherty  limestone,  never  gives  any  evi- 
dence of  a  fragmental  character.  North  of  this  belt  is  the  Upper  slate  mem- 
ber. This  is  in  places. several  times  as  thick  as  the  three  lower  members 
combined,  but  the  whole  width  is  included  within  a  single  belt  because  it  is 
everywhere  substantially  alike,     It  is  a  graywacke,  graywacke-slate,  mica- 


4  THE  PENOKEE  IRON  BEAEING  SERIES. 

slate,  or  mica-schist,  which  is  chiefly  composed  of  quartz  and  feldspar.  Like 
the  Quartz-slate  member,  in  thiu  section  it  usually  reveals  its  fragmental 
character.  The  area  of  rocks  situated  at  the  same  geologic  horizons  as 
these  four  belts,  between  Grogebic  lake  and  the  middle  of  T.  47  N.,  R.  44  W., 
Michigan,  does  not  make  up  so  plain  a  succession,  so  that  this  part  of  the 
district  is  separated  from  the  four  belts  just  spoken  of  as  the  Eastern  area, 
and  under  this  title  is  given  separate  treatment. 


CHAPTER   T. 


By  R.  D,  Irving. 


GEOLOGICAL  EXPLORATIONS  AND  LITERATURE. 

Geological  exploration  in  the  district— Barnes  (1847);  Whitney  and  Barnes  (1847);  Randall  (1848); 
Whittlesey  (1849  and  1860);  Laphara  (1858);  Brooks  and  Piimpelly  (1871);  Irving  (1873,  1876, 
1877,  1885) ;  Wight,  Sweet,  and  Wright  (1875) ;  Wright  (1876) ;  Chamberlin  (1877) ;  Conover  (1878) ; 
Rominger  fl882);  Van  Hise  (1884-1887).  Object  of  the  work  under  the  U.  S.  Geological  Survey. 
Annotated  list  of  the  literature  of  the  subject. 

As  in  the  case  of  most  other  reo-ions  on  the  south  side  of  lake 
Superior,  the  first  geological  explorations  made  in  the  Penokee  country 
date  quite  far  back,  but  no  attempt  at  detailed  work  was  made  before  that 
done  west  of  the  Montreal  river  by  the  Wiscon.sin  survey,  1873-1878. 
East  of  this  river,  in  Michigan,  the  only  approach  to  a  detailed  study  prior 
to  that  by  the  authors  of  the  j^n-esent  volume  was  by  Dr.  C.  Rominger, 
then  state  geologist  of  Michigan,  in  1882.  Dr.  Rominger's  results  have  not 
yet  been  published,  but  he  has  been  kind  enough  to  send  me  a  manuscript 
copy  of  that  portion  of  his  last  report  which  covers  this  district.  While 
this  report  is  unaccompanied  by  maps,  and  while  Dr.  Rominger's  locations 
of  specimens  are  not  closer  than  the  quarter  section,  it  yet  contains  much 
valuable  material. 

I  add  a  few  notes  with  regard  to  each  of  the  geologists  who  have 
personally  investigated  any  portion  of  the  Penokee  district,  arranging 
these  notes  chronologically,  the  date  after  each  name  being  the  time  of 
exploration. 

Barnes  (1847).— Mr.  George  0.  Barnes  was  one  of  the  assistants  on  the 
U.  S.  Geological  and  Mineralogical  Survey  of  the  lake  Superior  land 
district,  then  under  the  direction  of  Dr.  C.  T.  Jackson,  and  subsequent  to 


6  THE  PENOKEB   IRON-BEARIiSTG   SERIES. 

1848  under  the  joint  charge  of  Messrs.  J.  W.  Foster  and  J.  D.  Whitney. 
Mr.  Barnes  appears  to  have  been  the  first  geologist  to  enter  any  portion  of 
our  district.^  In  the  summer  of  1847  he  accompanied  the  township  land 
surveyors,  noting  and  locating  all  rock  exposures  met  with  on  the  lines 
surveyed,  from  a  point  on  the  Ontonagon  river  where  it  crosses  the  east  line 
of  T.  49  N.,  R.  41  W.,  Michigan,  southward  for  28  miles;  then  westward  12 
miles,  north  12  miles,  west  6  miles,  nqrtli  6  miles,  and  east  6  miles  to  the 
northeast  corner  of  T.  47  N.,  R.  43  W.,  Michigan;  and  thence  northward 
to  Grogebic  lake.  Mr.  Barnes's  exploration  was  thus  chieiiy  in  the  granitic 
and  gneissic  region  in  the  southern  part  of  the  southern  end  of  our  district. 
One  of  his  lines,  however,  crossed  the  iron-bearing  .slates,  but  without 
affording  him  any  indication  of  their  existence.^ 

Whitney  and  Barnes  (1847).— Later  in  the  Same  scasou  Mr.  Barnes  accom- 
panied Mr.  J.  D.  Whitney,  then  one  of  the  chief  assistants  under  Jackson, 
in  a  second  trip  into  the  country  between  Gogebic  lake  and  the  Montreal 
river.^  Their  coui'se  was  from  the  mouth  of  Black  river  to  the  northwest 
comer  of  T.  48  N.,  R.  46  W.,  Michigan  ;  thence  along  the  range  and  town- 
ship lines  (then  the  only  surveyed  lines  in  the  region),  south  12  miles,  east 
6  miles,  north  12  miles,  and  west  5  miles  to  the  Black  riAJ^er,  on  the  north 
line  of  T.  48  N.,  R.  46  W.,  Michigan.  This  route,  as  also  that  previously 
traversed  by  Mr.  Barnes,  is  indicated  on  Fig.  1.  Thus  Messrs.  Barnes  and 
Whitney  made  in  1847,  in  all,  three  traverses  of  oiu- district,  the  easternmost 
one  lying  6  miles  west  of  lake  Grogebic,  and  the  westernmost  2  to  5  miles 
east  of  the  Montreal  river.  Whitney  also  saw  nothing  on  his  trip  to  sug- 
gest to  him  the  existence  in  the  region  of  any  other  formation  than  the 
granites  to  the  south  and  the  eruptive  greenstones  of  the  copper-bearing 
series  to  the  north.  Accordingly  this  portion  of  their  district  was  mapped 
by  Messrs.  Foster  and  Whitney  with  the  granites  to  the  south  and  the  erup- 
tive greenstones  to  the  north  coming  directly  into  contact  with  one  another, 
without  any  intervening  slates.     This  mapping  was  rej^roduced  on  ail  later 

'  Diary  of  lielfl  work  for  the  summer  of  1847,  in  Report  on  the  Geological  and  Mineralogienl  Sur- 
vey of  the  Mineral  Landa  of  the  United  States  in  the  State  of  Michigan;  by  G.  T.  .Jackson,  United 
States  Geologist.  Senate  Docs.,  1st  sess.  31st  Cong.,  1849-50,  vol.  iii,  No.  1,  pt.  3,  pp.  371-605,  also 
627-801.. 

=  See  p.  34. 

'  Whitney's  diary,  in  same  report  as  that  of  Mr.  Barnes  .just  referred  to,  pp,  33,  34. 


•     GEOLOCrlCAI.  EXPLORATIONS    AND   LiTEKATURE.  7 

g-eolofiMciil  maps  ol'  flic  Inkc  Suporior  roiiutry  until  after  the  exploration 
li\-  Puiiipclh  ;iii(l  Hrooks  in  1872. 

Randall  (1848).— In  1848  l)r.  A.  Randall,  one  of  the  assistants  of  Dr.  D. 
D.  Owen  on  the  U.  S.  Geological  Survey  of  Wisconsin,  Iowa,  and  Mimie- 
sota,  accompanied  the  linear  surveyors  along  the  fourth  principal  meridian 
from  lake  Superior  southward.'  Dr.  Randall  appears  to  have  been  the 
first  to  note  the  existence  in  this  region  of  any  of  the  rocks  of  the  iron- 
bearing  series,  he  having  observed  exposures  of  lean  magnetic  ore. 

Whittlesey  (1849  and  i860).— Col.  Charles  Wliittlcscy  was  one  of  the  "heads 
of  sub-corps "  on  the  survey  above  referred  to'-^  under  Dr.  D.  D.  Owen. 
After  Dr.  Randall's  discovery  of  iron  ore  on  the  fourth  principal  meridian 
in  1848,  Col.  Whittlesey  followed  the  iron-bearing  slates  from  the  meridian 
westward  to  the  vicinity  of  English  lake,  or  Lac  des  Anglais,  as  it  was 
then  called  by  the  French  voyageurs.  '  Col.  Whittlesey's  report  covers  all 
of  the  region  drained  l^y  the  Bad  and  Montreal  rivers  in  northern  Wiscon- 
sin. It  is  quoted  at  some  length  below.  Eleven  years  later  Whittlesey 
made  a  somewhat  more  thorough  study  of  the  same  region  for  the  Wiscon- 
sin state  survey  then  organized  under  James  Hall.''  His  earlier  work, 
however,  was,  of  course,  no  more  than  a  very  rough  reconnaissance.  It 
could  not  indeed  have  been  more,  considering  the  difficulties  of  travel  in 
the  region  and  the  fact  that  there  existed  in  it  at  the  time  but  a  single  sur- 
veyed line,  that  of  the  meridian  above  mentioned.  These  difficulties  con- 
sidered, we  must  credit  Whittlesey  with  having  accomplished  a  good  deal. 
In  fact,  to  his  earlier  work  for  Owen's  survey  and  later  work  for  the  Wis- 
consin survey  was  due  all  really  definite  information  at  hand  with  regard 
to  the  geology  of  the  Penokee  country,  previous  to  the  investigations  of 
the  Wisconsin  survey  of  1873-79. 

Lapham  (1858).— In  September,  1858,  Dr.  I.  A.  Lapham,  afterward  chief 
of  the  Wisconsin  Geological  Survey,  made  rapid  trips  along  the  Penokee 
range  from  Bad  river   at  Penokee   gap    eastward  as    far   as  the   fourth 

'  Report  of  a  Geological  Survey  of  Iowa,  WIsconsiu,  aud  Miimesota,  David  Dale  Owen,  1852 
pp.  151,  153,  155,  444;  see  also  C.  AVhittlesey  in  an  article  on  the  Penoliie  Mineral  Eange,  Proc.  Bost. 
Soc.  Nat.  History,  Vol.  ix,  1862-'63,  pp.  235-244. 

2 Op.  cit.  p.  XV. 

3  The  Penokie  Mineral  Range,  Proc.  Bost.  Soc.  Nat.  Hist.,  vol.  ix,  1862-63,  pp.  235-244. 


8  THE  PENOKEE  lEOlSr-BEARINa  SERIES. 

meridian,  and  westward  to  the  end  of  the  range  in  the  vicinity  of  English 
lake.'  Dr.  Lapham's  object  was  an  economic  one  mainly,  his  investigation 
being  made  in  behalf  of  a  Milwaukee  iron  company.  Dr.  Lapham  located 
a  number  of  the  exposures  of  the  lean  magnetic  ore  that  makes  up  so  much 
of  this  range  and  some  of  the  exposures  also  of  the  associated  rocks.  These 
exposures  he  indicated  on  a  simple  map  accompanying  his  report.  His 
map  was  thus  the  first  attempt  at  a  definite  platting  of  any  portion  of 
this  interesting  belt. 

Brooks  and  Pumpeiiy  (1871).— In  the  fall  of  1871  Messrs.  T.  B.  Brooks  and 
R.  Pumpeiiy,  then  engaged  for  the  State  of  Michigan  in  a  geological  survey 
of  the  so-called  upper  peninsula  of  that  State,  made  a  rapid  reconnaissance 
along  the  iron  belt  from  the  passage  of  Bad  river  through  the  Penokee 
range  in  Wisconsin  eastward  to  the  Ontonagon  river  in  Michigan.  No 
definite  mapping  was  attempted,  but  some  very  interesting  new  conclu- 
sions were  reached,  as  announced  shortly  afterward  in  publications  below 
quoted  from. 

Irving  (1873,  1876,  1877,  1885).— The  writer's  studies  in  the  Penokee  region 
began  in  June,  1873,  under  the  auspices  of  the  Wisconsin  Geological  Sur- 
vey, then  recently  revived  under  the  direction  of  Dr.  I.  A.  Lapham.^  Sub- 
sequently this  work  was  continued  under  the  same  anspices  during  the 
seasons  of  1876  and  1877,  under  the  direction  of  Dr.  T.  C.  Chamberlin, 
the  successor  of  Dr.  Lapham  as  state  geologist.^  The  attempt  was  made 
to  do  such  detailed  work  as  would  furnish  the  necessary  material  for  the 
construction  of  maps  and  the  determination  of  the  true  order  of  succession 
of  the  rocks  of  the  region.  The  results  obtained  were  finally  published  in 
1880  in  full,  with  numerous  maps  and  other  illustrations,  in  the  third 
volume  of  the  Greology  of  Wisconsin.  This  report  covers  not  only  that 
portion  of  our  present  district  which  is  included  within  the  State  of  Wis- 
consin, but  also  all  of  northern  Wisconsin  lying  north  of  T.  43  N.  and 
east  of  R.  6  W.     Many  of  the  results  contained  in  this  report  are  made 

'  The  Penokee  Iron  Range,  Trans.  Wis.  State  Agr.  Soc,  vol.  v,  p.  391-400;  also  Report  of  the 
Directors  of  the  Wisconsin  and  L.  S.  Mining  and  Smelting  Co.,  Milwaukee,  1860,  pp.  22-37. 

^Geol.  of  Wis.,  vol.  II,  pp.  7,  8. 

3  Annual  Report  of  Progress  and  Results  of  the  Wisconsin  Geological  Survey  for  the  year  1876, 
pp.  13-18.     Annual  Report  of  the  Wjaconsin  Geological  Survey  for  the  year  1877,  pp.  17-25. 


(JKOLOGK^ATi  EXPLORATIONS  AND  LITERATURE.  9 

use  of  ill  the  prcscnr  voluiiic.  The  routes  tbllowetl,  so  tar  iis  tliey  traverse 
our  jivesciit  district,  and  the  exposures  located  iin^  all  indicated  on  tlic 
detailed  maps  herewith.  'I'he  writer's  visit  to  this  district  in  IS.S,'')  was  made 
parth'  tor  the  purpos<^  of"  supervising  the  field  work  then  going-  on  uu<ler 
Prof".  C  R.  Van  Hise  for  the  U.  8.  Geological  Survey  and  partly  for  the 
purpose  of  studying  in  detail  certain  jioints  along  the  contact  between  the 
iron-})earino-  series  and  the  ffneissic  rocks  south  of  it. 

Wight,  Sweet,  and  Wright  (1875).— In  1875  the  coutrol  of  the  Wisconsin 
Geological  Survey  passed  into  the  hands  of  Dr.  O.  W.  Wight.  By  his 
order  Messrs.  E.  T.  Sweet  and  C.  E.  Wright  made  together  an  extended 
geological  reconnaissance  in  northern  Wisconsin,  being  accompanied  by 
the  chief  geologist  in  person.^  In  the  course  of  this  reconnaissance  the 
party  crossed  the  Penokee  range  at  the  passage  of  Bad  river,  where  a 
cross-section  of  the  iron  series  was  attempted.  The  results  obtained  were 
subsequently  published  in  brief  by  Mr.  Sweet." 

Wright  (1876).— In  January,  1876,  Dr.  Wight  was  superseded  in  the  direc- 
tion of  the  Wisconsin  Survey  by  Prof.  T.  C.  Chambei'lin,  who  invited  Mr. 
Wright  to  continue  his  study  of  the  western  portion  of  the  Penokee  range, 
with  particular  reference  to  the  occurrence  there  of  iron  ores.  Accordingly 
Mr.  Wright,  accompanied  by  Mr.  F.  H.  Brotherton,  a  well  known  lake 
Superior  woodsman  and  explorer,  spent  the  weeks  from  August  22  to 
October  3,  1876,  in  making  a  detailed  examination  of  that  portion  of  the 
iron  series  lying  between  Bad  river  and  lake  Numakagon.  Mr.  Wright's 
results^  were  published  in  the  form  of  an  itinerary,  as  below  noted,  accom- 
panied by  a  series  of  maps  showing  the  positions  of  rock  exposures,  etc. 
Mr.  Wright's  routes  of  travel  and  the  exposures  located  by  him  are  platted 
on  the  detailed  maps  accompanying  the  present  volume. 

chamberiin  (1877).— In  the  fall  of  1877,  Prof.  T.  C.  Chamberlin,  accom- 
panied by  A.  D.  Conover,  carried  the  detailed  mapping  of  the  Penokee 

1  Geol.  of  Wis.,  Tol.  11,  pp.  72-79. 

2 Notes  on  the  Geology  of  Northern  Wisconsin,  l-rane.  Wis.  Acad.  Sci.  and  Letters,  vol.  iii, 
pp.  40-53. 

'  Annual  Report  of  Progress  and  Eesulta  of  the  Wisconsin  Geological  Survey  for  the  year  1876, 
pp.  18-23.     Also  Geol.  of  Wis.,  vol.  m,  pt.  iv,  pp.  239-301. 


10  THE  PENOKEE  lEON-BEAEING  SEEIES. 

series  from  the  Potato  river  eastward  to  the  Michigan  boundal'y  at  the 
Montreal  river.  ^ 

conover  (1878).— In  the  summer  of  1878,  Mr.  A.  D.  Conover  made  a  some- 
what extended  trip  through  the  country  between  the  Montreal  and  Bad 
rivers  for  the  purpose  of  filling  up  certain  gaps  left  by  work  of  previous 
years.  His  routes  were  mostly  north  of  our  present  district,  the  upper 
portion  of  which  they  touched,  however,  in  T.  44  N.,  R  2  W.,  and  T.  45  N., 
R.  2  W.,  Wisconsin.^  Mr.  Conover's  results  were  embodied  in  my  final 
report^  on  the  eastern  lake  Superior  district.      •    ' 

Rominger.  (1882).— Dr.  C.  Rominger,  acting  in  his  capacity  of  State  geol- 
ogist of  Michigan,  made  a  study  of  the  region  between  the  Montreal  river 
and  Gogebic  lake  in  the  summer  of  1882,  spending  three  weeks  in  that 
field.  While  Dr.  Romingei''s  results  have  not  yet  been  published,  they  are 
embodied  in  a  manuscript  report  of  which  he  has  been  kind  enough  to 
furnish  me  a  copy. 

Van  Hise  (1884-1887).— The  particular  investigation  upon  which  the  present 
volume  is  especially  based  was  begun  in  1884.  The  object  was  the 
extension  of  the  detailed  work  done  by  the  Wisconsin  survey  east- 
ward from  the  Montreal  river  to  lake  Grogebic.  Nothing  very  definite 
was  then  known  with  regard  to  the  latter  district  save  that  there 
existed  in  it,  contrary  to  Foster  and  Whitney's  reports,  an  eastward 
continuation  of  the  Penokee  Iron  series.  It  was  then  thought  that  a 
single  season's  work  would  be  sufficient,  with  the  aid  of  the  published 
results  of  the  Wisconsin  Survey  and  of  my  own  familiarity  with  the  coun- 
try west  of  the  Montreal  river,  to  furnish  the  material  needful  for  such  an 
accoiint  of  the  Penokee  series  as  it  would  be  desirable  to  include  in  a  final 
report  on  the  Huronian  of  this  part  of  the  lake  Superior  region.  Accord- 
ingly, to  Prof  C.  R.  Van  Hise,  who  had  already  acquired  a  thorough 
acquaintance  with  the  original  Huronian  of  the  north  shore  of  lake 
Huron  and  with  the  iron-bearing  series  of  the  Marquette  and  Menominee 
districts,  was  assigned  the  work  of  making  a  detailed  survey  of  the  disti'ict 


'  Annual  Report  of  the  Wiscousiu  Geological  Sui\'ey  for  the  year  1877,  pp.  25-28.     See  also  Geol. 
of  Wis.,  vol.  Ill,  pt.  Ill,  p.  56,  atlas  PI.  xxvi. 

'Annual  Report  of  the  Wisconsin  Geological  Survey  for  the  year  1878,  pp.  5-6. 
2  Geol.  of^Wis.,  vol.  iii,  pt.  iii,  pp.  53-214. 


GEOLOGICAL  lOXl'LOlIATIONS  ANJ3  LITEKATURB.  11 

between  tlie  Montreal  ri\('r  and  liiko  Gog'ebic.  In  this  work  ]w.  was 
occupied  diirinj^-  the  inontlis  of  ,luly  iiud  Auj^'ust  of  1S,S4,  returning  witli 
a  large  colhu'tion  and  copious  notes.  His  routes  of  tra.vel  an*  indicated 
on  the  accompanying  detailed  maps. 

During  the  following  fall  months  Prof.  Van  Hise  carefully  studied  the 
material  collected,  a  large  number  of  thin  sections  having  been  made;  and 
the  locations  of  exposures,  made  always  by  pacings  from  section  corners  or 
quarter-posts,  were  platted  on  a  large  scale  map.  This  study  served  chiefly 
to  couvince  us  that  the  whole  belt,  including  the  Wisconsin  portion,  was 
worthy  of  still  further  research.  Although  certain  very  interesting  new 
conclusions  were  reached,  still  more  important  lines  of  study  were  sug- 
gested. The  Wisconsin  Survey  work  was  done  at  a  time  when  the  micro-  ' 
scope  was  only  just  coming  into  use  in  this  country  as  a  petrographical 
instrument,  and  consequently  numerous  important  petrographical  points 
were  missed  by  that  survey;  while  its  type  collection  had  meantime  been 
destroyed  by  fire.  But  what  more  particularly  urged  us  on  was  the  great 
value  of  the  ii-on-bearing  series  of  this  region  as  a  type  series.  Accordingly, 
it  was  decided  that  Prof  Van  Hise  should  spend  another  season  in  the 
region,  not  only  in  filling  up  gaps  in  his  own  work  of  the  previous  season, 
but  in  carefully  revising  the  Wisconsin  Sixrvey  work  and  extending  it  when 
possible.  On  this  work  he  was  engaged  from  June  26  to  September  4,  in 
which  time  he  succeeded  in  covering  the  entire  distance  from  lake  Gogebic 
to  lake  Numakagon.  As  has  already  been  said,  the  writer  joined  this 
party  for  a  time  during  the  summer,  making  a  special  study  of  the  southern 
contact.  The- routes  followed  and  exposures  located  .are  all  indicated  on  the 
accompanying  detailed  maps. 

During  the  winter  following  this  field  season  the  material  collected  was 
carefully  studied,  and  it  became  evident  that  additional  field  work  was 
needed,  certain  points  having  been  overlooked,  and  certain  others  which 
could  not  then  be  determined  had  become  determinable  by  the  great, 
activity  in  mining  and  prospecting  which  during  this  winter  and  the  follow- 
ing season  went  on  along  the  Penokee-Gogebic  range.  Accordingly,  Prof 
Van  Hise  returned  to  this  field  June  16,  1887,  and  was  there  continuously 
occupied  until  Jul)^  25,  taking  advantage  of  all  of  the  light  which  had  been 


12  THE  PENOKEE  lEON-BEAEING  SERIES. 

thrown  upon  the  structure  of  the  region  by  mining  operations,  and  examin- 
ing again  many  of  the  more  important  exposures.  Among  other  things, 
the  eastern  termination  of  the  Gogebic  range  had  not  been  ascertained,  but 
exploring  operations  enabled  Prof  Van  Hise  to  determine  that  the  horizon- 
tal Eastern  sandstone  overlies  the  Penokee-Grogebic  series  just  west  of 
Grogebic  -lake.  Later  -in  the  same  season,  August  15,  in  company  with 
Prof  Van  Hise,  I  visited  the  district  to  examine  personally  the  numerous 
intei'esting  localities  which  had  been  discovered  or  developed  by  mining 
since  the  previous  season. 

During  this  season's  field  work  was  noticed  the  remarkable  association 
between  the  ores  and  an  altered  greenstone,  which  the  miners  called  either 
*  soapstone  or  diorite  dike,  and  the  conclusion  was  reached  that  between  the 
two  there  must  be  some  genetic  connection.  Mr.  J.  Parke  Channing, 
mining  inspector  for  Gogebic  county,  was  induced  to  investigate  the  rela- 
tions of  the  ore  bodies  and  dike  rocks.  This  work  he  did  during  the  latter 
part  of  the  season,  reaching  some  very  interesting  conclusions.  Mr.  Chan- 
ning has  also  given  a  good  deal  of  attention  to  all  new  points  shown  hj 
mining  development  of  the  region  up  to  the  present  date,  and  has,  with 
great  kindness,  promptly  furnished  us  information  as  to  them. 

In  this  connection  should  be  mentioned  the  fact  that  Mr.  J.  M.  Long- 
year,  who  is  in  charge«of  the  lands  of  the  Lake  Superior  and  Portage  Ship 
Canal  company,  a  large  part  of  which  lie  between  lake  Gogebic  and  the 
Montreal  river,  kindly  allowed  us,  after  the  work  of  ISSfi  was  completed, 
to  make  use  of  the  notes  as  to  locations  of  exposures  obtained'  by  parties  of 
woodsmen  under  his  direction.  These  parties,  crossing  each  section  a 
number  of  times,  made  notes,  not  merely  as  to  timber  and  topography,  but 
also  located  and  collected  specimens  from  every  exposure  encountered, 
with  a  view  to  discovering  indications  of  the  existence  of  iron  ore  deposits. 
Going  over  these  notes  and  specimens  carefully,  Prof  Van  Hise  has  been 
able  to  add  to  the  information  obtained  by  himself.  Just  how  far  his  own 
studies  have  extended  between  lake  Gogebic  and  the  Montreal  river  will 
be  seen  from  his  routes  of  travel  indicated  on  the  accompanying  maps. 


GE(»L(m;I('A1.   EXl'LOUATIUNS  AND   LITEKATUEE. 


LITERATURE. 


13 


Tlio  following  is  a,  list  of  all  publications  which  we  have  met  with  in 
any  way  referrinf>-  to  the  Ponokee-Gog-eljic  district.  Hciiifr  arrang-ed  chron- 
ologically, the  notes  and  (juotations  attached  to  the  name  of  each  work  will 
serve  to  show  more  fully  than  has  been  done  in  previous  2)ages  the  history 
of  our  subject,  up  to  the  time  of  the  special  investigations  upon  which  the 
present  volume  is  particularly  based.  The  dates  are  the  years  of  publi- 
cation. 

1S49. 

Whitkey  (J.  D.).  Letter  to  Dr.  C.  T.  Jackson,  in  Jackson's  Eei)oit  on  the 
Mineral  Lands  of  Lake  Superior,  Senate  Documents,  1st  session,  30th  Congress,  vol. 
II,  No.  3,  1847-'48,  p.  223-230. 

Whitney's  two  traverses  of  the  Gogebic  iron  belt  have  already  been 
mentioned  and  indicated  on  Fig.  1.  In  this  report  we  find  the  first  men- 
tion of  the  results  of  his  observations  on  the  two  lines  followed,  as  also 


S''™  ^«  "        xn  w? ~~SM -—an 

Fig.  1.— Eeproduction  of  Barnes  and  Whitney's  geological  map  of  region  between  AgogBbic  lake  and  Montreal  river. 

of  the  observations  of  Mr.  Barnes  along  a  line  12  miles  farther  to  the 
east.  The  exact  courses  of  these  traverses  are  given  in  a  volume  subse- 
quently referred  to,  from  which  also  is  taken  the  geology  of  the  map  of 
Fig.  1.  From  this  map  it  will  be  seen  that  no  suspicion  was  raised  in 
Whitney's  mind  as  to  the  existence  in  this  region  of  his  "Azoic  slates," 
these  slates  appearing  under  this  name  over  a  large -part  of  the  very  map 
of  a  portion  of  which  Fig.  1  is  a  reduced  copy.  The  trappean  rocks  which 
belong  to  what  we  now  designate  as  the  Keweenaw  or  copper-bearing 
series,  are  made  to  lie  directly  against  the  ''granite  and  syenite,"  without 


14  THE   PENOKEE   IRON-BEAEING  SERIES. 


intervening  slates.      The   same   conclusion  is   indicated  in  the  following 
remarks  quoted  from  this  letter  (p.  230) : 

On  crossing  the  soiitliern  edge  of  the  trap  range,  we  no  longer  find,  on  proceeding 
soiith,  a  belt  of  sandstone  similar  to  that  on  the  north,  as  is  the  case  between  the 
Portage  and  the  Ontonagon.  The  only  rocks  which  we  have  found  in  place  are 
granite  and  greenstone;  not,  however,  arranged  in  regular  order,  but  scattered  con- 
fusedly without  order  of  superposition  or  direction.  The  whole  country  to  the  south, 
as  seen  from  any  one  of  the  high  points  of  the  trap  range,  as  far  as  the  eye  can  reach, 
seems  to  be  almost  an  unbroken  plain.  On  traversing  it,  it  is  found  to  be  made  up  of 
alternate  swamps  and  low  ridges  of  granite  and  greenstone,  which  are  sometimes 
liiddeu  by  soil  and  covered  with  pine,  hemlock,  and  especially  sugar  maple.  Some- 
times, again,  the  granite  rocks  rise  with  almost  vertical  walls,  yet  never  to  any  con- 
siderable height,  from  the  midst  of  the  swamps,  and  are  covered  only  by  thick  moss. 
It  will  be  impossible  to  draw  any  line  of  demarcation  between  the  granite  and  green- 
stone rocks,  since  they  occur  together  constantly,  neither  being  confined  to  *any 
particular  portion  of  the  district. 

The  Avhole  of  this  country  is  almost  inaccessible  from  its  swampy  nature;  neither 
does  it  promise  to  be  of  aily  value  for  its  mineral  contents,  since  we  have  never  found 
any  other  ores  than  a  iaw  scattered  magnetic  masses  of  iron  ore  and  iron  pyrites; 
neither,  however,  in  any  considerable  quantity. 

1850. 

Baknes  (George  O.).  Diary  of  field  work  for  the  summer  of  1847,  in  Report 
on  the  Geological  and  Mineralogical  Survey  of  the  Mineral  Lands  of  the  United 
States  in  the  State  of  Michigan,  by  C.  T.  JacksSn,  XJ.  S.  Geologist,  Senate  Docs., 
1st  sess.,  31st  Cong.,  1849-'50,  vol.  in,  No.  1,  pt.  in,  pp.  371-605,  also  pp.  627-801. 

Although  a  further  and  final  report  was  published  by  Dr.  Jackson  in 
1852,  this  one  is  essentially  the  closing  report  of  his  work,  covering  the 
period  up  to  the  time  of  his  resignation  of  the  survey  into  the  hands  of 
Messrs.  Foster  and  Whitney.  Mr.  Barnes's  diary  of  field  work  is  given  in 
this  report.  The  larger  part  of  the  region  west  of  Gogebic  lake  was  at 
the  time  without  surveyed  lines.  His  course  of  travel  and  his  failure  to 
discover  a  series  of  iron-bearing  slates  have  already  been  noted.  Mr. 
Barnes  crossed  the  Grogebic  belt  once  and  made  an  extended  trip  in  the 
granite  region  farther-  south.  Later  in  the  same  season  (see  diary  pages 
738-740)  Mr.  Barnes  accompanied  Whitney  in  his  traverses  of  the  Gogebic 
district,  and  designated  the  positions  of  a  number  of  rock  exposures  ou  the 
•  lines  followed. 


C.EOLOdlCAL  EXPLORATIONS  AND  LITERATFTRE.  ]5 

Whitney  (,I.  I ).).     I  )i;iiy  in  .saiu.'  loporl  will,  tliaL  of  Mr.  Btirues  just  referred  to. 

In  Whitney's  diary  ..f  (iuM  work  (pp.  738-740)  an  ax;c.(.unt  is  given  of 
the  journey  made  hy  him  in  company  with  Mr.  Barnes.  His  course  of 
travel  has  ah-eady  been  given  and  mapped,  Fig.  1.  The  observations  made 
on  three  Hues  of  travel  by  Barnes  and  Whitney  in  the  country  between 
Gogebic  lake  and  the  Montreal  river  are  all  the  basis  for  any  state- 
ments with  regard  to  this  region  made  by  Jackson  and  by  Foster  and 
Whitney  in  their  respective  reports.  All  later  published  statements  with 
regard  to  the  geology  of  this  region,  up  to  the  time  of  the  rapid  trip  made 
by  Brooks  and  Pumpelly  in  1871,  depend  also  upon  these  few  observa- 
tions by  Whitney  aud  Barnes.  It  is  interesting  to  note  that  although  these 
gentlemen  found  nothing  in  the  region  but  granite  and  greenstone,  they 
must  have  passed  on  at  least  two  of  these  lines  within  a  short  distance  of 
large  exposures  of  slate  and  jaspery  iron  ore. 

FosTEK  (J.  W.)  and  Whitney  (J.  D.).  Synopsis  of  Explorations  of  the  Geolog- 
ical Corps  iu  the  Lake  Superior  Land  District  in  the  Northern  Peninsula  of  Michigan, 
Senate  Docs.,  1st  sess.,  31st  Cong.,  1849-'50,  vol.  in,  No.  1,  Pt.  iii,  pp.  605-626. 

This  is  the  first  report  made  by  Messrs.  Foster  and  Whitney  after  they 
had  superseded  Dr.  Jackson  in  the  control  of  the  Geological  Survey  of  the 
Mineral  Lauds  of  the  United  States  in  Michigan.  In  it  are  embraced  the 
results  of  work  done  by  these  gentlemen  in  the  capacity  of  assistants  to 
Dr.  Jackson.  Among  other  results  thus  obtained  were  those  by  Whitney 
and  Barnes  in  the  summer  of  1847,  in  the  country  west  of  lake  Gogebic. 
It  appears  evident  that  no  further  work  was  done  in  that  district  for  this 
report,  or  for  their  final  report  below  noticed,  by  either  of  these  gentlemen 
or  by  any  of  their  assistants. 

Accompanying  this  report  are  several  geological  maps,  one  of  which, 
entitled  "The  District  between  Portage  Lake  aud  Montreal  River,"  drawn 
on  a  scale  of  3  J  miles  to  the  inch,  includes  that  portion  of  the  district  which 
forms  the  subject  of  the  present  volume  as  far  west  as  the  Montreal  river. 
This  is  the  map  referred  to  above  in  connection  with  Whitney's  letter  of 
1847  to  Dr.  Jackson. 


16  THE  PENOKEE  IRON-BEARmG  SERIES. 

1851. 

PoSTEK  (J.  W.)  and  Whitney  (J.  D.).  Report  ou  the  Geology  of  the  Lake 
Superior  Laud  District,  Part  2,  The  Iron  Rejjiou,  together  with  the  General  Geology, 
Senate  Docs.,  special  session,  32nd  Cong.,  Washington,  1851,  vol.  iii,  No.  1,  406  pp., 
with  maps. 

This  is  Foster  and  Whitney's  final  report  on  tl3.e  iron  regions  of  the 
upper  peninsula  of  Michigan.  In  it  several  brief  references  are  made  to 
the  region  w^est  of  Grogebic  and  south  of  the  trappean  beds  of  the  Copper 
range.     We  quote  as  follows  (p.  39): 

Farther  west  another  granite  belt  starts  from  the  head  waters  of  the  Ontonagon 
river,  and  thence  extends  to  the  western  limits  of  the  district,  intersecting  the  head 
of  Agogebic  lake  and  crossing  the  Montreal  river  about  15  miles  from  its  mouth. 
Southward  it  forms  the  watershed  between  the  rivers  of  lake  Superior  and  the  Mis- 
sissippi and  passes  beyond  the  limits  of  this  district  into  Wisconsin.  It  is  probable 
that  this  belt  is  a  continuation  of  that  first  described,  but  we  have  not  been  able  to 
trace  the  continuity.  There  is  an  interval  of  20  miles  where  the  surface  of  the 
country  becomes  nearly  horizontal  and  is  strewn  with  accumulations  of  clay  and 
gravel,  burying  up  the  subjacent  rocks. 

In  the  extreme  western  portion  of  the  district,  west  of  range  40,  granite  is  the 
predominating  rock  below  the  southern  boundary  of  township  47  and  is  associated 
with  a  hornblende  rock  which  sometimes  assumes  a  slaty  structure.  The  granite  is 
mostly  a  binary  compound  of  feldspar  and  quartz,  the  former  largely  predominating 
and  giving  a  reddish  tinge  to  the  whole  rock;  mica  is  present  only  in  very  small 
quantity,  while  hornblende  and  chlorite  are  occasionally  scattered  in  minute  i)articles 
through  it.  Nearly  the  whole  of  the  granitic  region  in  this  part  of  the  district  pre- 
sents the  most  forbidding  and  desolate  aspect.  Though  it  forms  the  most  elevated 
portion  of  the  country,  being  the  watershed  between  lake  Superior  and  the  Missis- 
sippi, it  is  low  and  swampy  and  filled  with  numerous  lakes,  of  which  over  fifty  were 
crossed  by  Mr.  Burt  in  surveying  the  boundary  line  between  Lac  Vieux  Desert  and 
the  Montreal  river.  There  are  occasional  elevations,  which  are  dry  and  wooded  with 
sugar  maple  and  which  undoubtedly  are  covered  with  a  good  soil,  but  the  larger  por- 
tion of  the  region  presents  almost  interminable  cedar  swamps,  in  the  midst  of  which 
the  granite  and  hornblende  ridges  rise,  with  precipitous  walls,  rarely  to  more  than  50 
feet  in  height  above  the  surrounding  country.  These  ridges  are  generally  very  nar- 
row, and  their  sides  are  covered  with  a  thick  coating  of  moss  and  lichens.  Nothing 
can  exceed  the  desolate  solitude  of  this  region.  Not  even  the  Indian  traverses  it;  it 
is  destitute  of  game  and  its  stillness  is  never  broken  except  by  the  crashing  of  the 
tornado  through  the  dense  forest,  tearing  up  the  trees  and  piling  them  together  so 
as  to  present  an  almost  impassible  barriei',  as  if  still  further  to  repel  the  intrusion  of 
man  into  a  region  so  little  fitted  for  his  reception. 


GEOLOGICAL  EXPLORATIONS  ANT)  LITEKATTTRE.  17 

The  {>raiiitr  nl  l  lie  whnlc  of  this  pmtiiiii  of  I  lie  district  is  vory  (M)arse  grained  aud 
crystalline  and  is  cliaraili'i  i/.cd  by  a,  predomiuance  of  feldspar  and  an  almost,  entire 
absence  of  mica.     ( I'p.  47.  4.S.) 

The  .statuiueuts  in  the  ab()ve  quotations  rest,  of  course,  upon  the  few 
obser,vations  by  Messrs.  Wiiitney  aud  Barnes.  It  is  manifest  that  no 
thought  of  the  existence  of  iron-bearing  slates  between  hike  Gogebic  aud 
the  Montreal  river  was  entertained.  This  is  the  more  singular  since  the 
discoveries  of  iron-bearing  slates  by  Mr.  Whittlesey  west  of  the  Montreal 
river  (subsequently  noted)  were  known  to  Messrs.  Foster  and  Whitney,  as 
appears  from  their  statement  (page  51). 

1853. 

Whittlesey  (Charles).  Geological  Report  on  that  portion  of  Wisconsin  bor- 
dering on  the  south  shore  of  Lake  Superior,  surveyed  in  the  year  1849,  under  the  direc- 
tion of  David  Dale  Owen,  U.  S.  Geologist,  by  Charles  Whittlesey,  liead  of  subcorps. 
In  Eeport  of  a  Geological  Survey  of  Wisconsin,  Iowa,  and  Minnesota,  by  David  Dale 
Owen,  U.  S.  Geologist,  Philadelphia,  1852. 

It  seems  that  Dr.  Oweu  himself  must  have  crossed  the  westernmost 
end  of  the  district  with  which  we  are  concerned  near  Nimaakagon  lake. 
This  appears  to  have  been  in  1848  or  1849,  but  no  observations  of  impor- 
tance were  made  (pp.  159-160).  Dr.  Randall,  one  of  Owen's  assistants, 
appears  to  have  made  the  first  discovery  of  iron-bearing  slates  in  this 
district  while  following  the  Fourth  Principal  meridian  in  1848  (p.  444). 

Col.  Whittlesey's  own  exploration  was  made  in  1849,  in  which  year  he 
followed  the  belt  of  iron-bearing  slates  from  the  vicinity  of  Montreal  river 
to  English  lake.  The  following  quotation  will  serve  to  show  what  were  his 
views  as  to  the  general  structure  of  the  whole  Bad  river  country  in  Wis- 
consin : 

The  accompanying  map  and  sections  are  intended  to  represent  at  one  glance  the 
principal  features  in  the  geology  of  this  region.  The  extent,  elevation;  and  relative 
thickness  of  the  various  formations,  as  well  of  solid  rock  as  the  looser  earthy  deposits, 
will  there  appear  in  a  more  compact  and  intelligible  form  than  I  could  give  them  by 
written  descriptions,  however  elaborate. 

There  are  four  formations  or  great  classes  of  rocks  shown  on  each  section.  These 
all  appear  in  the  same  order  of  succession,  reckoning  from  the  lake  southerly,  and  may 
be  grouped  thiis : 

MON  XIX '^ 


18  THE  PENOKEE  IKON-BEAEHSTG  SEEIES. 

1.  Sedimentary. 

a.  Eed  sandstone. 
h.  Black  slate. 
('.  Conglomerate. 

2.  Tra])pous  liocTcs,  or  those  of  volcanic  origin. 

a.  Black  and  red  amygdaloid  and  greenstone  trap . 

b.  Augitic,   hornblendic,  and   feldspathic  rocks,  embracing  syenite  and 

granites  of  the  same  age. 

3.  Metamorphosed  Rocks. 

a.  Hornblendic  slates. 

b.  Iron  slates. 

c.  Black  slates,  iu  large,  thin,  rectangular  sheets. 
^7.  Talcose  slates,  with  quartz. 

c.  Slaty  quartz. 

4.  Granitic. 

a.  Syenite,  and 

b.  Granite,  occupying  the  country  south  of  the  mountain  range  or  uplift, 

are  the  oldest  rocks  seen.  (P.  425.) 
The  Sedimentary  and  lyneous  Rods. — The  relative  a tje  of  the  rocks  beneath  the 
clay  and  drift  is  a  subject  upon  Avbich  a  prolonged  discussion  would  be  in  place  if 
theoretical  consideration  might  be  introduced  here  at  large.  The  granites  and 
syenites  of  the  interior  are  no  doubt  the  most  ancient  rocks  of  the  district.  After 
the  protrusion  of  those  extensive  interior  granitic  masses  many  successive  changes 
have  occurred,  but  in  what  precise  order  is  a  question  not  easily  determined.  The 
immense  sandstone  deposits  of  the  basin  of  lake  Superior  must  have  been  subse- 
quent to  the  granites  of  Wisconsin,  Chippewa,  and  Montreal  rivers,  and  probably 
rested  on  them.  Since  that  era  a  prolonged  and  intense  internal  igneous  action  has 
taken  place,  and  the  trap,  hornblendic,  and  greenstone  masses  have  been  ejected,  and 
also  with  them  irregular  protrusions  of  recent  granite  and  syenite.  The  metamorphic 
slates  have  been  elevated  during  these  convulsions,  and  the  sedimentary  rocks  thrust 
away  to  the  northward  and  tilted  uj)  at  high  angles. 

The  old  granites  and  syenites  have  been  rent,  and  fluid  matter,  suck  as  quartz  and 
hornblende,  inserted  iu  the  fissures  and  between  the  beds.  Along  the  northern  por- 
tion of  the  Penokie  range  an  outburst  has  taken  place,  as  it  were,  between  the  sedi- 
mentary rocks  and  their  ancient  basis,  on  a  line  from  the  Montreal  to  Lac  des  Anglais; 
but  the  overflows  have  not  been  confined  to  one  volcanic  effort.  The  black  and  red 
trap,  against  which  the  conglomerate  abuts,  is  doubtless  due  to  a  different  effort  from 
that  which  produced  the  greenstone  trap-rocks  that  rise  between  the  East  fork  of 
Bad  river  and  the  Montreal.  The  augitic,  hornblendic,  and  syenitic  mountains 
between  the  East  fork  aud  the  main  stream  differ  in  form,  in  chemical  constitution, 
and  bedding  or  stratification  from  either  the  greenstone  or  black  trap, 


Semef-nfoyic 
Coriff7artierez^ 

I 


ami  compact 


Trap  9 
FclspatTiic 

JBdrnhJerule 
Jlochs 

Qiioj'tz, 
.3-ort  ari£l  "Ihlv 
X  offBi  xd^e  Gr- 


MocTc  Of'. 


JRedf^enz^ 


3&frtfu/ortz7(, 


nTsfeet 


^37  feet 


?oo  feet 
soiif^et 


asoiket^ 
73ofeet 


7ra3  fce^t 


7s^ee6 


73  of  eat 


(J.  Jh'j'zt 


87/ fe^ 


S^siefeet' 


SciZsccrrvSli/yc. 


jQ7at'eet 


J)£a-7o  flrnty 

,Auffitic 
JRocTos 

iSom^Zenetic 
MjOcTcs 


ffoTTthJ^n^zc 

Tblcose, 
jBoTTtMem^'c 

fvitJt  7n^f^^/ro7T^ 

Siervttic  rr??^ 


ist  josfi.,2,71^  eeft. 


w.       Tyler-'sFork 
,  «5       ■^S4'feet 


S77  feet 


7osofeet 
TylersF<7rk 
914' feet 
lyier^sFoT-h 

oS'f'feet 
uoofeet  ^ 
ct7i€i3:Rt7^-s?ves 


Conglomercde. 

Sletck,-  tzn^- 

G-T-eyTTzzp 

Siervite 

.SbrnbTeTJ^Szc 

RocJts 

&ra7zzte 

BtrribZern^iC 

RocTts 

Slaclv  Slate 

SurrMendeSlate 

Wagneticlrcfn, 

Altered-T/ilcose 
Elates, 


aae  i'eei 


Che^womi^on^try 


JiIasJikBgFcrr'7t, 


iEastFo-rh 


I'dOs  ofiyierilW-h 


Pewabic  Itange 
naei'ket 


GEOLOGICAL  EXPLORATIONS  AND  LITERATURE.  19 

Prof.eediiif;' almit;  I  lie  iikiihiIiijii  ihI^cs  (if  I  lie  iiorl  liciii  |i!irl  of  tin' range,  between 
the  main  stioain  and  llic  oiitk't  (tf  Ijacdcs  Anji'lais,  we  cni-ouiilcr  dtlior  varieties  (»f 
rocks,  t'cldsiialliic,  jjrauit-ic,  and  iKii'nblcndic  in  their  coiiipositiun,  a|)i)iircntly  an 
independent  ii|iiilt  or  ontbnrst.  Along-  this  whole  line,  however,  the  nietainoiphie 
rocks  of  I  lie  sunthern  I'idgos  of  the  range  arc  eontinnons  IroMi  near  the  Montreal  to 
I'M',  des  Anglais.  They  have,  at  ditt'ercnl  times,  been  i)ushcd  over  tlu^  granites  at  the 
sonth,  distorted,  broken,  and  tilted  np  in  diflercnt  degrees,  bnt  always  in  the  same 
direction.  The  northern  portion  of  the  range  exhibits  to  my  mind  evidence  of /o«r 
periods  of  igneous  action,  producing /(>«/•  formations  of  I'oeksof  a  trappose  cast,  which 
I  have  rei)resented  sej)arately  on  the  map. 

They  are:  1st,  black  iind  red  trap;  2d,  greenstone  trap,  embracing  or  gradu- 
ating into  massive  hornblende  and  syenite  at  the  west;  M,  angite  and  hojiiblcjidc 
rocks  in  mass,  also  embracing  granite  and  syenite;  4th,  granite,  syenite,  and  coarse 
hornblende  rocks,  north  of  Lac  des  Anglais. 

But  how  to  decide  the  order  or  relative  age  of  these  protrusions!  It  appears 
that  the  same  materials  under  difl'erent  circumstances  of  fluidity,  pressure,  and 
rapidity  of  cooling  may  take  all  these  forms. 

At  present  I  can  only  place  these  four  varieties  in  one  ffroup,  filling  a  geological 
epoch  of  no  great  duration,  and  place  it  between  the  era  of  the  red  sandstone  depos- 
its and  the  metamorphic  uplifts;  for  it  is  by  the  appearance  of  this  group  that  both 
those  systems  have  been  j)ushed  aside,  one  to  the  north,  the  other  to  the  south. 
Whether  the  schistose  rocks,  before  their  upheaval  and  metamorphosis,  were  older 
or  newer  than  the  sandstone  I  do  not  decide;  but  both  the  schists  and  the  unaltered 
sedimentary  rocks  are  more  ancient  than  the  above  group  numbered  from  one  to  four 
(pp.  429-430). 

The  geological  map  mentioned  in  the  above  quotation  was  never 
printed ;  but  its  main  features  were  embodied  in  Owen's  general  geologi- 
cal map  of  the  northwest.^  PI.  iv  is  a  reproduction,  save  as  to  the  omis- 
sion of  colors,  of  the  original.  In  a  similar  manner  Whittlesey's  four  cross- 
sections  of  the  Bad  river  country  are  reproduced  in  PL  iii. 

In  Whittlesey's  classification  of  the  formations  of  the  Bad  river 
country,  above  given,  the  red  sandstone  (Iff)  includes  horizontal  sandstones 
belonging,  as  our  belief  is,  to  the  Potsdam  sandstone,  and  also  vertically 
placed  sandstones  belonging  to  the  Keweenaw  or  Copper  series.  To  the 
latter  series  belong  Whittlesey's  formations  from  Ih  to  2b,  inclusive.  His 
group  of  naetamorphosed  rocks  (3)  is  about  equivalent  to  the  iron-bearing 
series  Avhich  forms  tlie  sul)ject  of  the  present  volume.     His  group  of  granitic 

I  See  volume  of  iUustrations  aooompanyiug  tlie  Report, 


20  THE  PENOKEE   lEOISr-BEARING  SERIES. 

rocks  (4)  iucludes  the  granitic,  gneissic,  and  schistose  rocks  which  lie  to 
the  south  of  the  Penokee  range  and  form,  as  he  says  and  as  we  also  believe, 
the  oldest  division  of  the  region.  We  are  not  sure  that  we  fully  under- 
stand Whittlesey's  statements  at  the  close  of  the  quotation  above,  as  to  the 
relative  ages  of  the  different  rocks  of  tlie  region.  It  seems  that  he  would 
say  that  the  various  eruptive  rocks  of  the  Keweenaw  series  Avhich  occupy 
the  belt  of  country  between  the  red  sandstone  on  the  north  and  the  iron- 
bearing  slates  on  the  south  are  all  of  them  of  an  intrusive  nature,  having 
been  thrown  to  the  surface  after  the  formation  of  the  red  sandstone,  which 
they  have  thrust  northward,  at  the  same  time  pushing  the  slaty  series 
to  the  south.  His  position  as  to  the  more  recent  origin  of  these  trajipean 
rocks  relatively  to  the  red  sandstone  is  one  which,  of  course,  can  not  be 
maintained.  It  has  been  abundantly  proved  by  the  work  of  the  Wis«onsin 
Survey  (1873-1879)  that  these  eruptive  rocks  antedate  the  red  sandstones. 
In  large  measure  they  originated  as  surface  flows;  and,  moreover,  certain 
of  them  have  yielded  the  most  of  the  fragmental  material,  coarse  and  fine, 
of  which  the  sandstones  are  built. 

The  following  quotations  have  more  especial  reference  to  the  iron- 
bearing  formation: 

There  is  a  continuous  mountain  chain  from  the  Montreal  river  to  Bladder  lake, 
the  xnolongation  of  the  Porcupine  mountain  range  in  Michigan.  I  have  called  it  the 
PenoMe  range,  tliis  being  the  Indian  word  for  iron,  which  is  found  in  its  westerly- 
portion  in  great  force  (p.  434). 

The  most  easterly  appearance  of  magnetic  iron  which  I  obseiVed  was  in  fissile 
black  slate,  about  4  miles  west  of  the  Montreal  trail,  along  which  the  section  No. 
4  W.  is  made.  The  bed  lies  back  of  the  trappose  range,  about  16  miles  from  the 
lake,  in  a  protrusion  of  metaniorphic  slates,  the  argillaceous  portions  merely  tinged 
with  iron.  About  4  miles  along  the  strike  of  the  beds,  southwest  by  west,  the  bed 
was  seen  by  Mr.  Eandall  in -1848,  in  the  fourth  principal  meridian,  in  township  44 
north,  18  miles  from  the  lake.  From  thence  I  and  my  assistant,  Mr.  Beesly,  an  active 
woodsman  and  faithful  and  acute  observer,  traced  it  at  moderate  intervals  along  the 
uplift  to  the  west  end  of  "Lac  des  Anglais,"  or  about  15  miles,  to  where  the  rango 
terminates.  Here  the  metamori>hic  slates  that  first  show  themselves  between  the 
Montreal  river  and  the  Montreal  trail  on  the  east  sink  beneath  the  level  of  the 
country  and  are  replaced  by  syenitic  rocks. 

By  examining  the  sections  ]Sro>s.  1,2,  3,  and  4  W.,  attached  to  this  report,  the 
position  of  the  iron-bearing  rocks  will  be  found  to  be  the  same  in  each,  and  the 


(iK()L()(!l»'Al.   KXI'LOKATIONS  AND   LlTKi; ATIUIR  21 

details  of  the  i-i»cky  beds  iil)()vt'  and  liclow  Mu-  iniii  arc  also  lite  same,  so  that  wc.  may 
with  coiilidtMKM^  l<rouoiiu(ic  it  to  1m>.  a  (Miiitiiuioiis  bed  IVoiii  tlic  meridian  wcsiwardto 
Lat'  des  Anglais.  Its  thickiu'ss,  richness,  and  value  vary  very  much,  but  wc  loniid  it 
more  or  less  developed  whenever  we  cross(Ml  the  ranse  and  could  get  a  view  of 
the  rock. 

The  geolofificiil  relations  of  the  ironbeaiinj;-  strata  are  exhibited  in  the  two  fol- 
lowing sections,  the  first  taken  near  the  trail  that  passes  over  the  Pewabic  range 
between  the  forks  of  the  Tyler  branch  of  Bad  river,  the  second  south  of  Lac  des 
Auglais. 

On  the  Pewabic  range  the  strike  of  the  beds  is  east  by  north;  the  dip  north  by 
west  80°  to  850.  The  beds  of  quartz  are  of  great  thickness— 2(»(l  to  250  feet.  Near 
the  junction  of  the  cpiartz  and  talcose  slate  the  latter  assumes  the  aspect  of  novacu- 
lite.  The  iron  bed  is  schistose  in  its  structure  and  is  composed  of  magnetic  oxide, 
sometimes  alternating  with  beds  of  (juartz.  The  total  thickness  of  the  talcose  slate 
is  not  seen;  it  must  be  very  thick  and  is  traversed  by  numerous  veins  of  quartz.  Its 
dip  and  strike  are  variable. 

The  bed  of  magnetic  iron  ore  south  of  Lac  des  Anglais  is  of  extraordinary  thick- 
ness  25  to  GO  feet.    The  dip  here  is  northeasterly,  and  the  layers  variable  in  thickness 

that  alternate  with  quartz,  which  latter  repose  upon  hornblendic  slate,  running  down- 
ward into  talcose  slate.  Here,  as  well  as  on  the  Pewabic  range,  the  dip  and  strike 
of  the  beds  are  variable. 

The  metamorphic  strata  are  very  much  disturbed  throughout  this  range,  but 
agree  in  having  the  mural  faces  of  the  uplifts  to  the  south  and  southeast,  and  the  dip 
northerly  and  northwesterly  at  various  angles  of  from  5°  to  60°.  The  effect  of  this 
irregular  action  is  to  make  detached  ridges  and  crests,  sometimes  2,  3,  and  5  miles 
long,  thrown  up  at  different  elevations  and  inclinations. 

Sometimes  the  iron  stratum  is  composed  of  laminte  of  quartz  and  magnetic 
oxide,  alternating,  as  at  the  crossing  of  the  trail  between  the  forks  of  the  Tyler  branch 
of  Bad  river ;  also  south  of  Lac  des  Anglais. 

The  proportion  of  iron  and  quartz  is  very  variable,  but  the  separation  of  them  by 
mechanical  means  would  in  general  not  bo  difficult.  The  bands  of  ore  vary  from  mere 
thin  lamim*  to  a  thickness  of  12  and  even  18  inches,  presenting  sometimes  a  black 
surface,  contrasting  with  the  white  and  gray  color  of  the  quartz,  and  sometimes  a 
bright  metallic  gray  color.  The  thickness  of  the  m etal lifer (uis  portion  varies  in  the 
extreme  from  5  and  10  feet  up  to  50  and  70  feet,  and  at  the  passage  of  the  main 
portion  of  Bad  river  through  the  range  reaches  250  feet.  These  exposed  faces  fre- 
quently extend  beneath  the  surface,  where,  of  course,  no  estimate  can  be  formed  of 
their  entire  thickness. 

There  are  many  places  in  the  mountain,  west  of  Bad  river,  which  present  more 
than  50  feet  of  quartz  and  iron,  in  about  equal  proportions.  In  the  wild  and  deep 
ravmes  where  the  Bad  river  breaks  through  the  range  there  is  a  cliff  of  slaty  ore. 


22  THE   PEjSTOKEE   IRON-BEARING  SERIES. 

most  of  wliicli  comes  out  in  thin,  oblique  prisms,  with  well  defined  angles  and  sti'aight 
edges,  probably  300  feet  thick,  including  what  is  covered  by  the  talus  or  fallen  por- 
tions. I  estimate  more  than  one-half  of  this  face  to  be  ore,  and  in  places  the  beds 
are  from  10  to  12  feet  in  thickness,  with  very  little  intermixture  of  quartz.  There  are 
portions  of  it  not  slaty,  but  thick  bedded.  The  dip  of  the  laminae  is  mostly  north  and 
by  east,  80°  and  85°.  The  convulsions  that  have  occurred  at  this  point  have  thrown 
a  part  of  the  range  beyond  the  rest  of  it,  to  the  northward,  so  that  in  crossing  the 
river  and  passing  along  the  mountain  to  the  eastward  for  several  miles  the  fer- 
ruginous bed,  as  well  as  many  of  the  associate  strata,  were  not  visible  above  the 
general  surface  of  the  ground.  It  should,  however,  be  borne  in  mind  that  the  whole 
region  is  not  only  covered  so  thickly  with  timber  that  no  distant  views  can  be  had 
without  climbing  trees,  but  the  drift  often  conceals  the  rocks  over  a  large  proportion 
even  of  the  elevated  ridges.  In  addition,  the  rocks  themselves,  previous  to  the  era 
of  the  drift,  have  been  the  sport  of  giant  forces,  which  tossed  and  tilted  them  about 
at  various  angles  and  elevations,  realizing  the  table  of  Atlas  (pp.  444-440). 

It  will  be  noted  tliat  the  two  names  "  Penokie  "  and  "  Pewabic"  are 
used  in  the  above  quotation  for  the  bold  range  which  runs  from  the  Mon- 
treal river  to  the  vicinity  of  English  lake,  near  the  southern  boundar}^  of 
the  iron-bearing  formation.  Whittlesey  tells  us  in  a  later  piiblication,  sub- 
sequently noted,  that  the  word  "Penokie"  here  used  is  a  misprint  for 
"Pewabic"  (more  properly  "Biwabik"),  which  latter  term  is  the  Chippewa 
word  for  "  iron."  However  this  may  be,  the  former  term  in  its  more  iisual 
form  of"  Penokee"  has   since  become  thoroughly  fixed  by  general  usage. 

Whittlesey's  early  work  in  this  region  was,  of  course,  no  more  than  a 
very  rough  i-econnaissance.  However,  considering-  the  difficiilties  of  travel 
in  the  region  and  the  fact  that  there  existed  in  it  at  the  time  but  a  single 
surveyed  line,  the  fourth  principal  meridian,  we  must  give  Whittlesey  the 
credit  of  having  achieved  a  good  deal.  In  fact,  including  the  further 
examinations  made  by  him  some  ten  years  later,  and  below  noticed,  he  sup- 
plied all  the  information  of  any  value  obtainable  at  the  time  of  the  inaugur- 
ation of  the  Wisconsin  State  Survej^,  in  1873. 

1859. 

Lapham  (I.  A.).  The  Penokee  Iron  Range.  Wis.  State  Agricultural  Society 
Transactions,  1858-'59,  vol.  5,  pp.  391-400,  with  map. 

This  is  a  very  brief  and  general  account  of  the  Penokee  Iron  range, 

based  on  trips  made  along  it  by  Dr.  Lapham  in  September,  1858,  from  Bad 


CKOLoc.HAi,  i;\im,()i;ati()NS  and  litkkatimM':.  23 

riv(M',  at  I\'ii(tk(H'  '^-a]),  eastward  lit  tlio  fourth  principal  meridian,  and  west- 
wanl  to  the  end  ol'tlic  ranj>(' south  ot"  Euylisli  lake.  The  map  acconipauy- 
inji'  tlic  ])ap('r  is  topoji'raphical  only  and  is  vor\'  siniplo,  having-  Ix'on  coni- 
pilcd  from  the  V.  S.  Land  OfHcc^  township  plats  ;  tlicf  only  addition  on  it  to 
information  atiordcd  by  these  plats  being  the  course  of  the  crest  of  the 
range.  It  is  ti)  be  noted  that  Lapham's  publication  is  the  first  giving  any 
geological  facts  based  on  an  examination  of  the  region  subsequent  to  the 
completion  in  it  of  the  Land  Office  Survey. 

The  following  quotation  includes  all  of  geological  interest  given  in 
this  paper : 

This  remarkable  inountain  range  has  been  traced  from  a  little  east  of  the  fonrth 
principal  meridian  in  township  45,  in  a  direction  a  little  south  of  west,  across  three 
ranges  of  townships;  its  length  being  about  20  miles,  as  shown  on  the  accompanying 
map.  At  the  west  the  range  appears  to  slope  down  and  terminates,  but  toward  the 
east  its  extent  is  not  known.  The  highest  summits  are  about  1,200  feet  above  lake 
Superior,  or  1,800  feet  above  the  sea;  the  mean  height  is  100  or  200  feet  less.  Tyler's 
fork  crosses  the  range  at  a  place  called  "Tlie  Grorge,-'  and  Bad  river  crosses  at  Peno- 
kee,  through  a  gap  cut  down  to  a  depth  of  about  .'500  feet ;  the  river  here  having  an 
elevation  above  lake  Superior  of  668  feet. 

On  the  north  side  the  slope  of  the  range  is  moderate,  and  covered  with  "drift;" 
but  on  the  south  it  is  quite  abrupt,  and  steep,  rocky  precipices  occur,  looking  as  if 
they  had  at  some  remote  period  of  the  past  formed  the  shores  of  some  great  body  of 
water. 

What  gives  this  great  ridge  its  peculiar  interest  and  importance  is  the  immense 
stratum  or  bed  of  magnetic  iron  ore  which  it  contains,  extending,  with  varying  thick- 
ness and  value,  throughout  its  whole  length.  It  is  not,  therefore,  an  Iron  mountain 
simply,  like  those  heretofore  known  in  Missouri  and  elsewhere,  but,  as  its  name  imports, 
an  Iron  range;  as  if  mountain  masses  of  iron  had  been  passed  between  gigantic  rollers 
and  drawn  out  for  a  length  of  20  miles.  The  ore  is  found  in  a  very  ancient  chloritic 
slate,  so  ancient  that  it  is  supposed  to  have  been  deposited  long  before  the  existence 
of  vegetable  or  animal  life  upon  the  globe.  The  slate  rests  upon  a  light  colored 
quartz-rock,  which  usually  extends  to  the  base  of  the  range  on  the  south  side.  The 
ore  is  laminated,  like  the  .slate,  and  apparently  has  had  the  same  origin;  for,  as  we 
ascend  from  the  qnartx,-rock  the  slate  becomes  more  and  more  ferruginous  until  it 
passfes  into  pui-e  iron  ore.  This  change  is  so  gradual  that  it  is  often  difficult  to  deter- 
mine where  the  slate  ceases  and  the  ore  begins,  or  how  much  should  be  classed  as 
iron  ore  and  how  much  as  ferruginous  slate.  We  noticed  places  where  the  ore  had  a 
thickness  of  00  feet,  at  other  places  10,  and  wherever  we  could  get  access  to  the  rock 
at  the  proper  place  the  ore  was  found. 


24  THE   PENOKEE   lliON-BEAElNG   8EKIES. 

Above  tbe  ore,  that  is  north  of  it,  the  slate  has  been  hardened,  probably  bj^  some 
volcanic  agency,  into  a  compact  mass,  but  still  showing  traces  of  its  original  laminated 
structnre.  This  highly  indurated  rock  is  the  nucleus  of  the  ridge,  usually  ibrming 
the  crest  or  highest  part;  and  it  forms  the  north  slope,  except  where  covered  with  the 
bowlders  and  other  coarse  materials  of  the  Drift  formation.  If  we  may  judge  from 
the  polished  and  grooved  surfaces,  we  may  suppose  that  this  excessively  hard  rock 
has  resisted  the  action  of  the  powerful  currents  and  icebergs  that  once  flowed  over  the 
very  top  of  the  ridge,  which,  with  its  invaluable  beds  of  iron  ore,  was  thus  saved  from 
destruction. 

All  the  rocks,  including  the  ore,  have  a  considerable  dip  toward  the  north,  or 
toward  the  great  basin  of  lake  Superior;  and  they  are  always  found  in  the  same 
relative  position  in  regard  to  the  ore.  If  we,  at  any  new  locality,  could  find  either  of 
the  rocks  in  place,  we  at  once  knew  which  way  to  turn  to  find  the  ore. 

The  magnetic  ore  of  the  Penokee  Iron  range  contains  a  notable  and  much  varying 
proportion  of  silica  in  its  composition,  but  is  free  from  sulphur  and  other  deleterious 
qualities,  corresponding  in  this  respect  with  most  of  the  iron  ores  of  this  remote 
geological  epoch.  It  is  in  some  localities  so  highly  magnetic  that  particles  adhere  to 
the  hnumier  when  struck,  like  iron  filings  to  a  magnet;  and  the  compass  needle  as 
often  pointed  toward  the  east  or  west  as  to  the  north,  in  one  instance  being  entirely 
reversed,  the  north  end  pointing  to  the  south.  At  Penokee,  where  Bad  river  crosses 
the  range,  the  ore  exists  in  such  abundance  that  it  may  be  obtained  from  the  face  of 
the  hill,  much  as  stone  is  taken  from  an  ordinary  stone  quarry.  Large  masses 
that  have  fallen  from  the  cliiis  now  lie  loose  upon  the  surface,  and  will  supply  a  fur- 
nace for  many  years  before  it  will  be  necessary  to  resort  to  the  original  bed  (pp. 
394-396). 

I860. 

Lapham  (I.  A.).  Eeport  to  the  Directors  of  the  Wisconsin  and  Lake  Superior 
Mining  and  Smelting  Company,  in  the  Penokee  Iron  Eange  of  Lake  Superior,  with 
Reports  and  Statistics,  showing  its  Mineral  Wealth  and  Prospects,  Charter  and 
Organization  of  the  Wisconsin  and  Lake  Superior  Mining  and  Smelting  Company, 
Milwaukee,  1860,  pp.  22-37. 

This  is  a  pi-ivate  economic  report  based  on  the  same  exploration  as  the 
paper  immediately  preceding.  It  contains,  however,  some  further  general- 
izations of  interest.  Dr.  Lapham's  investigations  appear  to  have  been  con- 
fined almost  entirely  to  the  ferruginous  belt  and  its  immediately  adjacent 
layers,  Avhich  form  the  Penokee  ridge  proper.  The  higher  members  of  the 
series  received  only  very  shght  and  incidental  attention.  The  following 
quotation  gives  the  more  important  generalizations  reached: 


(IKOLOdlCAl.  K.VIM-OIIATIONS  AND   MTllKATUlM-:.  25 

1  will  now  |in>ceo(lt()t'X|>l;nii  the  }i'ool()fri«"il  rebitiiiiis  i>\'  this  vast,  lied  of  magnetic 
iron  (ire,  sliow  how  it  is  associated  witli  other  rocks,  and  its  mode  of  oeenneniM'  an<l 
eharacteristies. 

The  K'eix'i'iil  dii-eetion  of  tlie  lans'S  is  about  12°  north  of  east,  and  south  of  west' 
and  this  is  also  the  direction  of  the  strike  of  the  several  strata  of  which  it  is  composed. 
The  dip,  or  inclination  of  the  layeis  of  rock,  is  toward  the  north  and  at  light  angles  to 
the  strike.  The  dip  varies  from  ;30o  to  00°,  and  ui  a  few  eases  it  is  almost  perpendic- 
ular, or  1)0°.     West  of  Bad  river  the  dip  is  usually  Jpss  than  it  is  east  of  that  stream. 

The  rocks  with  which  the  ore  is  immediately  associated  are  of  the  kind  called 
Primary  or  Azoic,  having  been  formed,  as  is  supposed,  at  a  very  early  epoch,  and 
prioi'  to  t\it}  existence  of  aninuil  life  upou  the  earth. 

The  lowest  formation  uoticed  at  the  base  of  the  lange  on  the  south  side  was  a 
light  colored,  often  white,  quartz  rock,  consisting  of  minute,  closely  aggregated  grains 
of  quartz  or  sand.  Somjetimes  this  rock  forms  a  large  proportion  of  the  height  of  the 
cliff;  at  other  places  It  is  quite  subordinate. 

Next  above  the  quartz  rock,  and  often  alternating  wdth  it,  is  found  a  chloritic 
slate,  .  .  .  more  or  less  silicious;  in  some  localities  so  much  as  to  resemble  the 
novaculite  or  oil  stoue,  and  might  be  used  for  whetstones,  though  we  saw  none  ot 
much  tineuess  of  texture.  In  mauy  respects  this  slate  is  one  of  much  intei'est,  being 
often  associated  with  metallic  ores,  and  at  tliis  place  actually  passing  into  iron  ore-^ 
As  we  ascend  toward  the  north,  the  slate  gradually  becomes  more  and  more  ferrugi- 
nous, until  it  is  changed  to  a  very  pure  iron  ore.  .  .  .  It  is  often  quite  difficult 
to  decide,  by  the  external  characters  alone  where  the  slate  rock  ends  and  the  slaty 
iron  ore  begins,  so  gradual  is  the  change.  The  ore  possesses  the  same  slaty  character 
and  has  the  same  dip  or  inclination  of  its  layers. 

Immediately  above  the  ore  (or  north  of  it)  is  found  a  very  hard  rock,  ... 
with  obscure  marks,  indicating  its  original  slaty  structure,  which  is  supposed  to  be 
hardened  (metamorphic)  slate.  It  is  everywhere  tilled  with  thin  seams  of  highly 
magnetic  ore.  These  seams  .  .  .  vary  in  thickness  A'om  one-eighth  of  an  inch  to 
several  inches,  and  correspond  in  direction  and  dip  with  the  mass  of  the  rock.  They 
are  so  hardened  and  so  intimately  counected  with  the  rock  in  which  they  are 
imbedded  that  the  ore  cau  not  be  separated;  and  it  is,  therefore,  only  when  these 
seams  are  so  abundant  as  to  justify  the  smelting  of  the  whole  (ore  and  rock)  together 
that  they  possess  practical  value.  It  forms  the  crest  and  north  slope  of  the  range, 
except  where  covered  with  drift.     ... 

These  several  rocks — the  quartz  rock,  chloritic  slate,  magnetic  iron  ore  (for  this 
is  so  extensive  here  that  it  may  be  ranked  as  a  rock  formation),  the  "hard  rock,"  and 
the  drift — are  all  coextensive  with  the  range,  so  that  wherever  we  find  either  we 
could  with  certainty  predict  the  occurrence  of  the  others  in  the  jjroper  relative 
order.    ... 


26  THE  PENOKTEE  IRON-BEAEING  SERIES. 

It  is  supposed  by  geologists  that  slate  rocks  were  originally  deposited  from 
water,  and  that  the  layers  were  at  first  nearly  horizontal.  Whatever  may  have  been 
the  origin  and  original  position  of  this  slate,  the  same  must  be  assigned  to  the  iron 
ore,  the  same  causes  continuing  to  operate  throughout  the  whole  period  of  the  forma- 
tion of  the  slates  and  the  iron. 

These  strata  have  all  been  tilted  or  lifted  up  from  the  horizontal  to  nearly  a 
vertical  position,  without  otherwise  materially  modifying  their  structure  and  composi- 
tion. That  part  of  the  slaty  deposit  which  rests  upon,  and  was  therefore  of  later 
formation  than  the  iron,  has  been  subjected  to  still  other  influences  (probably  heat 
and  pressure),  which  have  transformed  the  fissile  slate  into  a  very  hard  subcrystal- 
line  rock. 

The  geological  relations  of  this  prolonged  ore  bed  are  quite  the  same  as  those  of 
the  nonmagnetic  ores  near  Marquette.  We  have  the  same  granitic  rocks  at  the 
north,  the  same  quartz  rocks  at  the  south,  and  the  same  slate  in  which  the  ore  i^ 
found.  Hence  we  must  assign  to  each  a  similarity  of  origin,  and  whatever  theory 
may  be  adopted  for  one  must  be  equally  true  of  the  other.     (Pp.  26-29.) 

The  following-  quotation  is  of  interest  as  showing  that  Dr.  Lapham 
was  the  first  to  notice  indications  of  the  occurrence  at  Penokee  gap  of  the 
fault  whose  existence  was  subsequently  demonstrated  and  details  worked 
out  by  the  Wisconsin  Survey.^ 

The  range  here  aiipears  to  have  made  a  sudden  offset  northward  in  passing  the 
river,  and  the  strata  are  more  or  less  disturbed,  the  dip  and  direction  not  being  so 
uniform  and  regular  -as  at  most  of  the  other  localities  examined. 

Dr.  Lapham  held,  with  regard  to  the  value  of  the  strongly  magnetitic 
rocks  of  the  Penokee  range,  the  usual  unfortunately  favorable  opinion  which 
later  experience  has  not  justified. 

It  will  be  seen  that  we  have  already  discovered  good  ore  in  such  quantities  as 
to  be  practically  inexhaustible,  situated  at  points  accessible  to  water  power  and 
having  bold  fronts,  rendering  it  comparatively  easy  to  be  quarried.  For  many  years 
to  come  only  the  richest  and  most  accessible  ores  can  be  brought  into  use,  rejecting — 
at  least  for  the  present — all  such  as  have  too  large  a  proportion  of  silica,  and  such  as 
are  not  in  a  condition  to  be  easily  and  cheaply  removed  from  the  natural  bed. 
Though  it  is  clearly  shown  that  the  ore  is  coextensive  with  the  range,  yet  it  must  not 
be  supposed  that  it  coustitutes  a  continuous  workable  mine  throughout  this  whole 
distance  of  20  miles.  It  is  only  at  the  points  indicated  where  the  ore  is  easily  worked 
and  where  water  power  is  at  hand  that  very  great  immediate  value  can  be  put  upon. 

■  Geology  of  Wiscousiu,  vol.  3,  pp.  150-152,  and  Atlas,  Plate  xxiii. 


(;k()M)(;i(;aI;  kaim,(»i;ati()NS  and  mtI':i;atuiM'I  27 

tlifsc  iiiiiit's,  t'oi'  tilt'  it'Msoii  t  liat  tlu'  (lilleveiicc  in  the  expciisc  of  (|iiariyiiit;'  tlic  ore 
tVoiii  tilt'  si(li>  1)1'  ii  liifjli  clilf,  and  of  niiiiiiii;'  it  ht'low  tlii'  siulaci'  of  tlid  j;r()iiml,  will  he 
sullii'iiMit  to  (lissi[»att'  all  tlif  prolits  that  can  be  inadt!  IVoiii  Mie  use  of  ores  so 
obtaiued.     (I'.  ;!.{.) 

ISO.'}. 

Whittli;skv  ((!harles).  The  Peuokie  Mineral  liaiige,  Wisconsin,  Proc.  Bost. 
Soc.  Niit.  Hist.,  vol.  IX,  l.S62-'(;.{.  pp.  235-244. 

The  work  doue  l)y  Col.  Whittlesey  in  184!)  in  coimectiou  with  Dr. 
Owen's  general  survey  of  the  northwest  antedated  the  linear  sm-veys  of 
this  region.  Eleven  years  later,  August  to  October,  18G0,  he  made  a 
further  examination  of  the  Bad  river  country,  under  the  auspices  of  the 
Wisconsin  Geological  Survey,  then  organized  under  James  Hall.  This 
time  he  had  the  advantage  of  the  linear  survej^s.  His  report  was  never 
published,  the  stirvey  being  very  short-Hved.  He  afterwards  published  a 
few  brief  details  in  reports  to  iron  companies  and  the  summary  contained 
in  this  pamphlet,  from  which  we  quote  quite  fully.  The  map  accompany- 
ing Whittlesey's  report  to  the  Wisconsin  Greological  Survey  was  not  jjub- 
lished  until  1880,  Avhen  it  appeared  in  an  appendix  to  the  third  volume  of 
the  Geology  of  Wisconsin,  but  it  is  properly  reproduced  here,  having  been 
prepared  in  1860.     (PI.  iv.) 

The  copper-bearing  strata  of  point  Kewenaw  (lake  Superior'!  extend  south- 
westerly across  the  boundary  of  the  state  of  Michigan  into  Wisconsin.  These  strata 
constitute  a.  long,  narrow,  and  bold  mountain  range  from  Copper  harbor  to  Long  lake, 
a  distance  of  100  miles.  There  are  no  stratigraphical  breaks  along  this  line,  the 
order  of  the  rock  being  everywliere  the  same.  The  dip  of  the  beds  is  always  north- 
erly or  northwest  and  the  strike  to  the  northeast  or  east,  the  general  line  of  outcrop 
being  northeast  by  east.  On  i)oint  Kewenaw  and  as  far  southwest  as  the  Akogebe 
lake,  on  the  west  fork  of  the  Ontonagon  river,  the  copper  veins  have  been  found 
valuable. 

Beyond  the  waters  of  the  Ontonagon,  in  the  same  direction,  veins  have  been 
discovered,  but,  after  limited  workings,  have  lieen  abandoned.  The  Montreal  river 
forms  the  boundary  between  Mk-hUjnn  and  Wiscomin,  and  as  early  as  the  year  1845 
mining  locations  were  made  on  its  waters  where  they  pass  the  range.  Locations  were 
also  made  upon  the  waters  ot  the  Bad  or  Mauvaise  river,  a  stream  with  numerous 
branches,  draining  the  country  from  the  Montreal  to  the  head  waters  of  the  Chip- 
peway  and  St.  Croix  rivers. 


28  THE  PBNOKEE  IRON-BEAEmG  SERIES. 

Historically  considered,  the  exploration  of  this  region  commenced  iu  the  year 
1840,  when  Dr.  Houghton,  as  a  commissioner  of  the  State  of  Michigan,  accompanied 
Capt.  Cram,  of  the  United  States  Topographical  Engineers,  who  was  then  surveying 
the  Menominee  and  Montreal  rivers. 

In  1840  and  1841  Dr.  Houghton  examined  the  rocks  on  both  these  streams  and 
the  country  between  their  sources.  I  am  in  possession  of  a  transcript  of  his  field 
notes  during  these  explorations.  In  184u-'4C  I  made  examinations  along  the  range 
across  the  Montreal  to  the  westward,  as  far  as  the  main  branch  of  Bad  river. 

Uj)  to  this  time  the  public  lands  in  this  part  of  Wisconsin  had  not  been  surveyed. 
The  fourth  i)rincipal  meridian  was  extended  northward  through  Wisconsin  to  lake 
Superior  iu  1848.  Dr.  A.  Randall,  one  of  the  assistants  of  Dr.  Owen  upon  the  survey 
of  the  Ghippeway  land  district  in  reference  to  mines  and  minerals,  accompanied  the 
linear  surveyors  along  this  line.  In  T.  44  N.,  Dr.  Randall  discovered  an  outcrop  of 
magnetic  iron  ore  and  brought  in  a  siiecimen.  The  next  season,  as  a  member  of  Dr. 
Owen's  corps,  I  made  an  exph)ratiou  on  the  western  branches  of  Bad  river,  crossing 
southerly  to  the  head  waters  of  the  Chippeway.  Near  Lac  des  Anglais,  and  thence 
easterly  across  the  middle  or  main  fork  of  the  Bad  river,  I  found  clifts  and  liluts  of 
siliceous  magnetite.  The  results  of  this  examination  may  be  seen  in  the  final  report 
of  Dr.  Owen,  published  at  Washington  in  the  year  1850. 

In  the  Chippeway  language  the  name  for  iron  is  imrabiJc,  and  I  thought  it  proper 
to  designate  the  mountains  where  this  nietal  exists  in  quantities  that  surprise  all 
observers  as  the  "Pewabik  range."  The  compositor,  however,  transformed  it  to 
Penokie,  a  word  which  belongs  to  no  language,  but  which  is  now  too  well  fastened 
upon  the  range  by  usage  to  be  changed. 

Soon  after  the  publication  of  Dr.  Owen's  report  the  excitement  of  1845-'46  in 
reference  to  copper  was  repeated  in  reference  to  iron.  The  government  was  at  last 
induced  to  make  surveys  of  the  region.  Preenaptors  followed  the  surveyors,  erect- 
ing their  rude  cabins  on  each  quarter  section  between  the  meridian  and  lac  des 
Anglais,  a  distance  of  18  or  20  miles.  The  iron  belt  is  generally  less  than  one- fourth 
of  a  mile  in  width,  regularly  stratified,  dipping  to  the  northwest  conformable  to  the 
formations,  and  having  its  outcrop  along  the  summit  of  the  second  or  southerly  range. 
Viewing  this  mountain  region  from  La  Pointe,  or  from  the  open  lake,  it  has  the 
appearance  of  a  single  crest.  Its  outline  against  the  sky  on  a  clear  day  is  very  dis- 
tinct and  regular.  Along  the  range  this  crest  line  is  nearly  level,  its  elevatiou  above 
the  lake  being  1,000  to  1,100  feet.  But  there  are  two  ranges,  known  in  the  country  as 
the  first  and  second,  or  the  "Copper"  and  the  "Iron"  range.  There  is  not  much 
difference  in  their  elevation.  The  copper  range,  being  nearest  the  coast,  covers  the 
iron  range,  which,  at  the  distance  of  30  miles,  is  visible  only  through  gaps  and 
notches,  the  whole  forming  one  blended  line  in  the  horizon.  To  the  south,  beyond  the 
iron  range,  the  country  is  lower  and  swampy. 


GEOLOGICAL  EXPLORATIONS  AND  LITEKATUKE.  29 

Two  roads  were  soon  constructed  tn  the  mineral  deposits  through  the  dense  ever- 
green forests  of  tiiis  hititudc.  One  of  them  conimcnccd  at  tlic^  lake,  near  the  mouth 
of  tlie  Montreal  river,  and  near  the  terniinalioti  of  tlic  fonvth  principal  meridian, 
extending  thence  south  and  not  far  fioiii  I  lie  meridian  line.  The  other  began  on 
Chegoimegon  bay,  at  Asiiland,  pursuing  also  a  southerly  eonrse,  and,  after  reaching 
the  second  range,  coiinecte<l  along  it  to  the  eastward  with  the  first  road,  passing 
the  cabins  of  the  ])reemi)tors.  In  1850,  Mr.  Daniels,  of  the  Wisconsin  Geological 
Survey,  and  Mr.  Lapham,  of  Milwaukee,  examined  the  iron  range  in  behalf  of  a  com- 
pany which  had  ma(U'  extensive  pnrchases  there  and  had  caused  a  survey  for  a  rail- 
way to  be  made  with  a  view  to  the  manufacture  of  iron  (pp.  1235-237). 

The  following  is  a  general  view  of  the  structure  of  the  formations  in  the  descend- 
ing order: — 

Formation  No.  1. — Potsdam  Sandstone.  On  the  Montreal  river,  .strike  northeast 
by  east,  in  places  N.  60°  E. ;  flij)  northwest  by  north,  75°  to  90?.  It  embraces  four 
members  a,  b,  c,  and  (1. 

Feet. 

rt.  Sandstone  proper,  corrected  for  bevel,  thickness 8,500 

1).  Alternations  of  sandstone  and  black  slate,  thickness 750 

c.  Conglomerate,   thickness 1,800 

d.  Alternations  of  trap  and  sandstf)ne,  thickness 800 

Total 11.850 

Formation  No.  2. — Trajipose,  in  two  memhers. 

Miles. 

a.  Brown  amygdaloid;  dip  and    strike  conformable    to  for- 
mation 1 ;  thickness  along  lac  Flambeau  trail 3 J 

h.  Compact  red  and  blue 2 J 

Total : 6 

Formation  No.  3. — Hornhlendic. 

Compact,  subcrystalline  and  slaty;   black  or  dark  colored; 

strike  N.  60°  E. ;  thickness  on  trail - 2^ 

Formation  No.  -1. — Siliceovs,  two  memhers. 

a.  Quartz,  slaty,  and  in  layers;  dark  colored,  but  less  than  for- 
mation 3 ;  thickness  variable ;  separated  from  6  by  a 
bed  of  magnetic  iron  and  iron  slate. 
h.  Quartz,  slaty,  in  layers  and  beds;  more  compact  and  lighter 
color  (gray  and  straw  color)  than  a;  iiovaculite;  strike 
N.  60°  to  650  B.;  dip  variable,  30°,  45°,  60°,  75°  to  the 

northwest ;  breadth  across  the  edges  on  trail 3^ 

Formation  No.  5. 

Granites  and  syenites  of  central  Wisconsin  (pp.  238-239), 


30  THE  PBlJfOKBE  lEOlST-BEARIFG  SERIES. 

1872. 

Bkooks  (T.  B.)  and  Pumpelly  (E.  ).  On  tlie  Age  of  the  Copper-bearing  Eocks 
of  Lake  Superior.    Am.  Jour.  Sci.,  3d  series,  vol.  iii,  1872,  pp.  428-432. 

Messrs.  Brooks  and' Pumpelly,  iu  the  fall  of  1871,  made  a  very  rapid 
trip  from  the  passage  of  Bad  river  through  the  Penokee  range  in  Wiscon- 
sin eastward  to  the  Ontonagon  river  in  Michigan. 

The  statements  below  quoted  from  this  paper  with  regard  to  the  coun- 
try between  the  Montreal  river  and  Gogebic  lake  contain  the  first  pub- 
lished announcement  of  the  existence  in  Michigan  of  an  eastward  continua- 
tion of  the  Penokee  Iron  series  of  Wisconsin.  The  width  of  the  iron-bear- 
ing series  often  greatly  exceeds  the  "  one-fourth  to  one-half  mile"  given  in 
this  quotation,  reaching  as  much  as  2^  miles  or  more,  while  at  other  times 
it  is  below  their  lowest  figure,  or  is  even  cut  out  altogether  by  the  overlap-' 
ping  of  the  overlying  Keweenawan  rocks. 

During  last  autumn,  traveling  sometimes  together  and  sometimes  apart,  we 
made  a  reconnaissance  of  the  country  between  Bad  river  in  Wisconsin  and  the  middle 
branch  of  the  Ontonagon  east  of  lake  Gogebic  iji  Michigan.  Our  route  was  chiefly 
confined  to  the  s^irface  of  the  upper  member  of  the  Michigan  Azoic,  which  we  have 
provisionally  considered  to  be  the  equivalent  of  the  Huronian. 

From  Penokie  gap  on  Bad  river  to  near  lake  Gogebic,  a  distance  of  nearly  60 
miles,  the  quartzites  and  schists  of  this  formation  are  tilted  at  high  angles  and  form 
a  belt  one-fourth  to  one-half  mile  in  width,  bordei'ed  on  the  south  by  Laurentian 
gneiss  and  schists.  On  the  north  it  is  everywhere  overlain  by  the  bedded  melaphyre 
(containing  interstratitied  sandstones)  of  the  Cupriferous  series.  These  form  ridges 
and  peaks  which  rise  200  to  300  feet  above  the  surface  of  the  Huronian  belt. 

These  ridges,  forming  the  "South  Mineral  range,"  unite  at  their  westeru  end 
with  the  Mineral  range  proper,  Avhich  forms  really  through  its  whole  length  the  tongue 
of  land  known  as  Keweenaw  point.  Between  these  two  ranges  lies  the  southwestern 
part  of  the  Silurian  trough,  which  has  been  mentioned  before  as  extending  inland 
from  Keweenaw  bay. 

Here,  as  there,  it  is  filled  with  the  horizontally  stratified  Silurian  sandstone, 
forming  a  generally  level  country.  For  a  distance  of  nearly  30  miles,  between  the 
Montreal  river  in  T.  47  and  lake  Gogebic,  we  found  the  Cupriferous  series  ai3par- 
ently  conforuung  in  strike  and  dip  with  the  Huroniau  schists,  and  both  uniformly 
dipijing  to  the  north  at  angles  of  50°  to  70°.  But  in  approaching  lake  Gogebic  from 
the  west  we  find  that  erosion  of  Silurian  or  pi'e-Siluriau  age  has  made  n  deep  inden- 
tation entirely  across  the  Cui>riferous  series  and  the  Huronian,  and  into  the  Lauren- 
tian, so  that  at  a  short  distance  west  of  the  la^ke  these  rocks  end  in  steep  and  high 


OE()T.<>(lI('AL   KXI'LOKATIONS  AND  LITKKATURE. 


31 


declivitie.s,  at  Mic  l>a,sc  of  wliicli  lies  the  level  (■(•iiiilry  ol'  tlie  Siliiiiau  saudstone,  in 
which  is  cut  the  hasiii  of  tlie  hiivc.  From  tliis  point  e:isl ward  this  iiiicient  erosiou 
had  made  giM-af  inroads  upon  the  continuity  of  the  (,ln|)riferous  and  okler  rocks  before 
the  deposition  of  the  Sihuian  sandstone.  The  nudapliyre  ridges  are  broken  into 
knobs,  or  arc  wanting,  and  no  Iluroidati  was  found  as  far  as  tlic  Ontonagon  river, 
7  miles  away,  and  the  linut  of  onr  observations.     (I'j).  JtoO—i'il.) 

1S73. 

Bkooks  (T.  B.).     Geological  Survey  of  Michigan,  Upper  Peninsula,  1869-1873, 
vol.  I,  New  York,  1873,  with  an  Atlas. 


Scale  13  rrUles  =J  inch  . 

Fig.  2 Reproduction  of  a  portion  of  Brooks  and  Purapclly's  geological  map  of 

the  upper  peninsula  of  Michigan. 

Ill  Part  I  of  this  volume,  chapter  vi  (pp.  183-I86)is  entitled  "Lake 
Gogebic  and  Montreal  River  Iron  Range."  This  gives  a  brief  outline 
account  of  the  occurrences  in  the  iron  belt  between  Montreal  y'whv  and 
Gogebic  lake.  The  account  is  somewhat  fuller  than  that  given  in  the 
paper  quoted  immediately  above,  but  is  based  on  the  same  rapid  examina- 
tion, and  contains  no  details.  On  the  general  map  of  the  Northern  penin- 
sula, given  in  tlie  atlas  which  accompanies  this  reixtrt,  the  Michigan  end  of 
the  Penokee  series  is  first  mapped,  Fig.  2  is  copied  from  the  western  por- 
tion of  this  atlas  map. 


32  THE  PENOKEE  lEON-BEAEIl^G   SERIES. 

The  ironrauge  under  consideration  may  be  regarded  as  the  eastern  prolongation 
of  the  Penokie  range  of  Wisconsin,  as  well  as  the  western  extension  of  the  Marquette 
series,  the  whole  being  Huroniau.  The  position  of  the  range  is  tolerably  well  defined 
by  magnetic  observations  and  notes  on  the  U.  S.  Land  OflBce  plats.  On  these  we  find 
mention  of  iron  and  magnetic  attractions  on  Sees.  7  and  8,  T.  47  N.,  R.  45  W.,  as 
also  in  Sees.  13  and  14  of  the  town  west.  The  belt  of  Huroniau  rocks,  as  made 
out  by  us,  extends  nearly  east  and  west  through  the  north  part  of  T.  47,  ranges  44, 
45,  46,  and  47,  crossing  the  Montreal  river  in  Sees.  16  and  21  of  the  last  named 
township.     Going  east,  the  range  was  lost  before  it  reached  lake  Gogebic. 

The  geological  boundaries  of  this  range  are  fortunately  of  the  most  irnmistak- 
able  nature  and  render  a  detailed  description  of  its  position  unnecessary. 

On  the  north  is  the  high,  broad,  irregular  ridge,  or  series  of  ridges,  constituting 
the  South  Copper  range,  the  rocks  of  which  are  greenish  and  brownish,  massive  and 
amygdaloidal  copper-bearing  traps,  their  bedding  being  exceedingly  obscure,  with 
occasional  beds  of  sandstone  and  an  imperfect  conglomerate.  The  strike  of  these 
rocks,  so  far  as  it  could  be  made  out,  was  east  and  west,  with  a  dip  to  the  north  at  a 
high  angle,  thns  .conforming  with  the  Huroniau  rocks  underneath. 

Against  and  over  the  copper  series  on  the  north  abut  the  horizontally  bedded 
lower  Silurian  sandstones,  which  are  beautifully  exposed  on  the  west  branch  of  the 
Ontonagon  river,  in  Sec.  23,  T.  46,  R.  41.  These  sandstones  form  the  surface  rock, 
and  occupy  the  broad  belt  between  tlie  two  copper  ranges  from  the  region  we  are 
describing  to  KeweenaAv  bay,  but  taper  to  a  point  before  reaching  the  Montreal  river 
in  going  west. 

On  the  south  of  tiie  Iron-bearing  rocks  are  a  series  of  granites,  chloritic  gneisses, 
and  obscure  schists,  Avhich,  except  the  latter,  are  unmistakably  Laurentian  in  their 
lithological  character,  and  are  nonconformably  overlaid  by  the  Huroniau  rocks.  The 
general  structural  relations  of  the  four  great  systems  here  enumerated  are  shown  in 
the  accompanying  diagram.     (Pp.  183,  184.) 

The  best  locality  in  which  to  study  the  character  of  the  iron  series  in  the  West 
region,  is  on  Black  river  and  its  tributaries,  especially  on  the  outlet  of  Sunday  lake, 
■  T.  47,  ranges  45  and  46.  Here  will  be  found  banded  ferruginous  jaspery  schists, 
chloritic  greenstones,  brown  ferruginous  slates,  black  and  gray  banded  siliceous  slates, 
siliceous  flag  ores,  several  varieties  of  quartzites,  and  clay  slate.  The  whole  series 
strike  east  and  west,  and  dip  north  away  from  the  granites  and  gneisses  and  under 
the  copper  rocks  at  an  angle  of  from  40°  to  90°.     (P.  185.) 

In  Part  ii  of  the  same  volume  (being  Prof.  Pumpelly's  report  on  the 
Copper-Bearing  Rocks),  Chapter  i  contains  the  same  statements,  reproduced 
almost  verbatim,  with  regard  to  the  ..Montreal-Gogebic  iron  belt,  as  are 
above  quoted  from  a  paper  pubhshed  jointly  by  Brooks  and  Pumpelly. 


GEULOCiUJAL  EXPLORATIONS  AND  LITERATURE.  33 

JuLiEN  (Alexis  A.).  Lithological  Dcsfrii)tious,  etc.,  of  ^51)  specimens  of  the 
niiroiiiiin  iiiiil  Lauiciitiari  Rocks  of  tlic  Upper  Peiiiusula,  appendix  A,  Geological 
Survey  of  Michigaii,  New  York,  1873,  vol.  ii,  i)p.  1-197. 

This  report  (•(mtaius  iiouinicroscoj)!*'.  lithological  descriptions  of  a  few- 
rock  specimens  collei'ted  by  T.  B.  Brooks  and  R.  Punipelly  in  the  Gogebic 
region  in  1871,  during  the  trip  already  several  times  referred  to. 

187<L-. 

Irving  (R.  D.).  On  the  Age  of  the  Copper-bearing  Rocks  of  Lake  Superior; 
and  on  the  Westward  Continuation  of  the  Lake  Superior  Synclinal.  Am.  Jour.  Sci., 
3d  series,  vol.  viii,  1S74,  pp.  46-5G,  with  a  map  and  section. 

This  is  a  paper  giving  an  account  of  several  conclusions  reached  after 
a  first  season's  work  in  the  Bad  rivei'  country  of  northern  Wisconsin.  Its 
main  object  is  to  set  forth  conclusions  on  the  points  indicated  in  the  title. 
The  following  paragraphs  are  quoted  as  bearing  on  the  district  which  forms 
our  present  subject: 

2.  The  Huronian  rocks  (II  on  the  map  and  section),  which  directly  overlie  the 
Laurentian,  and  unconformably,  as  shown  by  Brooks  and  Pumpellyfrom  observations 
made  by  them  in  Michigan  just  east  of  the  Wisconsin  line,  constitute  in  Ashland 
comity  a  continuous  narrow  belt,  whose  central  portion  is  the  Avell  known  "Penokie 
Iron  range,"  and  whose  width  never  exceeds  2  miles,  being  usually  much  less  than 
this.  This  belt  extends  without  break  into  Michigan,  almost  as  far  as  lake  Gogebic, 
where  the  rocks  are  lost  sight  of,  being  covered  by  accumulations  of  drift,  and  finally 
by  horizontal  Silurian  rocks.  They  do  not  reappear  until,  100  miles  farther  east,  the 
Marquette  iron  region  is  reached.  Here  they  are  fbund  again,  with  some  imi^ortant 
changes  and  covering  a  much  mder  extent  of  territory.  Toward  its  western  extremity 
the  Huronian  belt  appears  to  come  to  an  abrupt  ending,  the  underlying  Laurentian 
and  overlying  Copper-bearing  series  coming  together.  Farther  west,  however,  just  on 
the  west  side  of  Ashland  county,  are  two  isolated  belts  of  these  rocks,  in  every  way 
similar  to  the  main  area,  each  having  in  the  same  manner  its  central  ridge.  As  to 
the  continuation  still  farther  westward  of  the  Huronian,  nothing  whatever  is  known, 
as  is  indicated  on  the  map  by  an  interrogation  point.  The  rocks  of  this  group  in 
northern  Wisconsin  are  siliceous  schists,  talco-siliceous  schists,  black  slates  of  unde- 
termined composition,  white  quartz  rocks,  quartzites,  magnetic  and  specidar  schists 
of  various  kinds,  magnetic  and  specular  iron  ores,  diorites,  diorite  slates,  and  diorite 
schists.  The  beds  of  one  portion  of  the  group,  about  500  feet  in  actual  thickness  and 
continuous  for  over  30  miles,  are  impregnated  ttroughout  with  the  specular  and  mag- 
MON  XIX 3 


34  THE  PEjS^OKEE  IKON-BEARING  SERIES. 

7ietic  oxides  of  iron  in  proportions  varying  from  1  or  2  i>er  cent  up  to  60  and  80  per 
cent  of  the  whole.  The  entire  series  has  a  nearly  uniform  dip  we-stof  north,  generally 
at  a  very  high  angle.  The  thickness  never  varies  far  from  4,000  feet,  a  figure  obtained 
by  actual  measurement.     (P.  48.) 

The  estimate  thus  given  for  the  thickness  of  the  iron  series  falls  far 
short  of  the  truth,  as  the  writer  himself  was  able  to  show  after  further 
work.  The  mistake  arose  from  a  misconception  during  the  first  field 
season's  work,  of  the  nature  of  the  iip2>er  inica-schist  or  micaceous  quartz- 
ite  member  of  the  iron  series.  This  member  was  not  at  the  time  examined 
closely  and  was  supposed  to  belong  with  a  higher  series. 

Ikving  (R.  D.).  On  Some  Points  in  the  Geology  of  Northern  Wisconsin. 
Trans.  Wis.  Acad.  Sci.,  Arts  and  Letters,  vol.  ii,  1873-'74,  pp.  107-119,  with  map  and 
section.     (Published  1874.) 

This  is  essentially  the  same  paper  as  the  last  referred  to. 

isre. 

Whittlesey  (Charles).  Physical  Geology  of  Lake  Superior.  Proc.  Am. 
Assoc.  Adv.  Sci.,  1875,  24th  Meeting,  part  2,  pp.  60-72.  (Published  Salem,  Massachu- 
setts, 1876.) 

This  paper  is  published  only  in  abstract.  It  gives  in  outline  some  of 
the  author's  views  as  to  lake  Superior  geology.  It  contains  only  a  few 
very  general  and  incidental  references  to  the  Penokee-Gogebic  district. 

Brooks  (T.  B.).  On  the  Youngest  Hurouian  Rocks  South  of  Lake  Superior, 
and  the  Age  of  the  Copper-Bearing  Series.  Am.  Jour.  Sci.,  3d  series,  vol.  xi,  1876, 
pp.  206-211. 

This  paper  maintains  that  the  large  development  of  granite  south  of 
the  Menominee  river  in  Wisconsin  is  the  youngest  member  of  the  Huronian 
or  Iron-bearing  series  of  that  region,  and  then  proceeds  to  draw  a  parallel 
between  this  Menominee  granite  area  and  certain  granites  that  lie  north  of 
the  Penokee  range  in  the  vicinity  of  Bad  river,  Wisconsin,  which  he  main- 
tains are  in  the  right  position  to  be  again  the  uppermost  member  of  the 
Iron-bearing  series  of  this  region.     We  quote  from  his  argument: 

A  careful  consideration  of  all  the  facts  to  be  observed  in  the  Menominee  region 
confirms  me  in  this  hypothesis,  which  is  further  supported,  as  it  seems  to  me,  by 
observations  iu  the  Penokie  Iron  region  (Bad  river),  Wisconsin, 


(iKOLUClCAli  EXl'LDIiATlONS  AND  LlTKltATUItE.      '  35 

Col.  Whittlesey's  maps  aiul  sections,  given  in  Owen's  rep(nt,  1S53,  represent  a 
belt  (tC  gi'iinite,  syenite,  and  liornlileiule  rocks  us  dividing  the  I'enokie  series  (Flnron- 
lan)  from  the  overlying  (Jopperbearing  amygdaloidal  traps  and  sandstones,  which  lie 
to  the  nortli  and  nearer  the  lako. 

I  observed  tiiesc  rocks  at  several  points  in  1871,  and  noted  their  general  litho. 
logical  resemblance  to  the  Lanrentian,  as  well  as  the  almost  insurmountable  structural 
ditliculties  in  assigning  to  them  tliat  age,  and  recorded  in  my  notes  the  probability  of 
their  being  Upper  Iluronian.  Kowlaud  Irving  mentions  these  rocks  as  being  coarsely 
crystalline  aggregates  "chiefly  of  labradorite  and  orthoclase  feldspar,  hornblende, 
and  some  variety  of  pyroxene,"  with  occasional  evidences  of  bedding,  which  points 
toward  their  entire  conformability  with  the  underlying  Huroniau.  He  regards  them 
as  of  the  period  of  the  (Jopper-bearing  series,  constituting  its  lowest  and  oldest 
portion. 

Having  been,  so  far  as  I  know,  but  little  studied,  it  is  perhaps  impossible  at  this 
time  to  determine  their  age;  but  wliat  is  known  can  here  be  briefly  surveyed,  and  an 
inference  drawn,  which  will  not  be  without  value  in  directing  further  investigations. 

1.  The  general  lithological  similarity  of  this  granitoid  belt  to  the  Laurentian 
has  been  remarked.  It  has  quite  as  much  similarity,  if  not  more,  to  several  members 
of  the  Huroniau,  and  is,  I  believe,  not  identical  with  any  rock  known  to  belong  to  the 
Copper  series. 

3.  Its  geographical  extension  is  peculiar  in  this,  it  wedges  out  rapidly  to  the 
east  from  the  vicinity  of  Penokie  gap,  entirely  disappearing  at  the  Montreal  river, 
which  divides  Michigan  and  Wisconsin.  Prof.  Pumpelly  and  myself  traced  the 
boundary  between  the  Copper  and  Huronian  rocks  30  miles  farther  eastward  beyond 
lake  Gogebic,  without  again  observing  it,  which  we  should  certainly  have  done  if  it 
had  existed  there,  for  we  often  found  the  two  series  very  near  together,  although  the 
actual  contact  was  not  seen. 

3.  Not  only  does  this-  granitoid  formation  thin  out  and  disappear  in  its  eastward 
prolongation,  but  the  same  is  true  of  the  whole  Huronian  series,  the  belt  of  which 
becomes  narrow  as  followed  east,  and  finally  disappears  in  the  neighborhood  of 
Gogebic,  where  the  Laurentian  is  seen  very  near  the  Copper  series. 

4.  The  fact  that  the  granite  mass  does  not  cross  either  the  Copper  or  Huronian 
series,  or,  so  far  as  observed,  give  off  dikes  in  either,  renders  it  improbable  that  it  came 
into  its  present  position  as  an  eruptive  mass  subsequent  to  the  formation  of  both 
series  of  rocks. 

5.  The  various  ores  of  iron,  which  are  so  generally  and  abundantly  diffused  in 
the  Lower  and  Middle  Huronian,  are  entirely  absent  so  far  as  observed  from  the 
upper  three  or  four  members  as  developed  in  the  Marquette  and  Menominee  regions, 
and  also  in  the  Penokie  series  if  the  following  hypothesis  is  true;  but  they  occur  in 
aU  forms,  although  it  is  believed  not  abundantly  in  the  uppermost  exposed  member 


36  THE   PENOKEE   IRON-BEARING   SERIES. 

on  Black  river.  If  we  suppose  this  irou  to  liave  been  mostly  precipitated  as  a  car- 
bonate, then  we  might  expect  it  would  be  more  generally  dift'iised  through  the  rocks 
of  certain  epochs  than  those  materials  derived  frc^m  the  erosion  of  adjacent  coasts. 

There  is  evidently  but  one  hypothesis  which  will  reconcile  these  facts,  which  is, 
that  the  granitoid  formation  in  question  is  of  the  Huronian  jieriod,  and  probably  the 
youngest  member,  which  series  are  here  noncoiiformably  overlaid  by  the  Copper-bear- 
ing rocks.  I  conceive  that  this  view  is  supported  by  the  observations  iu  the  Menom- 
inee region  above  recorded,  and  suppose  this  Penokie  granitoid  formation  may  be  the 
equivalent  of  granitic  bed  XX  of  the  Huronian  series  as  developed  in  that  region. 
On  this  hypothesis  it  is  possible  that  the  valley  dividing  the  Penokie  range  proper 
from  the  granitoid  belt  may  be  underlaid  by  a  soft  slate,  the  equivalent  of  the  micace- 
ous schist,  bed  XIX.     (Pp.  207-208.) 

We  are  not  in  accord  with  Brooks  as  to  the  views  here  expressed.  We 
can  not  allow  that  the  granite  of  the  Menominee  is  Huronian  at  all,  inasmuch 
as  we  think  it  plainly  belongs  to  the  older  or  gneissic  series;  nor  can  we 
agree  with  the  statements  of  Brooks  as  to  the  granites  north  of  the  Penokee 
range.  As  to  the  Menominee  granite,  we  do  not  now  need  to  present  any 
arguments  in  defense  of  our  belief.  As  to  the  Penokee  granites,  we  may 
merely  repeat  the  substance  of  what  one  of  us  has  already  said  iu  several 
publications,  viz,  (1)  that  the  belt  of  "granite,  syenite,  and  hornblende  rocks" 
outlined  by  Whittlesey  really  has  no  existence;  (2)  that  the  granites  occur- 
ring iu  the  vicinity  of  Bad  river,  and  north  of  the  Penokee  range,  lie  in  no 
continuous  belt,  but  are  veins  and  masses  intersecting  the  gabbros  at  the 
base  of  the  Copper-bearing  series,  and  the  micaceous  quartzites  at  the  sum- 
mit of  the  Iron-bearing  series;  (3)  that  these  granites  are  manifestly  but  a 
phase  of  the  intrusive  reddish,  granitic  porphyries  which  mark  this  horizon 
both  north  and  south  of  lake  Superior;  (4)  that  being  so  manifestly  intru- 
sive, there  is  nothing  like  bedding  about  these  granites;  (5)  that  the 
"wedging  out"  to  the  eastward,  which  Brooks  speaks  of  as  characteristic  of 
his  supposed  granite  belt,  is  really  a  characteristic  of  a  great  gabbro  area 
here  occurring,  and  not  of  a  granite  area;  (6)  that  this  gabbro  mass  does 
traverse  the  Huronian  beds  in  a  most  noticeable  manner,  in  places  even 
cutting  them  out  at  the  surface  altogether;  and  (7)  that  at  the  southern 
margin  of  this  gabbro,  or  along  its  contact  with  the  Iron-bearing  slates,  it 
is  plainly  involved  with  and  intrusive  in  those  slates,  as  the  granite  is  in 
both  gabbro  and  slates. 


(;E0L0(1I(L\L  EXrLORATIOXS  AND  LITERATURE.  37 

In  short,  we  seo  n()tliin<>-  hero,  nor  indood  anywhere  in  tlie  lake 
Superior  country  to  warrant  the  view  tli;it  the  lron-hearin<<-  or  Huroniau 
series  has  an  npperinost  granitic  member.  Tlie  Menominee  granite  belt  we 
look  on  as  belong-iug  to  the  great  basement  gneissic  formation;  the  Penokee 
granite  belt  is  really  a  great  gabbro  area,  in  which  occur  isolated  and  limited 
intiiisions  of  granite,  granitic  por[)hyry,  and  allied  acid  rocks.  This 
gabbro,  with  red  acid  eruptions,  finds  its  exact  equivalent  as  to  stratigraph- 
ical  position  and  associated  intrusions  in  tlie  great  gabbro  belt  which  forms 
the  bold  range  of  hills  at  Duluth,  and  stretches  thence  far  to  the  northeast- 
ward into  the  interior  of  that  portion  of  Minnesota  which  lies  north  of  lake 
Superior.^ 

Brooks  (T.  B.).  Classified  List  of  Rocks  observed  in  the  Huronian  Series 
south  of  Lake  Superior.     Aui.  Jour.  Sci.,  3d  series,  a^oI.  xii,  187G,  pp.  194-204. 

This  paper,  as  its  title  indicates,  is  an  attempt  at  a  lithological  classifi- 
cation of  all  the  varieties  of  rocks  observed  by  Brooks  in  those  formations 
on  the  south  side  of  lake  Superior,  which  he  regarded  as  the  equivalents 
of  the  original  Huronian  of  the  north  shore  of  lake  Huron.  It  also  o'ives 
a  table  showing  the  "sequence  of  Huronian  strata  at  several  points  near 
lake  Superior,  with  hypothesis  of  equivalency."  The  districts  referred  to 
in  this  table  are  (1)  the  north  shore  of  lake  Huron,  (2)  the  Marquette  Iron 
district,  (3)  the  Menominee  Iron  district,  (4)  the  district  of  Black  river, 
Michigan,  and  (5)  the  district  of  Bad  river,  Wisconsin.  The  last  two  dis- 
trict are  within  the  area  now  under  description,  and  we  therefore  quote  the 
tah\e  so  far  as  it  gives  the  succession  of  strata  for  Black  and  Bad  rivers. 
We  do  not  now  discuss  Brooks's  hypothesis  of  equivalency  for  these 
sections  with  those  of  the  other  districts  referred  to.  We  need  merely  to 
say  that  we  can  not  accept  his  scheme  in  many  respects.  Indeed,  we  find 
too  many  things  that  we  can  not  agree  with  in  his  succession  for  the 
Penokee-Gogebic  series  itself  to  allow  us  to  accept  any  scheme  of  equiva- 
lency based  upon  it.  His  Bad  river  section  is  particularly  imperfect,  there 
being  great  thicknesses  of  rock  omitted  entirely,   while  the  granite  belt 

■Am.  Jour.  Sci.,  3d  series,  vol.  xi,  187G,  p.  493;  Geol.  of  Wis.,  vol.  in,  1880,  pp.  10,  13,  22,  3.5, 
44-46,  145-149,  167-183,  193-195,  233-237;  and  atlas  plates  xxi,  xxii,  xxlv,  xxv,  xxvi,  xxvii;  Monograph 
U.  S.  Geol.  Survey,  vol.  v,  1883,  pp.  37-57,  56-58,  144-145,  155-157,  158,  230-238,  266,  268-275. 


38 


THE  PENOKEE  lEON-BEAEING  SEEIES. 


placed  at  the  top  of  the  series  is  really  as  we  have  already  insisted,  but 
part  of  the  great  development  of  gabbro  which  lies  at  the  base  of  the 
Copper-bearing  series.  The  facts  given  in  this  tabulation  for  the  Black 
and  Bad  river  sections  are  based  on  the  observations  made  by  Brooks  on 
his  rapid  trip  through  the  region  in  1871,  above  referred  to.  In  a  later 
publication  he  accepted  the  Bad  river  section  given  by  R.  D.  Irving.^ 
The  following  is  the  portion  of  Brooks's  tabulation  referred  to- 


Black  river  series. 


Bad  river  series. 


Compact  greenstone,  with  green  cherty(?)  layers. 
Bright  red  specks  of  jasper  and  crystals  of 
pyrite. 

Greenstone ;  holds  grains  of  glassy  quartz  and 
appears  chloritic. 

Hematitic  and  magnetic  quarizt'se  flag  (like  Mar- 
quette flag  ores). 

Gray,  green,  and  brown  banded  ferruginous,  silice- 
ous stale,  with  strong  rhombohedral  cleavage. 

Grayish  and  greenish  handed  schist,  weathering 
brown,  apparently  cfeZoHtic,  -with,  jaspery  layers 
Contains  pyrites.  In  places  apparently  felsitic 
and  again  aphanitic. 

[Covered.] 

Ferruginous,    handed    (purple   and    green)   cherty 

so/list  (magnetic). 
Banded  ferruginous  ^'as^er  schist. 
Ferruginous,  siliceous  flags  (not  magnetic). 
Arenaceous  qiiartzose  schist. 
Eeddisli  quartsite. 

Hydrous  magnesian  or  argillaceous  schist. 
Greenstone. 

Banded  cherty  schist  and  schistose  cherty  hreccia, 

more  or  less  ferruginous.     84,  85. 
Compact,  hard  greenstone. 
Very  soft,  apparently  chloritic  greenstone. 
Gray,  banded,  slaty  schist. 

[Covered  100  steps.] 

Greenstone. 

^anAeA  ferruginous,  siliceous  schist,  strongly  mag- 
netic. 
Greenstone. 
Banded  ferruginous  slate. 

Massive  greenstone,  apparently  chloritic. 
Banded  ferruginous _/asi)er  schist. 


Red,  gray,  coarse  and  flue  grained  granitic  rock, 
rarely  schistose. 

Gi-eenstone  or  hornblende  rock,  apparently  chlo- 
ritic (somewhat  soft,  but  tough). 
[Covered  about  i  mile.] 


Clay  slate? 


Magnetic  amphiholic  quartzose  flags  and  quartzose 
magnetic  ore.  148.  Heavy  bed  forming  crest 
of  ridge. 

Black  clay  slate  without  oblique  cleavage. 


Gray  quartz  schist,  banded  with  occasional  1am- 
inse  of  magnetic  ore. 


Grayish  and  reddish  quartzose  schist. 
Gi-eenstone  (thin  bed). 

Soft,    light,    gray-green  slate,    probably  chloro- 
areillaceous. 


Gray  quartzose  schist,  faintly  banded. 

Gray  to  white,  massive  quartzite,     (Thin  bed.) 
Calcareous  rock? 


Amphibole  schist  and  perhaps  hornblendic  gneiss 
in  heavy  beds. 

[Covered.] 


1  Geol.  of  Wis.,  1880,  vol.  iii,  table  opp.  p.  450. 


(IKOT.OdlOAL  EXPLORATIONS  AND  LITI-niATUKH.  ,S9 

Sweet  (E.  T.).  Notes  on  tlio  Goology  of  Noitliern  Wisconsin.  Trans.  Wis. 
Acad,  of  Sci.,  Arts  and  Letters,  vol.  in,  lS7.^»-'70,  pj).  4()-.'J.'>. 

This  is  an  outline  account  of"  observations  in  northern  Wisconsin  made 
in  1873  and  187.^)  for  tlie  Wisconsin  State  Geological  Survey.  The  quota- 
tion below  gives  certain  facts  with  regard  to  tlie  Bad  river  and  Penokee 
gap  section.  It  should  be  said  that  I  afterwards  measured  this  section  in 
detail  for  the  Wisconsin  Survey  and  the  results  were  published  in  the  third 
volume  of  the  Geology  of  Wisconsin.  The  thickness  of  the  Iron-bearing" 
series  is  more  than  twice  as  great  as  supposed  by  Mr.  Sweet,  the  uppermost 
beds  being  far  above  the  uppermost  mentioned  by  him.  Two  magnetic 
belts,  moreover,  do  not  exist,  as  supposed  by  Mr.  Sweet,  the  supposed 
two  belts  being  the  same  belt  faulted  apart. 

The  junction  between  the  Laiu-entian  and  Huronian  is  in  the  southern  part  of 
Sec.  14,  T.  44,  R.  3  W.  At  this  point  Bad  river  passes  through  a  narrow  gorge 
having  nearly  vertical  walls  on  either  side.  In  the  left  or  northern  wall  of  the  gorge, 
fine  grained  white  quartz  with  a  vitreous  coating  and  slaty  siliceous  schist  occur, 
showing  a  strike  nearly  east  and  west,  and  dip  of  60°  to  the  north.  The  quartz 
represents  the  lowest  member  of  the  Penokie  system  examined  by  the  party  in  1873. 
Upon  examining  the  opposite  wall  of  the  gorge  siliceous  marble  was  discovered  for 
the  first  time  to  be  one  of  the  beds  of  the  Penokie  system,  lying  below  the  iron-bear- 
ing beds.  A  similar  arrangement  has  long  been  known  to  exist  in  the  Huronian  of 
the  Marquette  district,  which  has  led  to  the  suspicion  of  its  existence  in  Wiscon- 
sin. The  thickness  of  the  siliceous  marble  is  about  50  feet.  It  is  usually  fine 
grained  and  grayish  in  color.  Small  crystals  of  calcite  and  dolomite,  however,  can  be 
observed  irregularly  disseminated.  An  analysis  of  a  specimen  taken  from  the  ledge 
afforded  me  the  following  result : 

Per  cent. 

Carbonate  of  lime 50-52 

Carbonate  of  magnesia 33"41 

Insoluble  matter , -  -  13-85 

Oxide  of  iron 1'70 

Undetermined "52 

Total 10000 

The  analysis  shows  that  the  prcJper  name  for  the  rock  is  siliceous  dolomitic  mar- 
ble. In  the  Marquette  region  the  Morgan  furnace  limestone,  but  very  little  purer  than 
this,  has  been  extensively  used  as  a  flux.  One  hundred  feet  southeast  from  the  expo- 
sure of  siliceous  marble  there  is  a  large  ledge  of  gneissoid  granite  showing  a  weU 


40  THE  PENOKEE  lEON-BEARING  SERIES. 

defined  dip  of  77°  to  the  south,  and  strike  of  north,  75°  west.  In  following  the  strike 
west,  one  passes  within  25  feet  of  the  outcrop  of  siliceous  marble  which  has  a  northerly 
dip.  Between  100  and  200  feet  south,  on  the  line  of  the  railroad,  other  large  expo- 
sures of  gneissoid  granite  are  found  having  essentially  the  same  bedding  as  that  men- 
tioned above.  When  the  railroad  cut  is  completed  at  this  locality  the  absolute  June 
tion  of  the  Laurentian  and  overlying  Huronian  will  doubtless  be  exposed.  There  can 
be  no  doubt  of  the  unconformability  of  these  formations,  approaching  each  other  as 
they  do  with  a  persistent  opposite  dip  and  somewhat  different  strike.  Unconforma- 
bility has  been  shown  to  exist  between  the  Laurentian  and  Huronian  in  Michigan, 
but  this  is  the  first  time  that  it  has  been  proved  in  Wisconsin.  Northward  from  the 
granites  the  section  has  been  completed  for  over  1,600  feet.  In  this  space  are  included 
two  "  magnetic  ore  "  beds,  the  southern  130  and  the  northern  over  500  feet  thick. 
Directly  above  or  north  fi'om  the  northern  "  ore  "  bed  there  is  a  space  of  1,400  feet  upon 
which  exposures  have  not  been  found.  Above  this  blank  recent  railroad  excavations 
enabled  Mr.  Wright  and  myself  to  subdivide  and  extend  the  belt  of  400  feet,  supposed 
to  be  the  uppermost  member  of  the  Penokie  system,  into:  a,  siliceous  schists,  100 
feet;  6,  blank  (Bad  river),  75  feet;  c,  contorted  black  slate,  250  feetj  d,  diorites,  75 
feet;  and  e,  black  porphyritic  slates,  50  feet. 

Owing  to  the  heavy  deposits  of  drift  we  were  unable  to  find  exposures  for  1,300 
feet  north  from  the  black  porphyritic  slates. 

We  then  found  what  are  probably  the  latest  beds  of  the  Huronian  formation:  g, 
black  slate,  40  feet;  h,  quartzite,  about  250  feet;  i,  slaty  amygdaloid,  75  feet. 

The  thickness  of  the  formation  I  estimate  at  something  over  5,000  feet.  The 
dip  is  about  66°  to  the  north,  showing  entire  conformability  throughout.     (Pp.  42-44.) 

1877. 

iRViNGr  (E.  D.).  Report  of  Prof.  Irving.  In  Annual  Report  of  Progress  and  Re- 
sults of  the  Wisconsin  Geological  Survey  for  the  year  1876,  by  T.  C.  Chamberlin, 
pp.  13-18. 

This  report  contains  a  brief  account  of  the  progress  of  the  work  under 

R.  D.  Irving  in  northern  Wisconsin,  but  nothing  that  is  not  much  more 

fully  developed  in  the  third  volume  of  the  Geology  of  Wisconsin,  published 

in  1880. 

Wright  (C.  E.).  Mr,  Wright's  report.  In  same  publication. as  the  preceding, 
pp.  18-23. 

Contains  a  brief  preliminary  statement  of  results  obtained  in  the 
Penokee  region  in  1876.  The  same  results  are  given  more  fully  by  Mr. 
Wright  in  the  third  volume  of  the  Geolog}^  of  Wisconsin.  We  may  mei-ely 
quote  the  following  generalization  : 


(IKOUXilCAL  EXT'LOHATIONS  AND  LITERATURE.  41 

It  has  been  my  constant  aim,  and  still  ia,  to  correlate  the  Penokee  series  of  rocks 
with  those  of  Micliifian,  and  there  exists  in  my  own  mind  no  reasonahle  doubt  that 
tlie  rock  Ibrniations  of  thesi;  two  districts  are  the,  equivalents  of  each  other.  In  the 
Peuokee  we  have  the  limestone  and  quartzite  members;  tlie  belts  of  maf,nietic  schist 
interlamiuated  with  the  greenstones;  also  the  black  slates  and  mica-schists,  all 
o(!cupying  relatively  the  same  stratigraphical  position  as  in  the  Michigan  series. 
(Pp.  22-23.) 

Irving  (E.  D.).  Note  on  the  Age  of  the  Crystalline  Eocks  of  Wisconsin.  Am. 
Jour.  Sci.,  3d  series,  vol.  xiii,  1877,  pp.  307-309. 

The  object  of  this  note  is  to  oppose  Bradley's  view,  then  recently 
expressed,  and  indicated  also  on  his  geological  map  of  the  United  States, 
that  the  crystalline  rocks  of  Wisconsin  and  Michigan  may  be  altered  Lower 
Silurian.  It  is  a  general  outline  statement  of  the  succession  of  pre-Potsdam 
formations  in  northern  Wisconsin,  as  the  following  quotation  will  indicate: 

The  crystalline  rocks  of  Wisconsin  include  unquestionably  two  distinct  terranes, 
the  one  Ij-iug  unconformably  upon  the  other,  as  is  beautifully  shown  at  Penokee  gap, 
on  Bad  river,  in  the  lake  Superior  coimtry.  Here  a  white  siliceous  marble  of  the 
Huroniau,  overlain  by  hundreds  effect  of  distinctly  bedded  slaty  rocks,  and  dipping 
northward,  is  to  be  seen  within  20  feet  of  large  ledges  of  dark  colored  amphibolic 
gneiss,  whose  bedding  planes  dip  southward  and  strike  in  a  direction  diagonally 
across  that  of  the  more  northern  beds.  Tliere  are  no  doubt  instances  where  the  two 
series  are  difficult  to  separate,  similar  rocks  occurring  in  both  groups,  but  the  exist- 
ence of  the  two  is  incontestable,  and  their  uuconformability  with  the  unaltered 
Potsdam  equally  so.  The  facts  proved  thus  far  with  regard  to  the  older  rock  series 
of  Wisconsin  may  be  briefly  summarized  as  follows:  The  oldest  (I)  are  gneisses  and 
granites  with  other  rocks;  these  are  overlaid  unconformably  by  (II)  a  series  of  quartz- 
ites,  schists,  diorites,  etc.,  with  some  gneiss  and  granite;  these  in  turn  are  overlaid — 
probably  also  unconformably,  but  this  is  not  certainly  proven — by  (III)  the  Copper 
series,  which  includes  greenstones  and  melaphyres,  and  also  great  thicknesses  of  inter- 
stratified  sandstone,  melaphyres,  amygdaloids,  and  shales,  the  whole  having  a  thick- 
ness of  several  milps;  these  finally  are  unconformably  covered  by  (IV)  a  series  of 
unaltered  horizontal  sandstones  including  numerous  fossils,  many  of  which  are  closely 
allied  to  those  of  the  Potsdam  sandstone  of  New  York,  and  all  of  which  have  a  marked 
Primordial  aspect.  I  and  II  are  referred  to  the  Laurentian  aiid  Huroniau  systems  of 
Canada,  because  they  bear  the  same  relations  to  one  another  and  to  the  Copper  series 
that  these  systems  do.     (P.  308.) 

XS78.      . 

Irving  (E.  D.).  Eeport  to  T.  C.  Chamberlin,  State  Geologist,  of  work  done  in 
the  Penokee  region  in  1877,  dated  December  24,  1877.     In  Annual  Report  of  the 


42  THE  PENOKEE  IRON-BEAEING  SERIES. 

Wisconsin  Geological  Survey  for  tlie  year  1877,  by  T.  C.  Chamberlin,  Chief  Geologist, 
pp.  17-25.    Madison,  "Wisconsin,  1878. 

The  principal  work  of  the  season  of  1877  included  (1)  the  extension 
of  a  detailed  geological  section  begun  in  the  vicinity  of  Penokee  gap  during 
the  previous  year,  northward  5^  miles  to  the  railroad  track  near  the  cross- 
ing of  Silver  Creek,  Sec.  10,  T.  45,  R.  3  W.;  (2)  the  making  of  a  similar 
section  along  Bad  river,  somewhat  farther  east ;  and  (3)  a  detailed  map- 
ping and  magnetic  survey  of  the  Penokee  range  from  Bad  river  to 
Potato  river,  T.  45  N.,  R.  2  E.,  Wisconsin. 

The  plan  adopted  for  this  work  was  to  cross  the  iron  belt,  which,  although  quite 
sinuous  in  its  course,  preserves  still  a  general  east  and  west  direction,  curving  more 
and  more  toward  the  north  as  it  is  followed  eastward — fi-om  north  to  south  at  distances 
of  about  half  a  mile,  using  the  section  lines  as  much  as  possible.  Ou  each  of  the 
crossing  lines  stations  were  established  at  every  hundred  steps,  and  at  every  station 
the  aneroid  barometer,  the  variation  of  the  magnetic  needle,  and  the  time  were  care- 
fully observed,  a  simultaneous  series  of  barometrical  observations  being  carried  ou  at 
Ashland.  The  lines  were  begun  at  points  far  enough  to  the  south,  on  the  Laureutian 
rocks,  to  be  out  of  the  influence  of  the  irou  or  magnetic  belt  of  the  Huronian,  and 
were  extended  northward  far  enough,  not  only  to  be  out  of  the  influence  of  this  belt 
in  that  direction,  but  also  to  determine  the  presence  or  absence  of  any  other  similar 
belt  or  belts.  Some  of  the  lines,  moreover,  were  extended  farther  than  the  rest,  so  as 
to  pass  on  to  the  next  series  of  rocks,  allusion  to  which  has  been  made  above.  Other 
subordinate  lines  of  observation  were  frequently  run  across  the  sections  in  an  east  and 
west  direction,  and  all  the  lines  were  controlled  by  constant  reference  to  section 
corners  and  quarter  posts.  All  outcrops  were  of  course  examined  and  located,  and 
specimens  were  taken  for  subsequent  study,  particular  attention  being  given  to  the 
magnetic  belt  traversing  the  center  of  the  Penokee  range.  The  largest  outcrops 
are  found  where  the  several  branches  of  Bad  river  break  through  the  range  from  the 
southward.  At  each  one  of  these  gorges  the  work  was  carried  into  greater  detail,  in 
order  that  the  true  succession  of  the  various  layers  might  be  made  out. 

Many  interesting  facts  were  developed  during  this  detailed  work,  one  or  two  of 
which  may  be  mentioned  here.  The  exact  junction  of  the  Huronian  and  Laurentian 
series  was  found  at  the  gorge  of  Potato  river,  where  a  cliff- side  over  100  feet  in 
height  and  over  half  a  mile  in  length  is  traversed  near  the  middle  by  the  highly 
inclined  contact  line  between  the  "siliceous  slate,"  one  of  the  lower  members  of  the 
Huronian,  and  a  greenish  chloritic  gneiss  of  the  Laurentian.  The  siliceous  slate 
inclines  at  a  high  angle  to  the  north,  whilst  the  gneiss  layers  dip  to  the  south  and 
strike  in  a  direction  oblique  to  that  of  the  slate  layers.  It  is  worthy  of  note  that  the 
two  lowermost  layers  of  the  Huronian,  as  seen  at  Penokee  gap  and  for  many  miles  to 


(IIOOLOCICAL  KXI'LOUATIONS  AND   IJTKK ATIJIIK.  43 

the  eaistwaid,  the  ••  white  (|uait/."  and  ''siliceous  doloinitit'  inaii)]«',"  iire  here  ciitiiely 
alvsciit;  Imt  this  fact  is  ([uite  in  accord  witli  the  rehitioiis  everywhere  to  be  observed 
between  these  two  widely  distinct  rock  series.  Another  t'ac^t  of  iinportanc^e  is  the 
steady  lessening;  of  the  disturbing  iniluenee  exerted  on  the  magnetic  needle  by  the 
iron  belt  of  the  Iluroniaii,  as  it  is  followed  eastward.  In  its  more  western  extension 
the  variations  observed  on  and  near  this  belt  are  eonimonly  as  much  as  OQo  to  IStP, 
tlu^  disturbing  influence  extending,  moreover,  for  a  long  distance  to  the  north  and 
south  of  the  line  of  greatest  disturbance.  By  the  time  the  Potato  river  is  reached 
the  variations  never  approach  90o,  and  are  to  be  observed  along  a  very  narrow  belt 
only.  Still  farther  east  the  attraction  lessens  yet  more  rapidly,  and  on  the  Montreal 
river  you  have  yourself  observed  that  it  is  essentially  lost.  This  lessening  in  mag- 
netic attraction  does  not  necessarily  indicate  a  corresponding  decrease  in  the  amount 
of  iron  present  in  the  rocks  of  the  iron  belt,  but  rather  that  the  magnetic  oxide  is 
giving  way  more  completely  to  the  nonmagnetic,  or  sesquioxide,  which  is  always 
present,  in  greater  or  less  quantity,  even  Avhere  the  magnetic  attractions  are  greatest. 
The  outcrops  observed  bear  out  this  conclusion ;  for  a  considerable  quantity  of  very 
highly  manganiferous  red  hematite  is  to  be  seen  at  points  all  along  from  the  passage 
of  Tyler's  fork  eastward. 

Yet  another  point  of  interest  brought  oiit  by  this  year's  work  is  the  apparent 
demonstration  of  the  nonexistence  of  otlier  magnetic  belts  in  tlie  more  northern  or 
upper  portions  of  the  Huronian  series.  Hematite,  or  specular  ores,  may  exist  here, 
but  the  gaps  in  the  series  of  layers  have  now  been  so  largely  filled  up  that  it  appears 
probable  that  any  discoveries  of  ore  which  may  be  made  in  the  future  will  1)6  on  the 
already  known  magnetic  belt.     (Pp.  19-21.) 

Chamberlin  (T.  C).  Annual  Eeport  of  the  Wisconsin  State  Geological  Sur- 
vey for  the  year  1877.    Madison,  1878. 

• 

Pages  25  to  28  of  this  report  include  a  brief  account  of  an  examina- 
tion made  by  Prof.  T.  C  Chainberlin  of  tliat  pai't  of  the  Penokee  range 
which  lies  between  the  Potato  and  Montreal  rivers.  The  following,  as  to 
the  contact  of  the  basal  member  of  the  Iron-bearing  series  with  the  scliists 
south  of  it,  is  of  especial  interest: 

At  the  falls  of  the  Gogogashngun  a  most  interesting  section  may  be  made  out. 
The  falls  themselves  are  due  to  the  barrier  imposed  by  the  siliceous  schists  that  here 
form  the  lowest  exposed  member  of  the  Huronian  series.  By  going  back  from  the 
falls  a  short  distance,  guided  by  the  indications  of  the  loose  blocks  of  rock  on  the 
surface,  the  party  were  fortunate  enough  to  uncover,  at  their  first  attempt,  the  exact 
junction  between  the  Latirentian  and  Huronian  series.  The  Laurentian  membei'  con- 
sists of  a  peculiar  gneissoid  rock,  altogether  like  that  which  occupies  a  similar 


44  THE  PENOKEE  IRON-BEARING  SERIES. 

relation  at  Penokee  gap.  Its  strike  is  N.  07°  W.,  and  its  dip  49°  NE.  Tlie  Huroiiian' 
rock  lies  in  absolute  contact  with  this,  not  even  being  separated  by  a  fissure.  Indeed, 
at  one  poiut  the  siliceous  material  that  formed  the  Huronian  rock  had,  at  the  time  of 
its  deposition,  so  insinuated  itself  into  the  irregularities  of  the  surface  of  the  gneiss 
that  the  two  formations  are  interlocked,  and  n  hand  specimen  Avas  obtained,  one  portion 
of  which  is  Laurentian  gneiss  and  the  other  Huronian  schist,  the  two  being,  of  course, 
unconformable.  It  is  doubtful  whether  a  similar  specimen  has  ever  previously  been 
secured. 

The  base  of  the  Huronian  series  as  here  exposed  is  formed  by  gray  and  purple 
siliceous  schists,  interleaved  with  which  are  occasional  purplish  layers  of  a  clay-like 
texture.  Some  of  these  approach  a  pipestone  and  raise  the  question — which,  of 
course,  they  are  not  competent  to  answer — whether  they  are  not  the  approximate 
equivalents  of  the  pipestones  of  Barron  county,  which  sustain  a  somewhat  similar 
relation. 

The  general  strike  of  these  schists  is  N.  55°  E.,  and  their  average  di^J  about 
60°  NW.  By  comparison  with  the  Laurentian  strata  it  will  ]be  seen  that  the  two 
formations  strike  across  eacli  other  at  a  large  angle  and  dip  in  opposite  directions. 
(Pp.  20-27). 

Hunt  (T.  S.).  Special  Report  on  the  Trap  Dikes  and  Azoic  Rocks  of  South- 
eastern Pennsylvania.  Part  i,  Historical  Introduction,  Second  Geological  Survey  of 
Pennsylvania,  volume  E.     Harrisburg,  1878. 

Contains  in  a  general  historical  review  several'brief  references  to  cer- 
tain of  the  accounts  of  the  Penokee-Gogebic  district  previously  published 
and  above  noted.     Dr.  Hunt  had  not  himself  been  in  the  district. 

•  1879. 

Chambeelin  (T.  C).  Annual  Report  of  the  Wisconsin  Geological  Survey  for 
the  year  1878.    .Madison,  1879,  pp.  5-7. 

Contains  a  l^rief  account  of  the  woi'k  then  still  in  progress  under  R. 
D.  Irving  in  northern  Wisconsin,  and  particularly  a  statement  of  the  route 
followed  by  Mr.  A.  D.  Conover  in  making  certain  additional  exjilorations. 

Irving  (R.  D.).  Note  on  the  Stratigraphy  of  the  Huronian  Series  of  Northern 
Wisconsin ;  and  on  the  Equivalency  of  the  Huronian  of  the  Marquette  and  Penokee 
Districts.     Am.  Jour.  Sci.,  3d  series,  vol.  XVii,  1879,  pp.  393-398. 

This  article  calls  into  question  the  scheme  of  stratigraphy  for  the 
Penokee  district,  above  quoted  from  Brooks,  and  gives  also  a  preliminary 


(iliOLOdlCAL   KXI'LOltATlONa  A^'l)  LlTEUATintE.  45 

statement  of  certiuu  ix^sults,  further  noted  below,   in   connection  with  tlie 
final  report  ot"  the  Wisconsin  Survey. 

1880. 

Irving  (R.  D.).  The  (k-ologiciil  Structure  of  Northern  Wiscousiii.  In  the 
Geolosy  of  Wiscousiii,  vol.  iii,  pt.  i,  pp.  1-25,  with  outline  luap  auil  plate  of 
soctiouis.     Madison,  ISSO. 

As  the  title  indicates,  this  portion  of  the  final  report  of  the  Wisconsin 
Survey  gives  a  general  summary  of  the  conclusions  reached  as  to  northern 
Wisconsin  by  the  various  members  of  the  survey  corps  who  had  worked 
in  this  region,  including  the  author.  The  following  quotation  shows  the 
more  general  conclusions  reached  as  to  the  two  formations,  gneissic  and 
slaty,  with  which  we  are  particularly  concerned  in  the  present  volume: 

In  a  former  volume  of  this  report,  I  have  shown  how  the  Silurian  limestone  anil 
sandstone  formations  of  the  southern,  eastern,  and  western  portions  of  Wisconsin 
curve  concentrically  around  three  sides  of  the  Laurentian  nucleus  of  the  northern 
part  of  the  state.  On  the  northern  or  lake  Superior  side,  however,  we  find  an  alto- 
gether different  structure;  and  it  is  evident  at  once  that  the  Laurentian  nucleus  has 
constituted  a  barrier  between  the  lake  Superior  and  Mississippi  valley  regions  since 
pre- Silurian  times. 

Laurentian  system. — The  rocks  of  the  crystalline  nucleus  itself  are  referred  to 
the  Laurentian  of  Canada,  because  (1)  they  sustain  precisely  the  same  structural  rela- 
tions to  the  Huronian,  Keweenawan,  and  Lower  Silurian,  as  observed  in  the  case  of 
the  typical  Laurentian  of  Canada,  and  (2)  because  they  have  the  same  general  litho- 
logical  peculiarities  that  characterize  the  Canada  series.  There  can,  indeed,  be  no 
reasonable  doubt  that  they  are  directly  continuous  with  the  Canada  Laurentian. 
They  extend  to  the  shores  of  lake  Superior  in  the  vicinity  of  Marquette,  Michigan, 
and  appear  again  on  the  eastern  or  Canada  shore  of  the  lake.  The  separation  be- 
tween the  Wisconsin  Laurentian  and  that  of  Canada  is  therefore  only  a  superficial  one, 
the  connection  being  concealed  by  the  waters  of  the  lake,  and  by  the  overlying  Silu- 
rian depositions  in  the  eastern  extension  of  the  upper  peninsula  of  Michigan. 

In  Wisconsin,  the  northern  limit  of  the  Laurentian  approaches  most  nearly  to 
lake  Superior  on  the  Montreal  river,  which  is  here  the  state  boundarv — the  distance  to 
the  lake  shore  in  a  direct  line  being  only  13  miles.  From  the  Montreal  river  the  north- 
ern limit  trends  about  southwest  by  west,  and  on  Bad  river,  25  miles  farther  west,  it  is 
25  miles  from  the  lake.  From  Bad  river  the  course  is  in  general  but  little  south  of 
west,  to  Numakagon  lake,  T.  44,  E..  6  W.  Here  a  rapid  change  to  a  more  southerly 
direction  comes  in,  and  we  find  ourselves  following  the  western  side  of  the  Laurentian 
nucleus,  soon  to  be  bounded  by  the  regular  Mississippi  valley  formations.    The  south- 


46  THE  PENOKEE  IRON-BEARING  SERIES. 

ern  rim  of  the  lake  Sui^erior  trough,  at  an  elevation  of  1,000  to  1,100  feet  above  the 
lake,  lies  but  a  few  miles  soiith  of  the  northern  boundary  of  the  Laureutiau  area,  for 
about  50  miles  westward  from  the  Montreal,  after  which  it  passes  on  to  the  more  north- 
erly and  newer  formations. 

The  rocks  of  the  Laurentian  nucleus  have  already  been  partly  described  in  former 
reports.  Where  they  approach  lake  Superior  they  are  almost  wholly  gneiss  and  gran- 
ite. The  prevailing  rock  along  the  northern  border  is  a  dark  gray  to  black,  often 
greenish-black,  hornblende-gneiss,  in  which  the  hornblende  has  usually  been  more  or. 
less  completely  altered  to  chlorite.  This  alteration,  when  carried  to  any  considerable 
extent,  gives  a  greenish  tinge  and  greasy  feeling  to  the  rock,  and,  in  cases  of  extreme 
alteration,  there  is  a  passage  to  a  green  chloritic- schist.  The  associated  granites  are 
usually  light  pinkish- tinted  to  gray,  and  highly  quartzose,  a  frequent  gneissoid  ten- 
dency showing  their  sedimentary  nature.  These  I'ocks  have  a  nearly  due  E.-W. 
strike,  and,  near  the  northern  border,  a  high  southerly  dip.  They  are,  however, 
beyond  question  greatly  folded,  and  have  as  certainly  an  enormous  thickness. 

Muronian  system. — Lying  immediately  against  the  Laurentian,  and  very  sharply 
defined  from  it,  we  find,  extending  from  the  Montreal  river  westward  for  50  miles  to 
lake  Numakagon,  a  belt  of  schistose  rocks  which  we  refer  unhesitatingly  to  the  Can- 
ada Huronian,  and  Avhich  are  beyond  question  the  direct  westward  extension  of  the 
iron-bearing  series  of  the  upper  peninsula  of  Michigan.  This  belt  has  a  width,  in 
general,  of  from  1^  to  2J  miles,  and  includes  an  aggregate  thickness  of  strata  of  nearly 
13,000  feet,  with  a  number  of  well  marked  subdivisions,  several  of  which  are  persistent 
throughout  the  entire  length  of  the  belt.  These  subdivisions  may  be  briefly  summa- 
rized as  foUows,  beginning  below :  (1)  Crystallinetremolitic  limestone,  at  times  overlain 
by  a  band  of  white  arenaceous  quartzite,  and  at  times  absent,  the  next  formation  above 
them  coming  into  contact  with  the  Laiu*entian,  130  feet;  (2)  straw-colored  to  greenish 
quartz-schist,  and  argillitic  mica-schist,  often  novaculite,  410  feet;  (3)  tremolitic  mag- 
netite-scliists,  maguetitic  and  specular  quartzites,  lean  magnetic  and  specular  ores- 
forming  the  "  Penokee  Iron  range,"  ISO  feet;  (4=)  alternations  of  black  mica  slates  with 
diorite  and  schistose  quartzites,  and  unfilled  gaps,  3,495  feet;  (5)  medium-grained  to 
aphanitic,  dark  gray  mica-schists,  with  coarse  intrusive  granite,  7,985  feet — in  all, 
12,800  feet.  These  rocks  all  dip  to  the  northward,  the  angle  being  usually  high, 
but  lessening  toward  the  west,  and  trend  with  the  course  of  the  belt,  which  has 
numerous  minor  corrugations,  while  preserving  one  general  direction.  The  strike  direc- 
tions are  always  oblique  to  the  trends  of  the  underlying  Laurentian  gneiss,  proving 
the  unconformability  of  the  two  systems,  the  actual  contact  of  which  may  indeed  be 
seen  in  several  places. 

Westward  from  lake  Numakagou  the  Huronian  belt  is  lost  sight  of,  the  Lauren- 
tian gneiss  and  Keweeuawan  gabbro  and  diabase  coming  apparently  into  direct  con- 
tact wiifch  each  other. 


GKOLOGIOAIi  EXPLORATIONS  AND  LITKJiATURE.  47 

Tlif  rcffiriKr of  tlic  scliistosi'  scries  of  the  I'eiioUee  region  to  tlie  Iluroiiiaii  is 
jiustitied  by  tlie  tollowing  coiisideratioiis:  (1)  Tlicre  iippciirs  to  bo  a  direct  eoiitiiuuitiou 
with  the  irou-boaring  system  of  the  Maniuotto  region  of  Michigan;  ('2)  as  shown  on  a 
subseciuent  page,  tlie  grand  snbdivisions  of  tlio  Bad  river  and  Marcinetto  systems 
both  show  the  same,  relation  to  the  Laurentian  and  Kewcciiawau  systems  as  fonud  in 
the  Iluronian  of  Canada;  i.  e.,  are  newer  than  the  foinier  and  older  than  the  latter; 
(4)  the  Mar(inette  system  is  found  in  unconformable  contact  with  the  Lower  Silurian 
red  sandstone  of  lake  Sl^pel^or.  The  Marquette  Huronian,  in  its  southerly  extension 
in  the  IVIeuominee  region,  is  also  found  in  unconformable  contact  with  the  fossilifer- 
ous  primordial  sandstone  of  the  Mississi])pi  valley,  a  fact  which,  even  if  tbe  evidence 
were  not  amply  sufficient  without,  would  demonstrate  the  futility  of  the  attempts 
made  by  some  to  refer  the  whole  series  of  lake  Superior  crystalline  rocks  to  the 
Silurian.     (Pp.  5-7.) 

The  general  succession  of  formations  in  northern  Wisconsin,  from 
above  downward,  is  held  to  be  as  follows:  (1)  Lake  Superior  Sandstone; 
unconformably  followed  by  (2)  the  Copper-Bearing  or  Keweenaw  series ; 
unconformably  followed  by  (3)  the  Iron-Bearing  or  Huronian  series;  un- 
conformably underlain  in  its  turn  by  (4)  the  Gneissic  or  Laurentian.  In 
support  of  the  belief  that  there  is  an  unconformity  between  the  Kewee- 
nawan  and  Huronian  series,  Brooks's  arguments,  already  noted  above,  are 
quoted  and  indorsed,  after  which  the  author  proceeds  as  follows : 

I  conceive  that  further  evidence  of  nonconformity  is  afforded  in  the  Wisconsin 
region  by  tbe  following  facts:  (1)  In  the  Penokee  country  the  uppermost  beds  of  the 
Huronian  aregraclually  cut  out,  as  we  trace  them  westward,  by  the  gabbro  tbat  forms 
the  base  of  the  Keweenawan  series,  a  fact  which  appears  to  me  best  explained  by 
the  supposition  that  the  gabbro  covers  and  conceals  these  missing  beds.  (13)  There 
is  not  an  absolute  uniformity  in  dip  between  the  Huronian  and  Keweenawan  rocks 
in  this  region,  the  latter  standing  commonly  at  a  higher  angle.  (3)  West  of  lake 
Numakagon  the  diabases  and  otker  eruptive  rocks  of  the  Keweenaw  series  appear  to 
completely  cover  the  Hiu^onian  in  a  great  overflow.  Nevertheless,  the  approach  to 
conformity  in  Wisconsin  is  close,  and  were  we  to  draw  our  conclusions  fiom  this 
region  only,  the  nonconformity  could  hardly  be  regarded  as  proved.  There  are  no 
such  undulations  in  the  Hui'onian  of  the  Penokee  district  as  in  Michigan,  the  subor- 
dinate members  making  long  and  regular  bands  conforming  to  the  general  trend  of 
the  formation,  and  also,  in  a  general  way,  to  the  trend  of  the  several  belts  of  the 
Keweenaw  series.  Moreover,  the  lessening  in  dip  toward  the  west,  already  noted  as 
affecting  the  latter  rocks,  is  observed  also  in  the  underlying  Huronian  so  far  as  these 
can  be  traced  westward.     (P.  22). 


48  THE  PENOKEE  IRON-BEABING  SEKIES. 

Irving  (R.  D.).  Geology  of  the  Eastern  Lake  Superior  District.  In  Geology 
of  Wisconsin,  vol.  ill,  Pt.  3,  pp.  51-238,  with  6  atlas  plates,  18  volume  plates 
(including  7  colored  microscopical  plates),  and  4  figures. 

This  is  a  detailed  account  of  the  geology  of  that  portion  of  northern 
Wisconsin  which  is  drained  by  the  waters  of  Bad  and  Montreal  rivers  and, 
of  course,  covers  all  that  portion  of  the  belt  now  to  be  studied,  which  lies 
west  of  the  Montreal  river.  The  chapter  titles  are  as  follows:  Introduc- 
tion, Topography,  The  Laurentian  System,  The  Huronian  System,  The 
Keweenawan  or  Copper-Beaiing  System,  The  Lake  Superior  or  Potsdam 
Sandstone,  The  Quaternary  Deposits,  Appendices  A  and  B.  The  chapters 
on  the  Huronian  and  Laurentian  are  of  chief  interest  in  the  present  con- 
nection.    From  the  former  chapter  we  quote: 

The  Huronian  rocks  Ije  together  in  a  narrow  belt  ft'om  half  a  mile  to  3  miles  in 
width,  which  stretches  entirely  across  the  district,  ft'om  the  west  line  of  T.  41,  R.  5  W., 
to  the  Michigan  boundary  at  the  Montreal  river,  in  Sees.  24  and  13,  T.  46,  R.  2  E. 
The  total  length  of  the  belt  in  this  distance  is  about  46  miles.  The  wider  portions 
are  toward  the  east,  the  western  part  narrowing  in  places  to  as  little  as  a  mile  or 
even  half  a  mile  in  width,  as  in  the  sections  just  south  of  Bladder  lake.  The  total 
area  underlain  by  these  rocks  is  just  about  100  square  miles.    (P.  100). 

Then,  after  giving  in  brief  the  course  of  the  Huronian  belt  through  the 
various  townships  and  sections,  the  report  proceeds: 

The  southern  boundary  of  the  Huronian,  or  its  line  of  junction  with  the  Lauren- 
tian, is  a  very  sharply  defined  line,  on  account  of  tlie  bold  topography  and  frequent 
rock  exposures  of  the  Penokee  range.  Even  in  that  portion  of  the  Huronian  belt 
where  the  Penokee  range  disappears,  and  the  rocks  are  entirely  concealed  by  drift 
and  swamps,  the  magnetic  attraction  exerted  by  the  iron-bearing  member  of  the 
formation,  one  of  its  lower  layers,  serves  to  fix  very  closely  the  southern  boundary. 
On  the  accompanying  atlas  plates  this  boundary  line  is  laid  down  so  accurately  and 
the  facts  upon  which  it  is  based  are  there  detailed  so  faUy  that  no  further  explanation 
is  needed  here. 

In  addition  to  the  facts  given  on  the  maps  and  in  the  details  of  the  following 
pages,  it  is  merely  necessary  to  say  here,  with  regard  to  the  northern  bouudaryj  that 
it  does  not  follow  the  strike  of  the  Huronian  beds,  but  cuts  across  them  in  a  more  or 
less  irregular  way.  The  width  of  the  Huronian  does  not  vary  on  account  of  the 
thickening  or  thinning  or  disappearance  of  any  of  its  subordinate  layers,  but  the 
wider  portions  include  higher  layers,  which  are  wanting  in  the  narrower  portions. 
The  irregularity,  then,  of  the  northern  boundary  is  due  t(3  a  nonconformity  of  the 
overlying  rocks  with  the  Huronian.     It  should  also  be  said  that  these  overlying  rocks, 


GEOLOGICAL  EXri-OI.'ATIONS  AND  LITEl'.ATUKE.  49 

cliit'lly  i;;il)l>i-iis,  iirc.  (tf  iiiii  iKiioous  (irijjiii,  iiml  linvc.  ccrtiiinly,  in  some  places,  and 
(luite  possibly,  also,  in  oMiers  not  ye(<  Tei -oj^nized,  pciictralcd  fciio  Uui'ouia.u,  producing 
peculiar  ineguhuities  in  (he  line  of  Junction. 

The  main  lopograjthical  leatuicss  of  the  lluroiiiaii  belt  have  already  been  given 
in  another  connection.  It  is  only  necessary  to  notice  here  somewhat  more  definitely 
the  relation  existing  between  the  geological  structure  of  the  series  and  the  topography 
yf  the  strip  of  country  underlaid  by  it.  The  Iluronian  series  includes  a  succession 
of  bi'ds,  always  markedly  schistose  and  at  times  highly  slaty,  which  are,  for  the  most 
part,  inclined  at  a  high  angle  to  the  northward.  At  the  base  or  southern  side  of  the 
belt  are  luirrow  layers  of  crystalline  limestone  and  (luartzito,  succeeded  by  a  broad 
band  of  siliceous  slate,  some  400  feet  in  width,  above  which  there  is  again  a  much 
broader  band,  generally  as  much  as  800  feet  wide,  of  magnetic  and  specular  schists. 
Above  these  again  is  a  series  of  alternating  layers  of  mica  slates,  diorites,  quartz 
slates,  and  quartzites — the  latter  comparatively  ineouspicuous — which  reaches  a  thick- 
ness of  several  thousand  feet.  A  close  connection  may  be  traced  between  the  nature 
of  these  beds  and  the  features  of  the  surface. 

The  existence  of  the  Penokee  range,  which  marks  the  lower  side  of  the  Huronian 
belt  for  the  greater  part  of  its  extent,  and  which  has  already  been  described  in  some 
detail,  is  determined  by  the  broad  bed  of  magnetitic  quartzites  and  siliceous,  mag- 
netic, and  specular  schists  above  referred  to.  These,  by  virtue  of  the  superior  hard- 
ness and  power  of  resisting  chemical  action  conferred  on  them  by  their  siliceous 
ingredient,  have  remained  standing,  while  the  softer  beds  to  the  north  have  been 
worn,  for  the  most  part,  into  deep  valleys,  in  which  streams  run  parallel  to  the  trend 
of  the  Penokee  range,  being  impelled  to  their  courses  by  the  strike  of  the  under- 
lying rocks.  In  places  the  more  massive  diorites  and  quartzites  of  the  northern 
portion  of  the  series  rise  from  the  valley  in  abrupt  ledges,  but  they  never  constitute 
a  continuous  ridge  like  the  Penokee  range,  on  account  of  their  smaller  breadth  and 
inferior  resisting  power.  On  the  northern  side  of  this  valley  the  Huronian  beds  often 
extend  well  up  the  river  on  to  the  Copper  range,  being  i^rotected  here  by  the  massive 
rocks  of  the  Keweenaw  series,  which  bound  them  on  the  north.  The  south  slope  of 
the  Penokee  range,  again,  is  made  up  of  the  siliceous  schist  which  underlies  the 
harder  rocks  that  form  the  body  of  the  range,  and,  being  itself  generally  a  quite  soft 
and  easily  eroded  material,  the  southern  slope  is  often  iirecipitous,  or  at  least  very 
bold.  This  is  esijecially  true  of  the  middle  portions  of  the  range,  from  a  few  miles 
west  of  Bad  river  nearly  to  Tyler's  fork.  Further  east  this  layer  becomes  more 
quartzitic  and  harder  and  forms  itself  the  body  of  the  ridge,  the  overlying  beds  at 
the  same  time  losing  their  comparatively  great  resisting  power  by  a  change  in  com- 
position. In  some  places  in  the  eastern  extension  of  the  Huronian  belt  both  the 
siliceous  schist  and  the  overlying  beds  are  softer  than  the  Laurentian  below,  and  the 
crest  of  the  ridge  is  made  up  of  rocks  of  the  latter  series. 
IfON  XIX — —i 


50  THE  PBlSrOKEE  IRON  BEAEING  SERIES. 

A  variation  in  the  degree  of  northerly  dip  of  the  beds  of  the  Huronian  has  also 
very  measurably  affected  the  surface  features.  From  a  point  on  the  ridge  a  few 
miles  west  of  Bad  river  to  the  Montreal  the  angle  of  dip  is  always  very  high,  55°  to 
750,  while  farther  west  it  lessens  to  45°,  35°,  25°,  and  even  to  20°  or  15°.  In  these 
places  the  result  is  a  longer  front  slope  to  the  ridge,  and  a  very  steep,  frequently  bold 
and  precipitous  southern  face,  made  up  usually  of  heavily  bedded  quartzitic  iron  ore 
overlyiug  the  siliceous  schist,  which  now  loses  its  prominence  and  forms  only  the  foot 
of  the  southern  face.  The  entire  absence  of  the  Penokee  range  in  T.  44,  E.  4  W.  is 
perhaps  to  be  attributed  in  part  to  a  lessening  in  dip,  though  probably  chiefly  to  a 
change  in  the  character  of  the  lower  layers  of  the  series.     (Pp.  101-103). 

The  Huronian  series  consists  of  a  succession  of  more  or  less  highly  schistose  to 
slaty  beds,  which  reach  a  total  thickness  in  the  widest  part  of  nearly  13,000  feet. 
These  layers  all  stand  inclined  at  a  high  angle  to  the  north,  and  stretch  across  the 
country  in  outcrops  generally  parallel  to  the  southern  limit  of  the  formation,  some,  of 
the  more  prominent  ones  preserving  their  characters  across  the  whole  width  of  the 
district,  a  distance  of  about  45  miles.  Inclining,  as  they  always  do,  to  the  north, 
these  beds  are  without  folds,  and  the  series  is  only  limited  in  that  direction  by  the 
overlapping  of  masses  of  igueous  rocks  belonging  to  an  unconformable  system — 
the  Copper-bearing  or  Keweenawan  series — the  unconformity  being  one,  however, 
recognizable  only  on  a  comprehensive  survey  of  the  region,  and  not  by  any  observed 
contact  between  the  two  formations.  The  absence  of  any  folds  in  so  highly  altered 
and  inclined  strata  is  easily  explained,  if  we  regard  them  as  forming  part  of  a  great 
bend  underneath  the  trough  of  Lake  Superior  and  reappearing  on  the  north  side  of 
the  lake  with  a  reversed  inclination.  These  points  have  been  brought  out  on  a  prev- 
ious page,  and  need  only  to  be  alluded  to  here. 

The  degree  of  northward  inclination  of  the  Huronian  beds  is,  for  most  of  the 
course  of  the  formation,  fi'om  55°  to  75° ;  most  usually  between  65°  and  75°.  To  the 
west  of  Sec.  16,  T.  44,  E.  3  W.,  however,  the  degree  of  inclination  is  nearly  always 
much  less,  becoming  at  times  as  low  as  20°.  The  bends  in-  the  course  of  the  forma- 
tion have  already  been  noted  in  a  general  way.  Some  of  these  are  exceedingly  abrupt, 
as,  for  instance,  on  Sec.  10,  T.  44,  E.  2  W. ;  at  the  crossing  of  the  creek  in  sections  8 
and  17,  T.  44,  E.  2  W.,  and  at  several  places  in  the  western  extension  of  the  formation. 
These  bends  are  well  marked  out  in  the  rock  exposures  and  are  noted  in  detail  on  the 
accompanying  maps. 

At  the  i^assage  of  Bad  river,  the  strata  are  crossed  by  a  fault,  trending  about 
N.  170  W.,  the  layers  on  the  west  side  of  the  fault  being  thrown  800  feet  to  the  north- 
ward of  those  on  the  east  side ;  or,  regarding  the  throw  as  a  vertical  one,  the  western 
side  has  been  elevated  or  the  eastern  depressed  a  vertical  distance  of  over  1,700  feet; 
the  apparent  lateral  throw,  on  this  supposition,  being  explained  by  the  inclined  posi- 
tion of  the  strata.    This  fault  is  marked  in  the  topography  by  a  corresponding  set-off 


GEOLOGICAL  EXI'LOKATIONS  AND  LITEUATUKB.  51 

in  the  ronokcc  riiUK'Oi  wliicli  on  the  west  side  of  the  river  is  well  to  the  northward  of 
its  position  on  the  east  side.  It  is  also  well  marked  by  large  rock  exposures  on  either 
side,  and  also  by  an  abrupt  break  in  the  line  of  magnetic  attraction  caused  by  the 
magnetic  belt  of  the  formation.  The  facts  with  regard  to  it  are  all  detailed  on 
Atlas  Plate  xxiii,  and  also  in  the  descriptions  of  the  following  pages. 

The  following  tabulation  indicates  the  succession  of  layers  of  the  Hurouian  series 
in  the  vicinity  of  Bad  river,  so  far  as  made  out,  with  the  average  thickness,  surface 
breadth,  and  other  prominent  points  of  each  layer.  Several  of  the  lower  layers, 
inchiding  a  total  thi(;kness  of  some  1,500  feet,  have  been  traced  across  the  entire 
width  of  the  district.  The  higher  layers,  on  account  of  their  comparative  softness  and 
susceptibility  to  chemical  changes,  have  been  for  the  most  part  deeply  eroded,  and 
are,  moreover,  largely  buried  beneath  accumulations  of  drift  materials,  so  that  the 
exposures  are  comparatively  few  and  distant,  and  the  task  of  making  out  the  succes- 
sion becomes  much  more  difficult.     (Pp.  103-104.)     .     .     . 

Synopsis  of  the  Straligraphij  of  the  Hiironian  of  the  Penokee  region,   Wiscousiti. 

Average . 
thickness. 
Formation.  Feet. 

I.   Tremolitio  crystalline  limestone 90 

II.  (A)  Arenaceous  white  quartzito,  often  brecciated,  35  feet;  (B)  magnetitic  quartz- 
schist,  5  feet 40 

III.    Siliceous  slaty  schists;  including  quartzito,  "  argillitio  "  mica-schist,  and  novacu- 

lite;  all  having  much  quartz,  and  none  ever  showing  any  amorphous  material..  410 

IV.  Magnetic  belt;  including:  (a)  banded  magnetic  quartzite — gray  to  red  quartzite, 
free  from  or  loan  in  iron  oxides,  banded  with  seams,  from  a  fraction  of  an  inch 
to  several  inches  in  width,  of  pure  black  granular  magnetite,  only  rarely 
mingled  with  the  specular  oxide;  (b)  magnetitic  quartzite,  the  magnetite  in 
varying  j)roportions,  pretty  well  scattered  throughout,  and  mingled  with  the 
specular  oxide  in  proportions  varying  from  nothing  to  a  predominating  quan- 
tity; (c)  magnetitic  quartz-slate,  the  magnetite  pervading  the  whole,  and 
mingled  with  the  specular  oxide  as  before;  {d)  slate  like  (c)  but  largely  charged 
with  tremolite  or  actiuolite ;  (e)  arenaceous  to  compact  and  flaky  quartzite,  free, 
or  nearly  so,  from  iron  oxides ;  (/)  thin-laminated,  soft,  black  magnetitic  slate ; 
(g)  hematitic  quartzite,  the  iron  oxide  the  red  variety;  (/i)  garnetiferous 
actiuolite-schist,  or  eologite;  (i)  diorite,  which  is  restricted  to  the  western  end 
of  the  Hurouian  belt.  Kinds,  (a)  to  (d),  all  carry  much  pyrolusite,  or  other 
manganese  oxide.  These  varities  have  no  persistent  stratigraphical  arrangement, 
and  are  named  here  in  order  of  relative  abundance.     Total  thickness,  about.. .  780 

V.   Black  feldspathic  slate;  consisting  of  orthoclase  grains  imbedded  in  a  paste  of 

biotite,  pyrite,  limonite,  and  carbon 180 

VI.   Unknown,  always  drift-covered 880 

VII.   Dark  gray  to  black,  aphanitic  mica-slate,  having  a  wholly  crystalline  base  of 

quartz  and  orthoclase,  with  disseminated  biotite  scales 120 

VIII.    Unknown,  but  probably  in  large  part  the  same  as  VII 290 

IX.   Chloritic,  pyritiferous,  massive  diorite 150 

X.   Black,  aphanitic  mica-slate,  like  VII 25 


52  THE  PENOKEE  lEON-BEAEING  SEEIES. 

Si/nopsis  of  the  Stratigraphy  of  tlie  Haronian  of  the  Pcnokee  region,  Wisconsin — Continued. 

Average 
thickness. 
Formatiou.  ■  Feet. 

XI.   Covered,  but  iirobablj^  mica-slate 280 

XII.    Black  mica-slate;  ajiliaiiitic;  at  times  chiastolitic 225 

XIII.  Cliloritic  diorite-schist 35 

XIV.  Black  mica-slate,  like  XII,  often  cliiastolitic 375 

XV  to  XVIII.    Alternations  of  black  mica-.slates,  with  qiiartzites  and  ijuartz-schists 675 

XIX.    Greenstone-schist;  aplianitio;  the  hornblende  and  plagioclase  much  altered 260 

XX.   Covered,  but  probably  like  XXI - 525 

XXI.  Mica-schist;  from  aphanitic  to  medium  grained ;  including  bands  of  light  gray 
(luartz-schist,  the  mica  becoming  subordinate;  all  varieties  having  a  back- 
ground of  quartz ;  the  mica  wholly  biotite ;  penetrated  by  veins  and  masses  of 
very  coarse,  pink  to  brick  red  biotite  granite;  total  on  Bad  river,  4,960  feet. 
Seen  farther  east,  higher  layers,  2,500  feet ;  in  all .7, 460 

Total 12,800 

(Pp.  104-105.) 

The  numerous  local  details  of  this  volume  are  made  frequent  use  of 
in  the  following  ])ages  and  on  the  accompanying  maps. 

JuLiEN  (Alexis  A.).  Microscopic  Examination  of  Eleven  Eocks  from  Ashland 
County,  Wis('onsin.  In  the  Geology  of  Wisconsin,  vol.  iii,  pt.  3,  Appendix  B,  pp. 
224-238.     With  one  colored  microscopical  plate. 

This  paper  includes  detailSd  descriptions  of  the  following  rocks  :  (1) 
Chloritic  gneiss,  from  the  lower  gneisses  at  Penokee  gap  ;  (2)  crystalline 
limestone ;  (3)  tremolitic  magnetite-slate ;  (4)  black  feldspathic  slate ;  (5) 
chloritic  diorite  ;  (6)  magnetitic  mica-slate ;  (7)  chiastolitic  mica-slate ; 
(8)  diorite  greenstone-schist ;  (9)  magnetitic  mica-schist — all  from  the  Bad 
river  section  through  the  Iron-bearing  slates;  and  (10  and  11)  two  chryso- 
litic  diabases  from  the  overlying  copper  series. 

Wright  (C.  E.).  The  Hurouian  Series  West  of  Penokee  Gap.  In  the  Geology 
of  Wisconsin,  vol.  ili,  pt.  4,  pp.  239-301.     With  one  atlas  i)late  and  ten  volume  plates. 

This  report,  the  result  of  a  special  exploration  with  reference  to  the 
occurrence  of  iron  ores  in  the  slaty  series  west  of  Bad  river,  embraces 
three  chapters,  which  are  entitled,  respectively,  "  Dipping  Needle  and  Solar 
Dial  Compass,  and  the  Method  of  Employing  Them,"  "  Greological  Cross  Sec- 
tion of  Penokee  Iron  Range,"  and  "Special  Examination  of  Penokee  Iron 
Range  West  of  the  Grap."  The  first  of  these  chapters  calls  for  no  consid- 
eration from  us.     The  third  chapter  is  occupied  with  local  details,  aixanged 


(iEOLOGK^AL   KX  I'LOKATIONS  AND  LITEIIATURE.  53 

ill  the  form  of  a  diary,  with  the  results  of  a  microscopical  study  of  thin  sec- 
tions iirterspersed.  These  details  have  often  been  used  in  the  compilation 
of  our  maps  herewith,  most  of  the  important  outci-ops  described  by  Mr. 
Wright  having,  however,  been  revisited  for  our  work.  Wright's  route  of 
travel  also  is  laid  down  on  our  maps. 

The  second  of  Mr.  Wright's  chapters  is  of  a  more  particular  interest, 
because  in  it  he  gives  his  views  as  to  the  general  stratigraphy  of  the  Iron- 
bearing  series,  and  as  to  its  relations  to  the  adjoining  formations.  The  fol- 
lowing quotations  from  this  chapter  will  serve  to  shoAv  Mr.  Wright's  con- 
clusions ;  the  omitted  portions  are  mainly  microscopical  details: 

lu  this  geological  section  it  has  been  my  endeavor  to  correlate  the  Penokee  Iron- 
bearing  series — Lower  Hnronian — as  near  as  possible  with  those  of  Michigan,  assumed 
to  be  of  the  same  age,  as  they  uudonbtedly  are. 

We  will  adopt,  for  convenience  of  reference,  the  numbering  first  employed  in  the 
Geological  Report  of  Michigan,  1873,  to  designate  the  Lower  Hnronian  beds. 

The  first  rocks  we  wiU  .consider,  however,  are  those  of  th^  Laureutian  series, 
found  outcropping  on  the  south  side  of  the  Penokee  range.  These  rocks  are  granites, 
gneissoid  granites,  and  gneisses.  At  the  Gap,  they  have  a  strike  a  little  to  the  south 
of  west,  and  dip  from  65otb  80°  to  the  south.  The  granites  are  dark  gray  to  reddish, 
depending  on  the  predominance  of  the  mica  or  feldspar.  They  are  fine  to  very 
coarse  grained,  the  medium  grained  varieties  being,  however,  the  most  common.  The 
essential  mineral  ingredients  are  usually  easily  recognized.  The  bedding  i^lanes,  or 
"grain"  of  the  granite,  even  in  the  massive  varieties,  may,  generally,  after  careful 
examination,  be  made  out.    ... 

Orthoclase  is  the  prevailing  member  of  the  feldsijar  family.     .     .    . 

The  mica  is  chiefly  biotite  and  of  a  dark  color.     .     .     . 

The  gneisses,  like  the  granite,  vary  from  dark  gray  to  reddish.  Usually  they  are 
distinctly  laminated  with  the  layers  of  quartz,  feldspar,  and  folia  of  mica.  It  is  some- 
times slaty,  but  more  generally  it  is  heavily  bedded,  and  passes  almost  imperceptibly 
into  a  gneissoid  or  a  massive  granite.  The  dark,  fine  grained,  slaty  varieties  resemble 
hornblende-gneiss  and  hoi'nblende-schist;  in  fact,  I  have  been  obliged  to  make  micro- 
scopic sections  before  being  able  to  decide  which  it  was.    .    .    . 

Nonconformably  overlying  the  Laureutian  rocks  are  those  of  the  Huroniau 
series.  We  have  at  the  Penokee  Gap,  where  the  Wisconsin  Central  railroad  crosses 
the  range,  one  of  the  best  opportunities  I  have  ever  chanced  upon  for  observing  this 
interesting  fact.  This  nonconformability  on  the  Penokee,  I  believe,  was  first  noticed 
by  myself  in  1875.  The  Huroniau  series  at  the  Gap  are  plainly  bedded,  and  have  a 
strike  of  a  little  north  of  east,  and  dip  very  uniformly  06°  to  the  north. 


54  THE  PENOKEE  lEON-BEAEmG  SEEIES. 

The  lowest  member  which  I  have  seen  on  the  Penokee  range  is  a.  marble,  or 
dolomitic  limestone,  which  we  will  consider  as  No.  IV.  There  may  be  other  members 
below  the  marble,  as  is  the  case  in  the  Hnronian  rocks  of  Michigan,  but  I  have  never 
found  them. 

At  the  Penokee  gap  the  marble  is  siliceous;  in  color  light  drab  to  grayish  white, 
with  shades  of  green;  also  light  red  and  pink  not  uncommonly.  This  diversity  of 
colorsis  often  observable  in  a  hand  specimen.  The  marble  is  flue  grained,  and  strongly 
bedded,  or  massive,  depending  on  the  degree  of  metamorphism.  When  massive,  it  is 
usually  jointed,  and  weathers  to  a  light  drab.  Some  portions  of  the  rock  contain 
pale  greeu  to  almost  white,  radiated  bunches  of  actinolite,  resembling,  in  the  latter 
instances,  clusters  of  arragonite.     .     .     . 

No.  V.  Immediately  overlying  the  marble  at  the  Gap  is  a  quartzite,  varying  in 
color  from  grayish  white  to  white,  and  from  saccharoidal  to  vitreous  in  texture.  It  is 
massive  and  highly  jointed.     ... 

No.  VI.  ISText  in  order  above  the  qiiartzite  we  have  a  chloro- siliceous  schist. 
This  member  was  notable  for  preserving  its  individuality  wherever  we  found  it  out- 
cropping, and  was  therefore  easily  and  quickly  recognized.  It  has  a  dark  grayish 
green  color,  fine  grain,  and  is  jointed,  and  usually  more  or  less  slaty.  Along  the 
jointing  planes  it  is  often  finely  corrugated,  and  has  an  unctuous  feeling.  It  cleaves 
readily  into  thin  parallel  and  wedge-shaped  plates.  Examined  with  the  lens,  it  is 
difficult  to  distinguish  any  of  the  chlorite.     ... 

Nos.  VII,  IX,  and  probably  No.  XI,  are  represented  in  the  near  vicinity  of  the 
Gap  by  argillites  or  black  slates,  while  west  of  the  Gap,  from  10  to  14  miles,  as 
has  been  already  noted,  the  slates  are  replaced  by  diorites  and  hornblende  rocks. 
My  impression  is  that  these  slates  and  greenstones  do  not  vary  greatly  in  their  chem- 
ical composition,  the  texture  and  structure  being  due  to  different  conditions  or 
degrees  of  metamorphism. 

The  argillites  are  brownish  to  bluish  black  and  have  a  micro-granular  texture. 
Some  of  these  are  quite  slaty  and  cleave  freely;  others  are  more  compact.  The  slates 
are  thicker  and  the  fracture  uneven  to  conchoidal.  On  the  cleavage  planes  they  have 
a  bright,  lively  luster,  but  a  fracture  across  the  cleavage  is  of  a  dull  brownish  black. 
Disseminated  throiigh  the  slate  is  considerable  carbon,  which  appears  ragged  under 
the  microscope.  Numerous  slender  blades  resembling  microlites  of  feldspar  are 
visible.  Small  angular  grains  of  silica  are  present;  also  brownish  and  slightly 
dichroitic  leaves  resembling  biotite. 

The  diorites,  above  alluded  to  as  constituting  in  part  Nos.  VII  and  IX  of  our 
scheme,  are  massive  and  strongly  jointed,  rarely  ever  showing  any  signs  of  bedding. 
They  are  fine  to  coarse  grained  in  texture;  the  cleavage  planes  of  the  hornblende  in 
coarse  varieties  being  unmistakable ;  also  grains  of  magnetite.  The  other  mineral 
ingredients  are  not  so  readily  distinguished.     .     ,     . 


r.EOLOOICAI,  F,XrL( )ItyVTIONS  ANT)  IJTKTIATUEE.  55 

Rptiiniiii j;'  now  to  Xn.  I'/Z/oI'mir  cniss  section  we  liiid  it  and  No.  X  rojire- 
sciitcd  liy  actiiiolo-iiiiifiiu'fic  scliists.  Tlicsc  iiiciiiltcrs  ai«  more  or  less  iiiapietic, 
(leiieiidiiij;-  on  tlu^  iierceutafto  ol'  magnetite  an<l  otlier  conditions.     .     .     . 

The  niajinetie-  schist  Is  banded,  with  layers  of  lean  ore  inipregnated  by  actino- 
lite;  also  layers  of  arcMiaceous  (|nart/,  and  occasionally  a  thin  one  of  magnetic  ore. 
The  bands  vary  in  thickness  from  a  mere  line  to  one  or  more  inches.  Some  portions 
of  it  are  qnite  slaty  and  Jointed.  The,  textnre  on  a  fractnn^  is  nsnally  fine  grained. 
The  color  varies  from  gray  to  brownish  black.     .     .     . 

No.  XII,  usually  represented  in  the  Marquette  iron  district  by  a  banded  jasj^er, 
whicli  forms  the  "foot  wall"  of  the  iron  ore,  we  did  not  find  outcropping  west  of  the 
I'enokee  gap,  but  since  we  were  on  the  range  I  have  been  shown  by  exploring  parties 
sevei'al  s.amples  of  jasper  and  specular  and  magnetic  ore  (probably  No.  XIII)  which 
they  found  east  of  the  Gap.     .     .     . 

No.  XI V.  I  have  also  seen  si)eclmens  of  gray  quartzite  from  east  of  the  Gap, 
which  were  very  similar  to  that  forming  the  hanging  wall  of  the  Marquette  iron  ore 
mines,  and  for  this  reason  we  will  for  the  present  refer  it  to  No.  XIV.  The  next  num- 
ber in  the  regular  order,  No.  XV,  a  micaceous  argilllte  or  slate  is  found  outcropping 
in  Sees.  9,  10,  and  11,  T.  44,  R.  3  W.  The  outcrops  were  small  and  formed  at 
most  only  low,  narrow  ridges,  which  is  no  doubt  due  to  the  perishable  nature  of  the 
rock.    In  many  respects  it  resembles  the  argillites  described  under  Nos.  VII  and  IX. 

The  rock  is  dark  brownish  black,  has  a  dull,  slaty  texture,  is  strongly  jointed, 
cleaves  into  imperfect  slates,  the  thicker  ones  having  a  conchoidal  fracture.  Examined 
on  a  fresh  fracture  the  surface  appears*  thickly  sprinkled  with  very  minute,  shiny 
scales.     .     .     . 

Nos.  XVI  to  XIX.  Within  these  numbers  are  embraced  a  large  group  of  mica- 
ceous quartz-schists,  micaceoiis  slates  and  schists,  and  chiastolite  schist. 

These,  with  No.  XV,  no  doubt  could  consistently  be  comprised  under  one  head, 
but  in  order  to  reach  No.  XIX,  which  corresponds  very  nearly  with  the  chiastolite 
schist  of  the  Michigan  series,  we  will,  for  the  present,  retain  the  numbers  without 
attempting  to  classify  the  different  schists. 

The  most  important  member  of  these  is  the  micaceous  quartz-schist.  It  has  an 
immense  development  immediately  west  of  the  Gap.  It  is  of  a  dark  iron-gray  color, 
and  has  a  very  even,  fine  grained  texture.  It  weathers  to  a  light  drab.  Under  the 
lens  the  dark  colored  mica  is  plainly  visible.  Some  of  the  exposed  surfaces  are 
minutely  pitted,  owing  apparently  to  the  partial  decomposition  and  washing  out  of 
the  mica.  In  hand  specimens  the  structure  appears  massive,  but  uncovering  the 
ledge  the  fresh  surface  frequently  shows  a  slightly  banded  structure.    .     .    . 

This  comprises  all  of  the  rocks  which  I  am  satisfied  belong  to  the  Iron  series. 
Overlying  these,  apparently  nonconformably,  are  diorites,  uralites,  diabases,  hypers- 
thene,  and  granitoid  rocks.     The  diabases  are  massive  and  fine  to  very  coarse  grained. 


5(5  THE  PENOKEE  IRON-BEARmG  SERIES. 

In  some  of  them  the  crystals  of  labradorite  are  2  inches  across;  in  fact,  this  min- 
eral in  nearly  all  of  them  is  the  principal  one.  A  weathered  surface  of  the  rock  is 
frequently  rough  and  knotty,  with  prqiecting  grains  of  titanic  iron  ore.  The  augite 
is  best  recognized  under  the  microscope.  The  diorites  are  also  massive.  The  coarser 
varieties  are  easily  distinguished-  from  the  diabases  by  the  cleavage  planes  of  the 
hornblende.     (Pp.  248-255.) 

With  Mr.  Wright's  views,  thus  indicated,  as"  to  the  unconformities 
between  the  slaty  series  and  the  gneissic  and  diabasic  rocks  which  respec- 
tively underlie  and  overlie  it,  we  are  in  entire  accord.  With  his  schemes  of 
stratigraphy  for  the  Iron  series,  and  of  the  equivalency  of  its  members 
with  those  marked  out  by  Brooks  for  the  Marquette  Iron-bearing  series, 
we  are  in  many  respects  unable  to  agree,  as  will  appear  hereafter. 

Hunt  (T.  S.).  The  History  of  Some  Pre-Carabrian  Rocks  in  America  and 
Europe.-  Am.  Jour.  Sci.,  .3d  series,  vol.  xix,  1880,  pp.  268-283,  Read  before  the 
American  Association  for  tlie  Advancement  of  Science,  Sept.  1, 1879. 

This  paper  maintains'  the  Neptunean  origin  of  the  Archean  and  crys- 
talline schists,  including  also  the  "  olivines  and  serpentines,  and,  in  short, 
all  silicated  crystalline  stratified  rocks."  It  gives  also  an  outline  historical 
account  of  the  pre-Cambrian  formations.  In  the  course  of  this  account  a 
brief  reference  is  made  to  the  views  of  Foster  and.  Whitney  maintaining 
the  nondivisibility  of  the  lake  Superior  Archean,  and  to  the  opposing 
views  of  Logan,  Murray,  Kimball,  Credner,  Brooks,  Pumpelly,  and 
Irving,  the  author  himself  maintaining  a  divisibility  into  two  distinct  ter- 
ranes.  No  specific  reference  to  the  Penokee  region  is  made,  but  since  it 
affords  most  abundant  proof  of  such  a  divisibility,  Hunt's  essay  may  appro- 
priately be  noticed  here. 

Wadsworth  (M.'e.).  Notes  on  the  Geology  of  the  Iron  and  Copper  Districts 
of  Lake  Superior.    Bull.  Mus.  Comp.  Zool,  vol.  vii,  1880,  No.  1,  Cambridge. 

So  far  as  the  Iron-bearing  rocks  are  concerned,  the  main  objects  of 
this  volume  are  to  set  forth  certain  observations  and  conclusions  with 
regard  to  the  rocks  of  the  Marquette  Iron  region  of  Michigan.  Since  the 
Penokee  rocks  were  already  at  that  time,  by  common  consent,  regarded  as 
the  equivalents  of  those  of  the  Marquette  region,  the  conclusions  are 
extended  to  the  former  rocks  also.  The  following  quotations  will  show 
what  the  principal  ones  of  these  conclusions  are. 


(iHOLOCilCAL  EXPLOITATIONS  AND   LITKRATIIIMC.  57 

(1)  As  to  tilt'  structural  relations  ami  origin  of  the  iron  ores  and  asso- 
ciated jaspers: 

.The  olisci'vatioiis  and  figuras  o-ivou  in  the  ])rec(Mlins"  tflxt  sliow  conolnslvely  tlnit 
the  statements  of  Messrs.  Dana,  Kiniliall,  Ilnnt,  Brooks,  and  others,  tliat  the  iron  ore 
is  interstratilied  in  the  associated  sehists,  ai'C  incorrect,  and  only  retnrn  to  the  view 
ads'ocated  by  Mr.  Poster  iif  his  early  ])ablication.  So  far  as  g-eolo,iiical  science  has 
now  advanced,  the  facts  observed  can  only  be  explained  by  tiie  eruptive  origin  ot 
both  the  ore  and  jasper,  as  they  make  tiie  same  formation.  The  only  escape  from  this 
eonclnsion  is  the  supposition  that  the  ore  and  Jasper  have  b^en  rendered  plastic  in 
situ,  while  the  chloritic  schist  has  not  been.  Such  a  supposition  Mr.  Brooks'  was 
forced  iu  part  to  adopt.  That  the  ore  and  jasper  have  been  thus  rendered  plastic, 
while  the  schists,  quartzites,  and  other  associated  rocks  have  not  been,  is  too  absurd, 
chemically  or  geologically,  to  be  tolerated  for  a  moment  as  an  hypothesis.  .  .  . 
The  ore  and  jasper  show  that  they  are  the  intrusive  bodies  by  tlieir  breaking  across 
the  lamination  of  the  schists  and  other  rocks,  by  the  changes  that  take  place  in  the 
latter  at  the  line  of  junction,  by  horses  of  schist  being  jnclosed  in  the  ore,  by  the 
curvature  of  the  laniinatiou  produced  by  the  intrusion  of  the  ore  and  jasper,  etc.  Not 
the  slightest  sign  of  the  plasticity  or  intrusion  of  the  schists  relative  to  the  ore  or 
jasper  was  seen.  That  the  present  lamination  of  the  schist  existed  prior  to  the  intru- 
sion of  the  ore  and  jasper  is  shown  by  the  effect  of  tlie  hitter  upon  and  its  relations 
to  it.  That  this  lamination  is  the  original  plane  of  dejiosition  is  for  part  of  the  schists 
not  known;  but  whether  it  is  or  not,  it  has  been  taken  to  be  such  by  the  observers 
quoted  iu  the  establishment  of  their  theories,  and  they  must  abide  by  it.  The 
lamination,  however,  coincides  with  many  of  the  well  stratified  rocks  adjacent, 
and  iu  some  of  these  the  ore  and  jasper  were  unmistakably  intrusive.  The 
schists  that  retained  well  marked  stratification  planes  showed  in  some  places  extra- 
ordinary contortions,  oue  specimen  (293)  showing  a  synclinal  and  anticlinal  fold, 
requiring,  were  the  top  eroded,  the  counting  of  the  same  layer  four  times  in  the 
width  of  2  inches.  This  is  only  oue  case  out  of  numerous  ones  observed  (292,  292+, 
302).  In  the  fine  grained  detritus  composing  some  of  the  schists  it  is  quite  likely  true 
that  the  lamination  does  not  coincide  with  the  original  bedding ;  but  if  it  does  not, 
then  the  breaking  of  the  ore  across  any  chosen  plane  whatsoever,  except  the  lamina- 
tion i^lane,  can  be  shown  more  easily  than  in  the  former  case. 

The  ore  and  jasper  seem  to  have  been  erupted  in  huge  bosses  and  overflows,  as 
well  as  intruded  into  the  schist  in  the  form  of  long  arm  and  wedge-like  masses  or 
sheets.  On  account  of  the  banded  character  of  the  jasper,  and  the  intrusion  gener- 
ally being  nearly  iu  line  of  the  lamination  in  the  large  mass,  they  have  au  apparently 
stratified  character  to  those  who  believe  any  " striped "  rock  is  a  sedimentary  one; 

'Geol.  of  Mich.,  vol.  i,  1873,  pp.  139, 110. 


58  THE  PENOKBE  IRON-BEAEIKG  SBEIES. 

but  when  examined  in  detail,  and  in  x>laces  where  the  relations  can  be  seen,  they 
prove  to  be  eruptive  (pp.  66-68). 

(2)  As  to  an  unconformity  between  the  so-called  Laurentian  and 
Huronian,  referring  to  my  statement  as  to  this  relation  in  Geology  of  Wis- 
consin— 

He  states  that  a  perfect  case  of  nonconformability  exists  at  "Penoka  gap," 
Wisconsin,  to  which  we  have  before  referred;  but,  if  we  remember  correctly  Mr. 
Wright's  personal  statement  to  us,  neither  was  the  junction  seen  nor  the  kind  of 
junction  known  that  the  two  made  with  each  other.  It  is  too  fast  to  assume,  as  has 
been  done  by  Messrs.  Brooks,  Irving,  and  Wright,  that  the  strike  and  dip  of  a  foliated 
rock  is  the  strike  and  dip  of  its  stratification.  This  is  especially  the  case  when  the 
view  that  they  were  ever  stratified  is  still  a  disputed  point  (p.  25). 

We  have  heretofore  seen  that  the  view  that  the  "  Huronian"  unconformably  over- 
lies the  "Laurentian"  has  been  only  supported  by  the  fact  that  the  foliation  of  the 
latter  did  not  conform  in  its  dip  to  the  lamination  of  the  former.  This  proof  is  of  no 
value  unless  it  can  be  shown  that  both  rocks  are  stratified  and  in  situ.  That  the 
latter  is  not  so,  we  have  seen  in  numerous  localities.  Heretofore  the  two  systems 
have  not  been  observed  in  contact,  but  recently  statements  have  been  published  that 
their  junctions  have  been  seen  in  other  regions. '  The  statement  is  made  that  both 
rocks  are  stratified,  but  no  proof  is  adduced  to  show  on  what  the  conclusion  is  founded, 
and  although  the  contacts  were  said  to  show  beautifully,  nothing  was  pubMshed  indi- 
cating that  the  kind  and  manner  of  the  junction  was  observed.  It  would  seem  that 
even  here  the  decision  concerning  the  unconformability  was  based  on  the  foliation 
only  (p.  70). 

Ikving  (R.  D.).  The  Mineral  Resources  of  Wisconsin,  Trans.  Am.  Inst.  Min. 
Eng.,  vol.  VIII,  1880,  pp.  487-508,  accompanied  by  a  geological  map  of  Wisconsin,  and 
adjoining  portions  of  Michigan,  Minnesota,  Iowa,  and  Illinois. 

As  the  title  indicates,  the  object  of  this  paper  is  to  give  an  account  of 
the  mineral  resources  of  Wisconsin,  as  developed  to  date.  Prefacing  this, 
however,  is  given  an  outline  account  of  the  geological  structure  of  the 
state.  In  this  are  included  a  number  of  references  to  the  Penokee  district, 
but  nothing  is  given  of  interest  in  the  present  connection  that  is  not  included 
in  vol.  Ill  of  the  Geology  of  Wisconsin,  already  referred  to  at  length. 

Weight  (0.  E.).  Annual  Report  of  the  Commissioner  of  Mineral  Statistics  of 
the  State  of  Michigan  for  1880.    Lansing,  1881. 

'  Geol.  of  Wis.,  vol.  in,  1880,  pp.  98,  108, 117. 


r.EOT.OdTOAL  EXPLOITATIONS  AND  LITERATURE.  59 

Tliis  report,  ;is  also  latcrdiicw  by  the  same  autlior  (for  ISSI  ami  1882), 
and  l)y  his  successor,  A.  P.  Swincford  (lor  1883  and  1884),  contain  brief 
references  to  the  results  of  private  explorations  for  ii-on  in  tlie  Gogebic 
region.  They  contain  no  geological  information  of  significance.  Actual 
mining  develojjnumts  first  began  in  the  Gogebic  conntr)^  in  tlie  fall  of  1884, 
and  the  first  shi])inents  of  ore  of  imj)ortance  were  made  in  the  summer  of 
1886. 

18S3. 

Ohamberlin  (T.  C).  Geology  of  Wisconsin,  vol.  i,  1883,  ]>t.  i.  General 
Geology,  with  general  geological  map  of  Wisconsin. 

In  chapters  iv,  v,  and  vi  of  this  volume  Prof.  Ohamberlin  gives  a 
general  account  of  the  older  rocks  of  Wisconsin,  which,  in  accordance 
with  previous  publications  of  the  Survey,  are  regarded  as  divided  into 
three  distinct  formations — Laurentian,  Huroniau,  and  Keweenawan. 
While  in  a  considerable  measure  this  account  is  a  summary  of  conclusions 
already  announced  in  previously  published  volumes  of  the  Geology  of 
Wisconsin,^  it  still  embodies  some  later  conclusions,  as  also  the  most  com- 
prehensive statement  of  Prof.  Chamberlin's  own  Adews  on  these  formations 
yet  published.  The  following  extracts  will  serve  to  show  what  these 
views  are: 

(1)  As  to  the  Laurentian: 

Synoptical  notes  on  Laurentian  formation. — Name  derived  from  Laurentide  liills 
of  Canada.  Rocks  of  metamorphic  class,  mainly  gneisses.  Thickness  undetermined, 
but  gTeat.  Strata  much  folded  and  contorted.  Occupies  a  large  area  in  northern 
Wisconsin.    ... 

General  character  of  the  rocks. — We  have  already  referred  to  this  as  the  granitic 
foundation  upon  which  the  rocjc  structure  of  our  state  is  builded.  The  rocks  as  we 
now  find  them  consist  of  a  series  of  granites,  .  .  .  gneisses,  .  .  .  syenites, 
.  .  .  hornblendic,  micaceous,  and  chloritic  schists  and  allied  rocks.  With  these 
are  associated  igneous  diabases  .  .  .  and  similar  rocks,  together  with  diorites 
.  .  .  of  undetermined  origin.  Among  these  rocks  the  gneissoid  granites  vastly 
predominate,  so  that  the  whole  series  in  a  general  view  is  conveniently  termed 
granitic. 

Sedimentary  origin. — But  throughout  the  series  evidences  of  sedimentary 
accumulation  abound:  (1)  in  the  foliations  and  stratification,  (2)  in  the  alternating 

'  Vols.  II,  III,  aud  IV  of  this  series  antefLite  vol.  i  In  time  of  publication. 


60  THE  PENOKEE  lEON  BEARING  SEEIES. 

bands  of  varying  chemical  constitution,  (3)  in  the  verging  of  one  kind  of  rock  into 
another  laterally,  and  (4)  in  kinds  of  rock  not  known  to  be  produced  by  igneous 
agencies.  The  ^yhole  series  has  been  distorted,  folded,  and  crumpled  in  a  most  intri- 
cate manner,  and  the  rocks,  as  the  above  names  imply,  are  in  a  highly  crystalline 
condition.    It  is  manifest  that  the  series  was  not  so  formed  originally  (pp.  64-66). 

Thickness. — The  thickness  to  which  these  sediments  accumulated  was  something 
enormous.  In  their  present  crystalline  state  the  current  estimate  of  30,000  feet  is 
probably  not  too  great  for  the  exposed  portion,  though  the  original  Canadian 
measurement  on  which  it  is  based  included  beds  now  referred  to  the  Huronian  series. 
How  much  may  lie  below  is  not  known,  since  the  base  is  not  exposed.  So  great  an 
accumulation  could  only  have  taken  place  on  a  subsiding  bottom  (p.  69). 

Distortion  of  the  heds. — The  long  period  of  Laurentian  subsidence  and  sedi- 
mentation at  length  drew  to  a  close  and  the  accumulated  material  underwent  a  most 
extraordinary  transformation.  The  sands  and  clays  lay  originally  in  essentially  hori- 
zontal beds,  but  at  present  we  neither  find  horizontal  beds  nor  sands  nor  clays.  The 
strata  are  crumpled  and  folded  in  the  most  intricate  manner.  Not  only  have  the 
great  series  of  beds  been  arched  and  compactly  folded  iipon  themselves,  but  even 
the  thin  laminations  have  been  contorted  and  crumpled  in  the  most  remarkable  man- 
ner. The  axes  of  the  folds  in  the  region  of  northern  Wisconsin  run  mainly  north- 
east and  southwest,  varying  several  points  in  either  direction.  On  the  southwestern 
margin,  however,  there  is  a  tendency  to  a  more  westerly  and  northwesterly  trend, 
somewhat  parallel  to  that  margin  of  the  area  (pp.  72-73). 

Attending  metamorphism. — The  crystallization  of  the  material  is  strikingly  in 
harmony  -with  this  hypothesis  of  its  heated  condition.  The  sediments,  while  still  in 
their  horizontal  position,  doubtless  became  solidified  into  somewhat  firm  rock  (1)  by 
their  own  weight,  (2)  by  their  tendency  to  cohere,  and  (3)  by  the  agency  of  cementing 
infiltrations.  But  there  is  no  reason  to  suppose  that  this  induced  any  notable  degree 
of  chemical  or  crystalline  change.  But  in  their  j)resent  metamorphosed  condition, 
instead  of  compacted  sand  and  clay,  we  find  thoroughly  crystallized  rock,  in  the  form 
of  granites,  gneisses,  syenites,  hornblendic,  chloritic,  and  micaceous  schists.  These  ~ 
show  that  a  profound  chemical  change  has  taken  place,  wherein  the  matter  assumed 
new  combinations.  At  the  same  time,  compounds  of  like  kinds  collected  together, 
under  the  control  of  crystalline  forces,  and  assumed  the  form  of  definitely  crystallized 
minerals.  Sediments  that  may  originally  have  been  a  sandy  clay,  composed  of  silica, 
alumina,  and  potash,  mainly,  formed  granites,  gneisses,  or  mica-schists.  The  potash, 
alumina  and  quartz  united  in  part  to  form  orthoclase  feldspar,  or,  in  different  propor- 
tions, together  with  magnesia,  to  form  a  mica,  while  the  excess  of  silica  took  the  form 
of  crystalline  quartz.  The  minor  incidental  constituents  of  the  sediments  entered  into 
these  minerals  as  replacement  elements,  or  as  impurities,  or  formed  distinct  accessory 
minerals.    When,  as  in  some  cases  was  true,  there  was  a  larger  proportion  of  the 


GEOLOr.IOATi  lOXPLOKATIONS  AND  LITEKATUEE.  fi] 

hiisic  iiiiiteriiil,  as  lime,  iron,  etc.,  luniihlciidc  ami  allied  iniiicials  were  formed,  giving 
ris(^  to  syi'iiKic  rocks.  Where  lliesc basic  cleiiieiils  existed  in  stJll  larger  projiorfioiis, 
and  (lie  silica  was  relatively  less  abundant,  hornblendic  and  allied  rocks  were,  Ibrnicd, 
and  in  similar  ways  ot  liei'  variations  in  the  constitution  of  the  sediments  gave  rise  to 
other  variations  in  the  crystalline  results. 

The  changes  wore  not  carried  so  far,  in  most  cases,  a,s  to  destroy  all  traces  of  the 
original  bedding  of  the  sediment,  or  to  mix  the  material  of  adjacent  layers  in  any  nota- 
ble measure.  There  are  certain  massive  portions,  however,  in  which  nearly  all  dis- 
tinct trai-es  of  original  sediinentation  are  obliterated. 

To  conceive  in  detail  of  the  exact  method  by  whicli  these  remarkable  transfor- 
mations took  place,  lays  a  heavy  tax  upon  the  scientific  imagination,  and  certainly 
transcends  the  limits  of  demonstrable  science.  In  general  terms,  however,  the  meta- 
morpliism  may  quite  safely  be  said  to  be  due  to  combined  chemical  and  molecular 
forces,  acting  under  the  conditions  of  (1)  pressure,  (2)  heat,  and  (3)  moisture.  Beyond 
reasonable  doubt  the  strata  in  qirestion  presented  these  conditions,  while  undergoing 
the  distortions  ah'eady  described  (pp.  74-75). 

Igneous  phenomena  of  the  Laur.entian. — The  Laurentian  rocks  are  frequently 
traversed  by  dikes,  veins,  or  irregular  masses  of  intruded  rock.  These  are  most 
commonly  composed  of  granite,  but  are  sometimes  of  the  darker  basic  classes.  It  has 
not  been  determined  how  far  the  i^henomena  may  be  due  to  true  igneous  penetration 
from  below,  and  how  far  to  the  rendering  of  the  rock  of  certain  portions  of  the  series 
sufficiently  plastic  by  heat  and  moisture  to  be  forced  into  cracks  and  fissures  of 
adjacent  x)ortions.  In  either  case  the  essential  nature  of  the  action  was  the  same,  the 
difference  being  in  degree  of  liquefaction  and  the  source  of  material  (pp.  77-78). 

(2)  As  to  a  separation  between  Laurentian  and  Hnronian : 

We  have  Said  that  Laurentian  sedimentation  drew  to  a  close,  but  it  was  only 
because  the  elevatory  forces  just  described  forced  the  beds  up  from  the  ocean,  and 
prevented  further  accumulation  upon  them.  But  sedimentation  elsewhere,  did  not 
cease.  The  wash  of  the  land,  the  wear  of  the  waves,  and  the  settling  of  silts  beneath  the 
sea  continued  ceaselessly.  Even  while  the  great  elevation  was  in  progress,  the  land 
was  being  worn  and  beds  were  accumulating  in  the  adjacent  sea,  and  as  soon  as  it 
reached  its  loftiest  height  it  began  at  once  to  be  cut  down  and  carried  back  to  the  sea 
by  the  agency  of  the  great  leveler,  water. 

Of  the  sediments  formed  during  the  elevation  and  immediately  after — for  a  time 
whose  limits  are  yet  unknown— we  know  nothing.  They  are  deeply  buried  from  sight 
in  our  region,  and  if  their  equivalents  elsewhere  have  been  seen  they  have  not  yet 
been  determined  to  be  such.  So  far,  therefore,  as  the  details  of  the  history  are  con- 
cerned, it  is  an  unrevealed  chapter.  The  record  is  not  destroyed,  as  are  certain  pages 
of  human  history,  but  it  has  not  yet  been  reached  and  read  (p.  78). 


02  THE  PENOKEE  IRON-BEARING  SERIES. 

(3)  As  to  the  Huronian  : 

Synoptical  notes  on  Huronian  formation. — Name  derived  from  lake  Huron,  on  the 
north  side  of  which  the  formation  is  well  developed.  Known  in  Wisconsin  and  Mich- 
igan as  the  Iron-bearing  formation.  Probably  embraces  also  the  great  iron  deposits 
of  Missouri,  New  York,  and  Canada.  Consists  of  a  variety  of  metamorphosed  sedi- 
ments, embracing  quartzites,  limestones,  clay  slates,  micaceous,  hornblendic,  carbona- 
ceous, and  magnetic  schists,  and  diorites  and  porphyries  of  doubtful  origin.  Thick- 
ness 13,000  feet,  more  or  less.  Strata  arched  and  sometimes  folded,  but  not  usually 
closely  crumpled  and  compacted  like  the  Laurentian.  Constitutes  the  Penokee, 
Menominee,  and  Black  river  iron  ranges,  the  quartzite  and  porphyry  outliers  of  central 
Wisconsin,  and  the  quartzites  of  Barron  and  Chippewa,  and  probably  of  Marathon 
and  Oconto  counties.    Existence  of  life  probable.     ...    . 

Huronian  geography.— At  length  the  unrevealed  interval  gave  place  to  a  known 
era.  In  the  progress  of  erosion  and  subsidence  the  sea  advanced  upon  the  Lauren- 
tian lands,  and  separated  from  them  a  large  island  within  our  northern  boundaries, 
and  two  or  three  smaller  ones,  as  it  would  seem,  in  the  adjacent  territory  of  Michi- 
gan.    ... 

Local  characteristics— PenoTcee  region.— AXonfs,  the  Penokee  range  the  Huronian 
beds  are  found  abutting  against  a  wall  of  Laurentian  rock,  which  formed  the  ancient 
shore  line,  and  definitely  marked  the  southern  limit  of  the  primitive  Superior  sea.' 
Here  we  find  a  series  of  Huronian  beds  nearly  13,000  feet  in  thickness.  These  are 
now  upturned  and  metamorphosed,  but  the  history  of  their  formation  remains  for  the 
most  part  legible. 

The  Penokee  series— 1.  Limestone.— The  low«st  member  exposed  to  view  is  a 
crystalHne  magnesian  limestone  130  feet  in  thickness;  the  earliest  limestone  known 
in  our  series.  Its  bedding  and  its  association  with  aqueous  sediment  show  that  it 
was  deposited  beneath  water  as  a  calcareous  sediment.  The  source  of  its  material 
deserves  special  consideration.  The  student  will  perceive,  on  a  moment's  reflection, 
that  neither  the  simple  decay  nor  the  wear  of  the  adjacent  Laurentian  rocks  would 
give  a  material  made  up  almost  wholly  of  lime  and  magnesia,  for  the  Laurentian 
rocks  contain  these  ingredients  only  in  very  subordinate  quantity,  and,  furthermore, 
these  are  among  the  ingredients  removedr— not  left— by  decay.  The  ordinary  sedi- 
ments resulting  from  decay  and  wear  are  clays  and  sands,  not  limestone. 

In  later  ages  there  is  the  clearest  evidence  that  the  great  limestone  formations 
were  made  from  the  calcareous  remains  of  marine  life  in  ways  that  will  appear  more 
clearly  as  we  proceed.  It  is  probable  that  the  ancient  bed  of  limestone  under  con- 
sideration was  formed  in  a  similar  way,  although  no  distinct  traces  of  fossils  have 


1  This  was  not  then  a  vertical  wall  as  it  now  appears,  because  it  has  since  been  disturbed  in  common 
■with  the  Huronian  strata.  But  if  the  latter  be  depressed  *o  their  original  position,  the  Laurentian  slope 
where  observed  would  be  about  SO'^,  which  may  be  taken  as  the  declivity  of  the  Laurentian  shore. 


(iK()L(Hil(JAL  KXi'LOlfATlONS  AND  LITEKATITRB.  63 

yet  beiMi  (liscovorcd  in  it.  It  is  higlily  inagncsian,  and  is  a  dolomite  rather  than  a 
limestone  proper.  It  is  also  impure,  from  the  presence  of  siliceous  and  aluminous 
material. 

JDetrital  beds. — Overlying  this  formation  at  some  points  is  a  bed  of  white  gran- 
ular qnartzite,  which  indicates  that  the  deposition  of  calcareous  sediment  was  followed 
by  an  accumulation  of  ([uartz  sand. 

Upon  this  lie  beds  of  quartz-scliist  and  argillaceous  mica-schist,  having  together 
a  thickness  of  about  400  feet.  These  were  probably  originally  a  deposit  of  sand  and 
sandy,  calcareous,  and  magnesian  clay,  derived  mainly  by  ordinary  wear  and  decom- 
position from  the  adjacent  land. 

Above  these  is  a  thick  series  of  beds  of  iron-bearing  and  siliceous  schists  and 
quartzites,  which  now  form  the  crest  of  Penokee  Iron  range.  These  have  together  a 
known  thickness  of  about  800  feet.  They  appear  to  have  consisted  originally  of  beds 
of  fine  imiiure  sand,  with  lenticular  layers  of  iron  ore  thickly  sandwiched  throiigh 
the  mass. 

Origin  of  the  iron  ore. — The  origin  of  the  siMceous  material  can  be  confidently 
referred  to  the  atmospheric  decomposition  and  the  wearing  and  assorting  work  of 
streams  and  waves  acting  upon  the  granitic  and  other  siliceous  rock  of  the  adjacent 
Laurentian  land.  To  account  for  the  iron  ore  is  less  easy.  It  occurs  (1)  in  thin 
layers,  or  (2)  more  frequently  in  lenticular  masses  a  few  inches  in  thickness  inserted 
irregularly  among  the  laminations  of  the  schist,  and  (3)  in  scattered  particles  dissem- 
inated through  the  rock.  In  its  present  form  it  is  largely  magnetic  ore,  though  the 
specular  variety  is  present.  In  some  places  both  theee  forms  have  been  reduced  to 
hematite  and  limonite  by  subsequent  changes. 

The  manner  in  which  the  iron  is  associated  with  quartzose  material  bears  a 
somewhat  close  resemblance  to  the  way  in  which  magnetic-iron  sands  are  distributed 
through  the  quartz  sand  of  certain  beaches,  as  may  be  seen  at  many  points  on  the 
shore  of  lake  Michigan '  at  the  present  time,  and  as  is  reported  to  be  the  case  on  the 
coast  of  Labrador,  where  the  ocean  is  now  acting  upon  the  same  formation  that  the 
ancient  Huronian  sea  did  in  its  day  in  the  Penokee  region.  This  similarity  suggests 
a  like  derivation — an  explanation  applicable  to  many  of  the  features  of  the  deposit — 
but  it  does  not  very  satisfactorily  account  for  other  characteristics.  It  certainly 
seems  inapplicable  to  some  of  the  great  iron  deposits  that  occur  in  the  Huronian 
series. 

The  most  probable  explanation  of  the  massive  iron-ore  beds  in  general  refers 
their  origin  to  organic  agencies.  Meteoric  waters  charged  with  decomposable 
organic  matter,  percolating  through  the  soil  and  surface  rock,  change  its  iron  ingre- 
dient from  the  insoluble  to  the  soluble  form  and  bear  it  onward,  and  at  length  out 
into  some  adjacent  body  of  water,  into  which  the  drainage  is  discharged.     Here  it  is 

'  Vol.  II,  p.  239. 


64  THE   PENOKEE   IRON-BEARING  SERIES. 

reoxidized  by  free  contact  with  the  ntuiosphcrc  and  precipitated  in  the  insohibleform, 
and  thus  accumulates  in  beds.  Bog  ore  is  now  being-  deposited  in  this  mauuer,  and 
the  ores  of  the  Cliuton  and  Goal  periods  are  gcQcrally  attributed  to  similar  action. 
Little  licsitaucy  would  be  felt  iu  refcrriug  the  Hurouian  deposits  to  the  same  agency 
if  there  were  any  independent  evidence  of  the  prevalence  of  land  vegetation.  There 
is,  as  we  shall  ^ee,  independent  evidence  of  life,  but  it  has  not  usually  been  thouglit 
to  have  been  terrestrial.  Lowland  or  marsh  vegetation  would  probably  furnish  tlie 
requisite  conditions,  and  there  is  no  reason  for  doubting  its  existence,  except  the 
want  of  direct  evidence  of  it  in  this  and  the  succeeding  formations.  Notwithstanding 
this  doubt,  no  equally  satisfactory  explanation  of  the  origin  of  the  massive  iron  ores 
has  been  proposed. 

tSlute.^,  nchistn,  and  diorites. — Upon  the  magnetic  schists  there  repose  a  series  of 
black,  niica-bearing  slates,  alternating  with  diorites  {plagi-horn)  and  schistose  quartz- 
ites,  including  several  horizons  which  are  concealed  by  superficial  maferial  and  whose 
character  is  therefore  unknown.  Among  these  there  appear  to  be  included  those 
horizons  which  in  the  Marcpiette  region  bear  the  rich  iron  ores.  They  are  here 
doubtless  concealed  because  of  their  softness,  owing  to  which  they  have  been  more 
deeply  eroded  by  denuding  agencies.  Whether  these  horizons  are  iron-bearing  here 
remains  to  be  determined  by  actual  removal  of  the  drift.  The  mica-slates  were 
origijially  clay  beds,  probably  containing  some  carbonaceous  matter.  The  schistose 
quartzites  were  siliceous  sandstones  or  quartzose  clays.  What  the  diorites  were 
originally  is  yet  an  open  question,  it  being  maintained  on  the  one  hand  that  they 
are  metamorpliosed  basic  clays,  and  on  the  other  that  they  are  ancient  lava  flows, 
modified  by  long-continued  chemical  action.  This  series  reaches  a  total  thickness  of 
about  3,500  feet. 

Mica-schists. — Above  this  is  found  a  still  thicker  series  of  mica-schists,  which 
were  probably  once  mixed  clayey  sediments.  This  series  now  measures  nearly  8,000 
feet  in  thickness,  making  the  entire  group  of  the  region  embrace,  as  above  stated, 
about  13,000  feet  of  strata. 

It  will  be  observed,  in  glancing  over  the  whole,  that  the  great  mass  of  the  series 
was  formed  from  the  ordinary  sediments  arising  from  rock  disintegration,  and  that 
they  were  un([uestionabIy  derived  from  the  adjacent  Laurentian  land.  The  excep- 
tions to  this  statement  are  found  (1)  in  the  limestone,  probably  derived  from  the 
remains  of  marine  life;  (2)  in  the  iron  ores,  a  portion  at  least  of  which  probably 
arose  through  organic  action ;  and  (3)  possibly  the  diorites,  which  may  have  had  an 
igneous  origin.     (Pp.  80-84.) 

Succeeding  the  period  of  Hurouian  sedimentation,  whether  immediately  or 
somewhat  delayed,  there  was  an  era  of  upheaval  and  metamorphism,  analogous  to 
that  which  occurred  at  the  close  of  the  Laurentian  era.  It  produced  analogous,  but 
less  extreme,  effects. 


OEOLOdlCAIi  KXI'LOlIATlONa  AND  LITEUATURE.  65 

Metamoiyhinm. — None  of  (lie  oiiginal  (Icpiosits  now  renuiin  precisely  in  tluMr 
primitive  contlition,  tlion.uli  only  ;i  p(n'tion  of  (lieiii  liiive  been  so  trinislbiincd  that  the 
orij;iual  static  is  not  dearly  (lisccrnilile.  The  limestone  was  soiuewliat  eompaetedand 
remlored  more  crystallini',  and  seiitterod  erystals  of  tremolito  were  Ibrmed  by  the 
uuiou  of  lime  and  magnesia  with  siliea^ — in  other  words,  were  j;eneratcd  from  a  some- 
whi^t  silieious  portion  of  the  limesoonc.  The  great  sand  deposits  were  transformed 
into  (piartzite,  but,  for  the  most  part,  the  original  grains  and  pebbles  still  remain 
unobliteratcd,  while  in  some  instanees  line  laminations  and  beautiful  ripple  and  rill 
marks  are  excolleutly  preserved,  bearing  the  most  uneipiivoeal  testimony  to  their 
aqueous  origin.  The  iron  ores  associated  with  the  quartzites  and  silieious  schists  are 
now  found  largely  in  the  form  ot  magnetite  or  derivations  from  it.  If  they  did  not 
originally  exist  iu  that  state  (and  they  probably  did  not),  they  were  doubtless  trans- 
formed into  it  at  this  time  of  general  metamorphism.'  Probably  some  of  the  more 
massive  iron  deposits  iu  association  with  clay  and  cai'bonaeeous  schists,  as  those 
of  the  Commonwealth  and  Florence  mines,  were  only  compacted  and  dehydrated. 
Certain  substances  that  accumulated  incidentally  with  the  sand  of  the  series  now 
constitute  accessory  minerals  scattered  through  the  (juartzite,  as  pyrolusite,  novac- 
ulite,  mica,  and  others.  The  various  finer  silts,  clays,  and  mixed  sediments  were 
changed  to  slates  and  schists.  Iu  short,  the  whole  series  was  hardened,  compacted, 
and  in  some  measure  chemically  transformed  aud  crystallized.  The  changes  in 
these  respects,  however,  were  rarely  equal  to  those  of  the  preceding  Laurentian 
revolution. 

Bisturhance  of  heels.— In  respect  to  attitude,  great  changes  took  place.  Beds 
which  lamination,  ripple  marks,  and  other  characteristics  show  to  have  been  essen- 
tially horizontal  when  formed  are  now  found  arched  and  tilted  at  high  angles.  In 
the  Peuokee  region  the  strata  stand  at  angles  varying  from  20°  to  upwards  of  80°. 
In  the  Menominee  region  they  were  warped  and  folded  in  a  stillmore  striking  manner, 
and  stand  at  various  angles,  according  to  situation.  In  ceutral  Wisconsin,  instead  of 
close  folds,  immense  arches  were  formed.  The  Baraboo  quartzite  ranges  are  but  the 
insignificant  remnant  of  the  north  side  of  an  arch  of  gigantic  dimensions,  which 
swept  upward  to  an  altitude  approaching,  if  not  surpassing,  the  highest  existing  ele- 
vatious.i  Similar  broad  arches  were  formed  on  the  western  side  of  the  Laurentian 
island.     (Pp.  89-90.) 

(3.)  As  to  an  interval  between  Huronian  and  Keweenawan  :• 

Between  the  Huronian  and  Keweenawan  periods  an  interval. of  moderate  extent 
appears  to  be  iudicated  by  the  fact  that  the  beds  of  the  latter  repose  unconformably 
upon  those  of  the  former.    The  amount  of  this  unconformity  is,  in  Wisconsin,  but 

'  For  Fig-.,  see  vol.  ii,  p.  506. 
MON  XIX 5 


QQ  THE  PENOKEE  IROISr-BEAEING  SEEIES. 

slight,  though  it  appears  to  be  more  considerable  elsewhere.'  This  interval  was 
probably  entirely  occupied  by  the  disturbance  and  nietamorphism  of  the  Huroniau 
strata  above  described.  Indeed,  there  is  reason  to  think  that  this  was  only  partially 
accomplished  when  the  Kew^eenawan  eruptions  began.  Sedimentary  deposits  must, 
however,  have'  been  in  i)rogress  while  the  slow  upheaval  was  taking  place.  If  we 
could  reach  these  deposits  we  should  doubtless  find  them  in  no  very  essential  respect 
difterent  from  those  which  preceded  and  followed.  Prof.  Selwyu,  director  of  the 
Canadian  Geological  Survey,  as  the  result  of  his  studies  upon  the  equivalent  forma- 
tion at  the  east,  does  not  recognize  any  interval  between  the  two  series,  and  it  may 
be  that  what  is  but  a  moderate  break  in  Wisconsin  is  bridged  by  what  seejns  to  be 
an  essentially  continuous  series  in  the  eastern  region.     (Pp.  94-95.) 

Ieving  (E.  D.).  Lithology  of  Wisconsin,  Geology  of  Wisconsin,  vol.  i,  pt.  2, 
chapter  iii,  pp.  340-361. 

A  general  account  of  the  various  rock  spedes  of  Wisconsin  know^n  to 
date  (May,  1882).  So  far  as  the  Penokee  district  is  concerned  the  newer 
material  here  included  is  the  same  as  that  given  in  the  volume  next  referred 

to. 

Irving  (11.  D.).  The  Copper-Bearing  Rocks  of  Lake  Superior,  Monograph  U.  S. 
Geol.  Survey,  vol.  V,  1883,  29  plates  and  maps. 

This  volume  is  of  course  chiefly  devoted  to  the  Keweenawan  or  Copper- 
Bearing 'Series,  but  incident  to  a  discussion  of  the  relation  of  these  rocks 
to  the  older  formation  is  given  a  brief  summary  of  the  results  obtained  in  the 
Penokee  district  by  the  Wisconsin  Sm-vey  (pp.  391-392).  There  is  also  given 
a  map  (PI.  xxii.  Monograph  v)  of  the  region  extending  from  lake  Gogebic,  in 
Michigan,  to  Numakagon  lake  in  Wisconsin,  and  northward  to  the  shores  of 
lake  Superior,  on  which  the  entire  length  of  the  Penokee-Gogebic  belt  is 
shown,  the  Michigan  end  of  the  belt  having  been  platted  from  the  notes 
on  the  U.  S.  Land  Office  plats  and  from  statements  in  the  papers  of  Pumpelly 
and  Brooks,  above  referred  to.  The  belt  is  also  shown  in*  its  entire  extent 
on  the  general  maps  (Plates  i  and  xxviii.  Monograph  v)  which  cover  the 
entire  lake  Superior  region.  Other  points  discussed  having  bearing  on  our 
present  subject  are  the  relations  of  the  Penokee  series  to  the  Animikie  series 
of  the  north  shore  of  lake  Superior  (pp.  386,  392),  the  general  relations  of 

1  Fuller  data  than  are  given  in  the  Wisconsin  reports,  relating  to  the  unconformity  of  the  Huronian 
and  Keweenawan  series,  may  be  found  in  the  forthcoming  Monograph  of  Prof.  Irving,  on  the  Kewee- 
nawan or  Copper-Bearing  Series,  issued  under  the  auspices  of  the  U.  S.  Geol.  Survey.  See  also  the 
earlier  jiapur  of  Major  Brooks,  Am.  Jour.  Sci.,  3d  series,  vol.  xi,  1875. 


GEOLOGICAL  EX  TLOKATIONS  AND  LITERATURE.  67 

Keweeuiiwau  and  llurouiau  (\)\).  •402-40!)),  and  the  nature  (pp.  37-58), 
(trigin  (p.  144),  distribution  (pp.  231-233),  and  relations  to  the  Peuokee 
series  (p.  16())  of  the  coarse  grained  g-abljros  which  are  so  largely  devel- 
oped in  the  Had  river  country  of  Wiscousin. 

18M-1-. 

RoMlNGKR  (Dr.  (!.).  Geological  Report  on  the  Upper  Peninsula  of  Michigan, 
exhibiting  the  Progress  of  the  Work  from  1881  to  1884.  Manuscript  copy  of  Part  i  of 
vol.  V  (unpublished)  of  the  reports  of  the  Geological  Survey  of  Michigan. 

Through  the  kindness  of  Dr.  Romiugcr,  I  have  been  funiished  with  a 
transcript  of  that  portion  of  this  volume  which  refers  to  the  pre-Keweena- 
wan  rocks.  In  it  are  included  the  results  of  certain  studies  made  by  the 
author  in  the  Gogebic  district,  between  lake  Gogebic  and  the  Montreal 
river.  No  maps  or  other  illustrations  accompany  the  volume.  The  head- 
ings under  which  the  subject-matter  of  the  report  is  classified  are  the  names 
of  certain  of  the  several  rock  groups,  into  which  the  author  had  jireviously 
divided  the  pre-Keweenawan  rocks  of  the  Upper  peninsula  (vol.  iv,  Geol. 
Survey,  Mich.),  viz :  the  Granitic,  Dioritic,  Iron  Ore,  Arenaceous  Slate, 
and  Mica-Schist  groups.  Under  each  of  these  headings  is  given  the  addi- 
tional information  obtained  since  the  publication  of  the  previous  volume 
(iv)  of  reports,  for  the  entire  extent  of  the  Upper  Peninsula  of  Michigan. 

The  following  are  full  extracts  from  those  portions  of  the  report  which 
apply  to  the  Gogebic  district : 

Granitic  groiq). —  .  .  .  The  granites  bordering  the  south  side  of  the  Gogebic 
iron  range  and  of  its  continuation  into  Wisconsin,  the  Penokee  range,  came  under 
my  observation  during  the  progress  of  the  survey.  I  found  them  in  every  respect 
analogous  to  the  granites  of  the  Marquette  country. 

The  rocks  of  that  part  are  not  so  excessively  corrugated ;  the  upheaval  lifted 
the  strata  more  in  continuous  sheets,  and  belts  of  granite  intrusive  into  the  incum- 
bent strata  could  not  often  be  observed,  although  several  granite  seams  cutting. across 
dioritic  schists  were  seen  about  4  or  5  miles  west  from  the  shore  of  lake  Gogebic. 

Following  the  range  the  granite  is  not  always  found  iu  contact  with  the  same 
kind  of  strata.  Locally  heavy  quartzite  strata  are  iu  contiguity  with  it,  the  lower 
layers  of  the  quartzite  being  often  represented  by  a  couglomerate  tilled  with  rounded 
granite  pebbles  or  by  brecciated  quartzose  beds  crowded  with  orthoclase  crystals  and 
cemented  by  a  wax-colored  hydromicaceous  interstitial  mass,  which  rocks  resemble 
granite  so  much  that  it  is  difficult  to  distinguish  the  contiguous  beds.  These  rocks 
correspond  accurately  with  the  rocks  I  have  described  in  the  previous  report  as 


68  THE  PENOKEE  IRON-BBAEmG  SEEIES. 

occurriug  ou  the  contact  Hue  between  the  granite  and  quartzite  formation  in  the 
north  part  of  T.  47,  E.  25,  which  I  then  supj)osecl  to  be  quartzite  altered  by  its  con- 
tact with  the  granite  into  a  granite-like  rock.  Now  I  am  more  inclined  to  consider 
the  rock  as  a  recemeiited  mixture  of  granite  fragments  mingled  with  the  arenaceous 
material  which  formed  the  overlying  quartzite  beds.  Still  it  is  very  singular  that 
the  orthoclase  crystals  copiously  iiubedded  in  the  mass  have  all  sharp  outlines  and 
look  as  fresh,  as  if  they  had  been  formed  where  they  are  and  were  not  debris  of  a  dis- 
integrated granite. 

At  the  above  mentioned  locality,  in  the  NW.  ^  of  NB.  ^  of  Sec.  24,  T.  47,  E.  43, 
this  singular  rock  in  contact  with  the  granite  contains  locally  an  abundance  of  brown 
spar,  which  on  exposed  faces  of  the  rock  weathers  out,  leaving  behind  ochraceous 
matter,  which  tills  the  spaces  formerly  occupied  by  the  spar.     ,    .     . 

Only  a  short  distance  from  the  above  described  locality,  in  the  adjoining  Section 
23,  the  granite  is  seen  in  contiguity  with  dioritic  schists  of  a  brecciated  character, 
which  inclose  large  angailar  blocks  of  massive  diorite  of  various  qualities.  The 
granite  comes  there  also  in  contact  with  massive  diorite  belts  and  intersects  them  in 
dike  form. 

.  .  .  In  Sec.  13,  T.  47,  E.  46,  the  granite  is  found  in  close  proximity  to  cherty 
banded  ferruginous  beds,  inclosing  seams  of  good  iron  ore.  Below  these  iron-bearing 
beds  are  light  colored  kaolinitic  strata,  which  are  in  du'ect  contact  with  the  granite. 

Farther  west,  in  Section  15  of  the  same  town,  the  granite  comes  very  close  to 
the  ore-bearing  quartzite  formation,  in  which  extensive  exploring  pits  are  opened, 
but  I  had  no  opportunity  to  observe  in  this  jjlace  which  sort  of  rock  came  in  con- 
tiguity with  the  granite.  The  exjilorers  informed  me  that  diorite  formed  the  foot- 
wall  of  the  quartzites  in  which  the  ore  deposits  are  found,  and  that  the  diorite  joined 
the  granite  on  the  south  side.  Onward  to  the  west  the  explorers  made  to  me  the 
same  statements,  always  speaking  of  dioritic  rocks  intervening  between  the  granite 
outcrops  and  their  exploring  pits. 

Near  the  Montreal  river,  in  the  NW.  ^  of  Sec.  27,  T.  47,  E.  47, 1  found  granite 
in  immediate  contiguity  with  the  ore-bearing  quartzite  and  banded  jaspery  beds. 

In  Wisconsin,  above  the  island  in  the  Gogogashugun  river,  a  belt  of  schistose 
dioritic  rocks  intervene  between  the  granite  and  the  large  succession  of  light  colored 
slaty  rocks  which  form  the  island.  Above  these  slaty  rocks  are  quartzite  strata 
partly  brecciated  and  interwoven  with  seams  of  limonitic  iron  ore.  From  here  to 
Penokee  gap  I  did  not  make  any  observations  regarding  the  contact  line  of  the 
granite  with  other  rocks,  but  at  the  Gap,  in  the  bed  of  the  river  under  a  railroad 
bridge,  the  dire«t  superposition  of  crystalline  limestone  inclosing  tremolite  fibers  on 
the  granite  could  be  seen.  Above  the  limestone  succeeded  some  beds  of  quartzite, 
and  then  a  large  series  of  light  colored  silicio-argillaceous  schists,  which  most  likely 
are  identical  with  those  composing  the  island  in  the  Gogogashugun  river.     .    .     . 


C.IOOLlXilOAL   K\l'L()i;.\TI()N'S  AND  LITKRATURB.  69 

Dioritir  ijfoup. —  .  .  .  'I'lic  cxtciisimi  of  I  lie  smvcy  into  Mk;  (Jogcbic  dis- 
trict siiowod  to  me  ii  ix'vl'cct  :iiiiiii)j;y  in  tlio  slnictnrc  of  tlic  lliiroiiiiin  sorios  with  tlic 
iM;ir(liU'tte  or  the  Mt'iioniiiioc  icf^ioii.  Xortli  of  the  s'm"'ti!  range  previously  men- 
tioned, in  many,  l)iil  not  in  ;ill,  iociilitics  a  large  body  of  schistose  and  massive  dioritic 
rocks  overlies  it,  dipping  to  the  nortii,  nnd  forms  the  l)as<^  (Mi  which  the  iion-liearing 
rocks  repose.  iMore  rarely  the  dioritic  I'ock  Ix'lt  is  fonnd  missing  imd  the  iron  rocks 
follow  immediately  above  the  granite.  The  dioritic  rock  grcmp  tliere  amounts  to  a 
considerable  thickness.  Most  of  the  diorites  are  fine  grained,  and  some  of  them  very 
light  colored,  almost  totally  composed  of  granular  phigioclase.  Kocks  of  this  kind  are 
largely  exposed  along  the  north  lino  of  Sec.  23,  T.  47,  E.  44,  associated  with  singular 
compact  rock  belts  of  coarsely  brecciated  structure,  composed  of  large  and  small 
ang-ular  blocks  of  various  kinds  of  diorite  cemented  by  a  seme  interstitial  mass  very 
similar  in  composition  to  the  inclosed  dioritic  fragments.  In  this  cement  numerous 
milky  plagioelase  crystals  of  large  size,  or  also  rounded  concretionary  needles  of 
ieldspar,  have  segregated.  (Jalcspar  likewise  sometimes  enters  freely  into  the  composi- 
tion. The  cementing  groundmass  exhibits  a  distinct  fluidal  structure,  as  if  the  rock 
fragments  had  been  stin^d  into  it  while  it  had  the  plasticity  of  dough.  The  fresh- 
fractured  rock  resend)les  a  compact  porphyritic  diorite,  as  the  color  of  the  rock  frag- 
ments and  the  cement  does  not  differ  much,  but  on  the  weathered  faces  of  the  rock 
the  brecciated  composition  and  the  fluidal  structure  of  the  cement  become  very  obvious. 

Large  bluffs  of  the  same  kind  of  breccia  are  also  exposed  on  the  side  of  the  trail 
near  Mr.  Gillis's  camp  in  T.  47,  R.  43,  along  the  north  line  of  Section  23,-  it  forms  there 
the  foot- wall  of  the  galena-bearing  quartzite  formation,  whose  lowest  beds  are  a  coarse 
conglomerate  of  quartz  pebbles  of  various  color.  South  of  these  outcrops  a  large 
succession  of  massive  and  schistose  dioritic  beds  follows,  then  granite  follows  in  close 
contact  with  them. 

Iron  Ore  group.—  .  .  .  The  eastern  portion  of  the  range,  extending  from 
lake  Gogebic  to  the  Montreal  river  across  the  center  part  of  the  ranges  44,  45,  4G, 
and  47,  of  the  Town  tier  47,  did  not  prove  to  be  much  charged  with  iron  ore  this  side 
of  Sunday  lake,  but  west  of  it  so  far  as  Montreal  river  it  was  found  to  be  richer,  and 
in  a  number' of  localities  iron  ore  of  a  very  good  quality  and  in  paying  quantities  is 
so  far  demonstrated  to  be  ]n-esent;  but  no  actual  mine  has  yet  been  opened,  as  these 
remote  places  must  first  be  brought  into  communication  with  the  outside  world  by 
the  construction  of  a  railroad,  which  is  surveyed  but  only  partly  built  at  this 
time.    .    .    . 

The  ore-bearing  strata  displayed  in  the  Menominee  region,  on  the  north  side  of 
the  Quinnesec  ore  range,  are  in  all  probability  a  perfect  counterpart  of  those  of  the 
Gogebic  region.  Here,  as  well  as  there,  a  large  belt  of  limestone  forms  the  base  of 
the  series;  the  ore  in  both  localities  is  to  a  great  extent  limonitic  ore;  in  both  places 
are  graphitic  schists  associated  with  the  ore  deposits;  and  in  the  Penokee  region 


70  THE   PBNOKEE   IRON-BEARING   SERIES. 

the  immediate  succession  of  the  mica-schist  formation  above  the  ore  formation  is  a 
further  indication  of  the  younger  age  of  this  group.     ... 

■  Starting  from  the  landing  on  the  west  shore  of  Gogebic  lake,  situated  in  the 
center  of  Sec.  17,  T.  47,  R.  43,  on  an  old  Indian  trail,  we  meet  for  the  first  two  miles 
no  rock  exposures;  thence  repeatedly  in  the  hillsides  to  the  left  of  the  path  bluft's  of 
rock  are  seen  to  project,  which  on  examination  are  either  granite  or  the  brecciated 
diorite  rock  mentioned  in  the  previous  chapter;  farther  on  qnartzite  beds  are  seen  to 
underlie  the  surface,  on  the  right  hand  side  of  the  trail,  as  we  approach  the  mining 
camp  of  Mr.  Gillis,  situated  in  the  SW.  4  of  Sec.  14,  T.  47,  R.  43,  where,  by  natural 
and  artificial  denudation,  we  are  enabled  to  see  a  cross  section  of  about  800  or  900  feet 
of  strata,  which  dip  at  a  high  angle  to  the  north.  Tlie  aforesaid  brecciated  dioritic 
schists  are  seen  in  the  hillside  south  of  the  camp  as  the  lowest  rocks ;  on  them  sncceedsa 
belt  of  dark  conglomerate,  composed  of  quartz  pebbles  of  various  color,  and  of  granite 
pebbles  cemented  by  an  arenaceous  gronndmass,  in  which  is  a  considerable  amount 
of  feldspar  grains  besides  the  quartz  sand.  Then  follow  thick-bedded  gray  qnartzite 
layers;  on  them  rest  a  flesh-red  colored  compact  granular  quartz  belt,  which  by 
exposure  weathers  and  becomes  porous  and  absorbent,  like  an  ordinary  sandstone. 

Higher  still,  are  brecciated  qnartzite  layers,  composed  partly  of  chalcedonic 
quartz  masses,  and  intersected  by  irregular  fissure  seams  filled  with  galena. 

On  this  brecciated  qnartzite  belt  follow  thinly  lamlTiatcd  quartz  layers  of  very 
uneven  surface,  with  interposed  narrow  wedge  like  seams  of  black  shaly  material, 
which  causes  rapid  disintegration  of  this  belt  into  shelly  fragments.  Within  this 
series  occur  streaky,  interrupted  concretionary  seams  parallel. with  the  stratification, 
which  are  filled  with  galena. 

Higher  beds,  likewise  mainly  of  quartzose  nature,  are  even-bedded,  delicately 
striped  or  lineated  in  the  direction  of  the  bedding  by  the  alternating  intermixture  of 
linear  graphitic  seams,  with  the  granular  ([uartzose  feldsjjathic  gronndmass,  which 
besides  liolds  a  good  proportion  of  the  carbonates  of  lime  and  of  iron.  Weathered 
surfaces  of  the  white  and  black  striped  rock  are  therefore  rusty  brown.  In  some  of 
these  "layers  the  shaly  graphitic  material  predominates  over  the  quartzose,  which 
causes  them  to  be  softer  and  more  pliable;  the  upheaving  pressure  therefore  folded  them 
throughout  their  substance  into  innumerable  small  wrinkles,  as  we  often  observe  this 
same  phenomenon  of  corrugation  in  still  softer  sericitic  or  micaceous  schists  in  other 
geological  horizons.  North  of  these  beds  follow  uniformly  black  fine  grained  slate- 
rock  layers,  which  are  from  time  to  time  interlaminated  with  seams  of  harder  siliceous 
ledges,  likewise  black  colored  by  carbon.  The  aggregate  thickness  of  this  uppermost 
graphitic  slate-rock  belt  amounts  to  about  500  or  600  feet.  It  comes,  on  the  north 
side,  iu  direct  contact,  with  the  diabases  of  the  copper-lsearing  rock  group,  which 
appears  to  be  conformably  superimposed  on  it.     .     .     . 

The  galena-bearing  quartz  formation  and  the  graphitic  slate  series  above  it  are 
traceable  by  extensive  exposures  in  the  hillsides  along  the  trail  until  they  cross  the 


(}|'X)U)(il(;AI.   lOXI'LOlIATIONS  AMJ   IJTEKATURE.  71 

Presiitio  Islo  river,  in  SI'l.  \  ol'  Sw.  IT,  (he  bt'tl  of  which  strciiin  is  (sarved  tiiore  diajjo- 
iiiilly  across  the  stratilicatioii  into  thi'  hhwk  graphitic  slah's,  which  form  here  also  a 
ver\'  thick  succession  of  lieds. 

A  inih-  lartlierwcst.  in  tiie  SE.  :^()t'.Sec.  18,  T.  47,  M.  4.i,  I  mot  for  the  lirst  time  with 
outcrops  of  (lark  puriile-cqlored  banded  (luartzite.  beds,  formed  of  alternating  seams 
richly  impregnated  with  specular  ore  grains,  and  others  of  a  more  purely  (juartzose 
composition.  The  rock  belt  to  which  these  strata  belong  is  exi)osed  in  the  bed  of  a 
small  creek,  but  the  exposures  are  too  limited  to  otter  a  cross  section  giving  informa- 
tion of  the  thickness  of  this  belt  and  of  tlie  rock  adjoiiiing  it;  but  as  the  trend  and 
dip  of  the  beds  are  iu  couforniity  with  the  grai»hitic  slates  and  the  galena-bearing 
quartzites,  it  is  probabU'  that  they  belong  approximately  to  the  same  geological  hori- 
zon. West  of  this  creek  the  ti-ail  follows  the  south  line  of  Section  18,  and  t'len  of  13 
and  14  in  the  adjoining  township  as  far  as  the  Little  Presque  Isle  river. 

The  quotations  thus  given  from  Dr.  Roniinger  under  the  head  of  the 
Iron  Ore  group,  refer  essentially  to  those  slaty  quartzitic  and  partly  ferru- 
ginous rocks  which  lie  to  the  east  of  the  Presque  Isle  river  and  north  of  the 
o-reat  area  of  brecciated  greenstone-schist  subsequently  described.  (See 
Plates  II  and  x.)  On  account  of  the  difficulties  in  reading  the  structural 
relations  of  this  belt  the  quotations  are  made  quite  full.  The  remainder  of 
Dr.  Rominger's  remarks  under  this  heading  consist  only  of  unimportant 
descriptive  details  with  regard  to  the  exposm-es  of  ferruginous  rocks 
between  the  Little  Presque  Isle  and  Montreal  rivers. 

In  addition  to  the  foregoing,  the  following  general  remarks  are  woi'thy 
of  quotation  here,  since  they  define  Dr.  Rominger's  position  as  to  the  rela- 
tive ages  of  the  granitic  rocks  and  the  iron -bearing  slates  which  rest  upon 
them.  As  already  noted.  Dr.  Rorainger  had  previously  published  the  opin- 
ion (Geological  Survey  of  Michigan,  vol.  iv,  p.  6)  that  the  gTanitic  rocks  of 
the  Upper  Peninsula  of  Michigan  are  all  newer  than  the  various  schistose 
and  slaty  rocks  of  the  region  into  which  he  conceived  them  to  have 
erupted — as  did  also  Foster,  Whitney,  and  Wadsworth  before  him — and  to 
have  produced  the  crumplings  and  alterations  these  schists  now  present 
us  with.  Subsequently  to  the  publication  of  these  views  certain  consider- 
ations were  urged  on  Dr.  Rominger  by  the  present  writer,  going  to  show 
that  these  granitic  and  gneissic  rocks  are  really  older  than  a  large  portion 
of  the  schistose  and  slaty  rocks ;  for,  while  they  have  invaded  a  portion  of 


72  THE  PENOKEE  IRON  BEAEING  SEEIES. 

these,  the}"  have  yet  furnished  to  another  portion  an  abundance  of  frag- 
mental  material. 

The  remarks  quoted  below  are  in  no  part  a  reply  to  these  considera- 
tions. It  Avill  be  seen  that  Dr.  Rominger,  while  retaining  his  position  in 
the  main,  has  yet  modified  it  so  far  as  to  accept  an  older  granite  in  addition 
to  the  areas  which  he  regards  as  newer  tlian  the  schists  adjacent  to  them. 

In  a  previous  report,'  which  commences  with  a  description  of  the  geological 
structure  of  the  environs  of  Marquette,  I  have  spoken  of  the  occurrence  of  large 
areas  of  granite  some  tlistance  north  and  south  of  the  city  and  of  the  intermediate 
space  from  4  to  5  miles  in  width  as  being  occupied  by  a  large  body  of  massive  and 
schistose  dioritic  rocks.  These  in  turn  are  succeeded  upward  by  argillitic,  chloritic, 
and  hydromicaceous  schistose  layers,  inclosing  lenticular  seams  of  hematitic  iron  ore, 
■which  on  their  part  are  overlain  by  a  large  quartzite  formation  and  l)y  still  higher 
beds  of  siliceous  limestones  iuterstratified  with  argillitic  or  hydromicaceous  schists  of 
various  color,  some  of  them  intensely  impregnated  with  hematitic  iron  oxide. 

All  these  strata  I  described  as  being  steeply  upheaved  in  a  constant  axial  direc- 
tion from  east  to  west,  and  as  excessively  folded  and  corrugated,  suggesting  as  the 
principal  cause  of  these  disturbances  the  uprising  of  the  granite  into  a  synclinal 
trough  compressing  the  incumbent  sedimentary  layers. 

I  farther  stated,  that  particularly  the  lower  dioritic  portion  of  the  rock  beds 
inclosed  within  this  trough  was  found  intermingled  with  belts  of  granite,  partly 
parallel  to  the  stratiiication,  partly  transverse  to  it,  from  which  circumstance  I  inferred 
the  intrusive  nature  of  these  belts,  and  suggested  that  this  intrusion  occurred  con- 
temporaneously with  the  upheaval  of  the  granite  into  a  trough,  and  that  part  of  it  at 
least  must  have  been  then  in  liquid  or  plastic  condition. 

Generally,  a  solid  crust  of  granite  probably  served  as  a  substratum  on  which  the 
Huronian  sediments  were  laid  down,  but  occasion  is  not  often  offered  to  see  the  rocks 
in  contiguity  well  enough  exposed  to  allow  a  discrimination  as  to  whether  such  con- 
tact is  an  original  primary  one  or  resulted  from  subsequent  dislocation. 

The  existence  of  granite  as  a  surface  rock  at  the  time  the  Huronian  sediments- 
formed  is  proved  by  the  occurrence  of  belts  of  granite  conglomerate  and  breccia  in 
different  horizons  of  the  series. 

A  large  belt  of  conglomerate  formed  of  rounded,  water-worn  granite  pebbles  and 
schistose  rock  fragments,  cemented  by  a  matrix  of  similar  schistose  material  is  seen  in 
contact  with  a  granite  belt  in  the  south  half  of  Sec.  2,  T.  48,  11.  26,  but  this  instance 
is  not  a  satisfactory  example  of  the  deposition  of  sediments  inclosing  debris  of  the 
underlying  rock,  as  the  granite  pebbles  in  the  conglomerate  are  totally  diifereut  from 


1  Geol.  Survey  of  Mich.,  vol.  iv,  1881,  pp.  13-19,  22-39. 


(IKOLOCICAL  HXI'LOUATKJNlS  AND   r.lTIOItATlTRE.  73 

tlio  iiii(U'rlyiiif;j;r;init(',  wliicli  is  a  porithyritic  kind  largely  coiiiixtsed  of  iicryi)lo(Tys- 
tiillinc  R^lsitic  ^Liioimdiiiass  iiurlosiiit;-  (|iiai't./.  f;raiiis  and  oitlioclasc  crystals  of  larjjp.r 
si/.c.  This  ix'cuiiar  variety  orjiiaiiite  is  tyjtical  for  tlic  sinallci'  intrusive  belts,  and  most 
likely  the  .i;ianit<' in  this  casiM-ann^  in  contact  with  the  coiiiiloineratc  lielt  by  intiusion. 
Better  proof  tor  the  deposition  of  Ilnronian  sediments  on  a  base  of  t;niniteis  furnished 
by  another  locality  in  the  SH.  \  of  Sec.  32,  T.  -17,  K.  2fl,  where  several  knobs,  (ientrally 
coini>osed  of  massive  granite,  are  surrounded  by  a  mantle  of  coarse  granite  breccia, 
with  a  well  laminated  quartzose  material  as  a  cement.  This  breccia  is  (lonformably 
succeeded  by  a.  series  of  steel  gray  color(>d  shiiung  liydro-micaceons  slate  rocks  inter- 
laminated  with  heavy  belts  of  ligiit  colored  compact  ipiartzite.     .     .     . 

The  upheaval  of  the  granite  and  its  intrusion  into  the  overlying  strata  occurred 
in  all  probability  near  the  termination  of  thenuronian  period,  as  we  And  the  granite 
in  contact  with  any  of  the  Huronian  strata,  up  to  the  youngest,  and  these  always  in  a 
dislocated  position.     .     .     . 

The  dislocation  of  the  Huronian  beds  is  not  exclusively  due  to  the  upheavaland 
intrusion  of  the  granite,  as  numerous  other  intrusive  rock  belts,  dioritic  or  diabasic, 
intersect  the  granite  as  well  as  the  incumbent  beds. 

Whitney  (,I.  D.)  and  Wadsworth  (M.  B.).  The  Azoic  System  and  its  Proposed 
Subdivisions.  Bull.  Mus.  Oomp.  Zool.,  Harvard  Coll.,  whole  series  vol.  Vii  (Geological 
series  vol.  i),  1SS4,  pp.  505. 

Part  I  of  this  work  is  a  critical  "Synopsis  of  the  Evidence  on  which 
the  Rocks  of  the  Azoic  System  have  been  varioiisly  grouped  into  Distinct 
Divisions  by  American  Geologists,"  and  occupies  most  of  the  volume,  taking 
up  in  a  geographical  order  all  that  has  been  written  to  date  on  the  Archeau 
or  Azoic  geology  of  Canada  and  the  several  states  of  tlie  United  States 
within  wliich  these  ancient  rocks  come  to  the  surface.  Part  ii,  which  is  a 
"R(^sum(i  and  General  Discussion",  in  which  the  Authors  offer  "a  brief 
synopsis  of  the  conclusions  at  which  we  have  arrived  in  the  study  of  these 
older  rocks." 

To  any  geologist  who  had  concerned  himself  with  the  study  of  the 
pre-Cambrian  formations  in  the  years  immediately  preceding  the  appear- 
ance of  this  volume,  it  must  have  become  very  evident  that  sonae  sort  of 
a  critical  review  of  all  that  had  been  written  with  regard  to  these  ancient 
rocks  was  quite  necessary  to  future  progress  in  investigation.  For  ^aore 
than  one  reason,  what  had  been  written  prior  to  this  time  had  rep.iited  in 
the  greatest  of  confusion.  In  the  first  place  the  very  nature  of  the  rocks 
themselves  were  such  as  to  baffle  most  of  the  attempts  of  the  older  geolo- 


74  THE  PENOKBB  lEON-BEAEING  SERIES. 

gists  at  reaching  an  understanding  of  even  their  mineral  composition. 
Most  of  those  who  had  written  upon  them  up  to  this  time  had  been  unac- 
quainted with  the  newer  petrographic  methods,  which  indeed  are  only 
recently  reaching  any  very  satisfactor}-  development  so  far  as  the-  study  of 
these  difficult  rocks  is  concerned.  Then  again,  the  structural  problems  pre- 
sented by  these  rocks  ai-e  always  among  the  most  difficult  the  geologist  has 
to  deal  with.  Exposed  as  they  have  been  to  the  disturbing  forces  of  all 
the  enormous  lapse  of  ages  since  their  first  production,  their  occurrence  in 
anything  like  their  normal  position  is  the  rare  exception.  Faulted,  folded, 
squeezed,  internally  altered  in  every  possible  way,  and  intruded  in  every 
fashion  by  every  sort  of  eruptive  material,  the  difficulties  in  the  way  of  a 
correct  understanding  of  even  a  small  area  of  these  ancient  rocks,  are  often 
well  nigh  insurmountable.  Certainly  they  are  to  be  •  overcome,  only  by 
the  most  minutely  accurate  structural  work.  Approaching  them  from 
different  points  of  view,  the  geologist  who  has  acquired  his  experience 
among  the  unaltered  sediments  and  he  whose  ideas  have  been  developed 
mainly  among  the  more  modern  eruptives,  unrestrained  by  the  accurate 
knowledge  obtainable  only  through  the  modern  petrographic  methods,  and 
thi'ough  exact  structural  investigation,  have  amved  at  the  most  opposite 
conclusions  with  regard  to  the  structural  relations  and  genesis  of  these 
ancient  rocks. 

A  careful  sifting  of  all  that  has  been  written  upon  this  subject  was 
certainl)-^  very  desirable — a  sifting  which  should  not  attempt  to  destro}'  all 
information  gathered,  but  which  should  trj-,  so  far  as  possible,  to  separate 
what  is  inference  only  from  what  was  a  certain  result  of  accurate  observa- 
tion. In  the  case  of  the  present  work,  however,  it  becomes  very  evident 
within  the  first  few  pages  that  the  authors  have  a  distinct  theory  to  advo- 
cate with  regard  to  the  pre-Cambrian  formations,  a  theory,  in  fact,  for 
many  years  past  advocated  by  the  older  of  the  two  authors.  This  theory 
is  simply  that  the  pre-Cambrian  rocks  constitute  a  truly  Azoic,  confused 
intermingling  of  sedimentaries  and  eruptives,  which,  though  covering 
in  their  production  an  immense  lapse  of  time,  are  not  diAasible  on  any 
.  correct  geological  principle  into  chronologically  distinct  terranes.  All 
evidence  found  that  had  "been  jjresented  in  opposition  to  this  view  is  criti- 


(lEOLOGlCAL  KXl'LOliATlOISIS  AND  LlTKliATUJlE.  75 

cised  out  of  cxisteiicc,  aiul  tliis  ol'teii  with  pungency  and  lack  of  a  scientific 
spirit,  or  if  it  docs  not  \i((l(l  so  easily  W)  destructive  criticisms  is  disinisj:ied 
with  derision.  On  tlie  other  hand,  all  statements  of  previous  authors 
which  seem  in  an}'  way  to  support  tlic*  \i('w  aih'ocated  are  accepteil  with  a 
readiness  which  at  times  amounts  to  credulit}-.  Tliat  very  much  was  found 
among  the  older  writings  which  could  be  rejected  goes  without  saying, 
])articularl}'  so  since  these  older  writers  had  not  become  possessed  of  any 
of  the  modern  petrographical  knowledge,  aiid,  having  only  done  the  best 
that  the  development  of  the  science  enabled  them,  ver}-  frequently  and 
naturally  fell  into  serious  mistakes.  So  generally,  indeed,  has  the  opposi- 
tion of  writers  to  the  view  (jf  the  authors  of  this  volume,  or  an  occasional 
unavoidable  mistake,  been  seized  upon  for  their  condemnation  that  nearly 
all  geologists  who  had  previously  written  upon  the  pre-Cambrian  forma- 
tions, save  the  authors  themselves,  are  included  in  the  general  censure.  A 
satisfactory  critical  review  of  all  that  has  heretofore  been  published  on  the 
Archean  formations  of  America  therefore  remains  still  to  be  written. 
Unfortunately,  however,  it  doQs  not  now  seem  probable  that  such  a  review 
can  be  prepared  at  an  early  date,  for  the  author  must  not  only  be  unbiased 
and  read}'  to  go  where  trutli  leads  him,  but,  in  ord(Jr  that  the  review  niay 
have  any  considerable  value,  he  must  have  had  an  unusually  wide  experi- 
ence and  must  have  the  time  to  verify  on  the  ground  the  statements  of  the 
various  writers.^ 

The  following  quotations  from  Part  i  include  all  that  has  direct  refer- 
ence to  the  Peuokee-Gogebic  district  (pp.  495-497): 

Passing  now  to  the  Azoic  rocks  of  Wisconsin,  we  tiud  tluit  in  1876  Mr.  E.  T. 
Sweet  pointed  out  a  supposed  unconforraability  between  the  Laurentiau  and  Huroiiiau 
at  Penokee  gap,  stating  (Trans.  Wis.  Acad.,  vol.  ill,  1S75-'7G,  pp.  4'5-44): 

"When  the  railroad  cut  is  completed  at  this  locality  the  absolute  junction  of 
Laurentiau  and  overlying  Huronian  will  doul)tless  be  exposed.  There  can  be  no 
doubt  of  the  unconformability  of  these  formations,  approaching  eacli  other  as  they 
do  with  a  jjersistent  opposite  dip  aiid  somewhat  dift'erent  strike.     Unc(uiformability 

'The  above  paragraph  is  left  es.seiitially  as  it  was  written  hy  the  late  Dr.  Irving.  I  know  that 
now  Dr.  Watlsworfch,  a.s  .state  geologist  of  Michigan,  is  working  on  the  an<-ieut  rocks  of  lake  Superior 
in  a  systematic  anil  careful  manner.  This  later  Avork  has  led  Jiim  to  modify  or  abandon  many  of  liis 
earlier  views  concerning  them   as  well  as  the  pre-Cambriiin  of  other  regions. — C.  R.  Van  Hise. 


76  THE  PENOKEE  IRON-BEAKING  SERIES. 

has  been  shown  to  exist  between  the  Laiirentian  and  Huronian  in  Michigan,  but  this 
is  the  first  time  it  has  been  proved  in  Wisconsin." 

Of  the  same  supposed  unconformability  at  Penokee  gap  Prof..  R.  1).  Irving 
remarks  (Am.  Jour.  Sci.,  3rd  ser.,  vol.  xiii,  1877,  p.  308) : 

"The  crystalline  rocks  of  Wisconsin  include  unquestionably  two  distinct  ter- 
ranes,  the  one  lying  unconforniably  upon  the  other,  as  is  beaixtifully  shown  at 
Penokee  gap,  on  Bad  river,  in  the  lake  Superior  country.  Here  a  white  siliceous 
marble  of  the  Hm^onian,  overlaid  by  hundreds  of  feet  of  distinctly  bedded  slaty 
rocks  and  dipping  northward,  is  to  be  seen  within  20  feet  of  large  ledges  of  dark 
colored  amphibolic  gneiss,  whose  bedding  planes  dip  southward  and  strike  in  a  direc- 
tion diagonally  across  that  of  the  more  northern  beds.  There  are  no  doubt  instances 
where  the  two  series  are  difficult  to  separate,  similar  rocks  occurring  in  both  groups, 
but  the  existence  of  the  two  is  incontestable." 

In  the  third  volume  of  the  Geology  of  Wisconsin  (pp. ,  94,  98,  108,  116,  117, 
248-250)  accounts  of  the  unconformability  of  the  Lau.rentian  and  Huronian  are  given, 
but  the  kind  of  contact  when  seen  was  not  observed.  But  if  the  Laurentian  rocks 
are  eruptive,  then  of  course  there  would  be  unconformability.  The  proof  advanced 
was  that  the  foliation  of  the  granite  and  gneiss  dipped  at  a  different  angle  from  that 
of  the  Huronian  rocks.  Here,  as  in  the  case  of  the  Keweenaw  series,  tlie  Wisconsin 
geologists  failed  to  take  into  account  the  conditions  necessary  to  prove  their  points, 
while  Prof.  Irving,  without  giving  any  evidence  of  value,  made  out  a  beautiful  fault  — 
on  paper — at  the  Penokee  gap.  So  far  as  can  be  judged  from  the  evidence  presented 
by  these  geologists,  it  ai)pears  that  they  have  in  Wisconsin  the  same  structure 
as  exists  in  the  Azoic  of  Michigan,  namely,  a  series  of  mixed  sedimentary  and 
eruptive  rocks. 

Prom  the  following  extracts  it  will  be  readily  seen  that  there  are  no  other  tlian 
lithological  grounds  for  assigning  these  rocks  to  the  Huronian  and  Laurentian;  that 
they  are  two  distinct  formations  they  entirely  fail  to  prove.     .     .     . 

In  1880  Professor  Irving  gives  as  the  reasons  for  assigning  the  rocks  which  are 
placed  in  the  Laurentian  in  Wisconsin  to  that  system,  their  "  close  lithological  sim- 
ilarity— the  only  marked  difference  being  the  absence  of  crystalline  limestones  in  the 
Wisconsin  area — of  similar  structural  relations  to  the  Huronian,  Keweenawan,  and 
Lower  Silurian  systems,  and  of  probable  direct  continuity  with  the  Canada  Lauren- 
tian through  the  upper  peninsula  of  Michigan  and  underneath  the  waters  of  lake 
Superior."     (Trans.  Am.  Inst.  Min.  Eng.,  vol.  viii;  1880,  pp.  480,  481.) 

Of  the  Huronian  in  the  same  article  it  is  stated  (p.  483) : 

"The  rocks  of  this  series  have  been  called  Huronian  by  Brooks,  and,  in  the 
writer'sjudgment,  correctly  so,  on  account  of  their  sinularity  to  the  Canada  Huronian, 
with  which  they  not  improbably  have  a  direct  connection  underneath  the  Silurian  of 


(;i:()lA)(il<'AI>   KXI'LOUATIONS  AM)   MTKKATIIKK.  77 

tho  e;i.storii  piii't  of  the  pcniiisul;i,  Iml  nim-c  especiiilly  bccai.'si'  Miey  evidently  0(!cupy 
the  siiiiie  ^'eological  interval  as  the  ty[>ical  (Jaiiadiaii  .scries,  exhiliitinj;'  the  same  noii- 
coulorniit^y  with  an  uiuleilyiTin-  {^neissie  and  granitie  system." 

It  appears,  then,  thali  tlie  only  evideuec  that  the  Wisconsin  f;colof;ist.s  have  that 
llie.  Lanreiitiaii  and  lluronian  are  what  Miey  pnrjiort  to  be  is  Iitlio]o};ieal;  and  they 
have  advanced  no  sonnd  aryiimeut  showing-  that  they  form  distinct  ages  in  the  Azoic 
system.  The  relation  of  the  two  snpposed  series  is  net  that  which  is  seen  when  the 
Paleozoic  comes  in  contact  with  the  Azoic,  or  what  it  would  be  naturally  were  the 
Uuroiiian  laid  down  on  tlie  i»reexistiug  Laureutian.  The  contacts — when  these  con- 
tacts have  been  tigured — appear  rather  to  be  those  made  by  eruptive  rocks  with 
prior  existing  ones.  The  geologists  before  mentioned  have  assumed,  not  proved,  the 
sedimentary  metamorphic  origin  of  all  the  rocks  in  question,  and  on  the  correctness 
of  that  assumption  depends  their  argument.  They  have  failed  to  observe  the  phe- 
nomena of  the  contact  when  seen  beyond  the  mere  fact  of  a  different  dip  to  the 
foliation  observed.  In  fact,  they  have  failed  to  prove  any  of  the  points  essential  to 
establishing  their  conclusions. 

Since  the  questions  of  the  unconforraabiHty  and  fauh  at  Penokee  gap 
and  of  the  general  separability  of  the  Penokee  Iron  series  from  the  more 
southerly  gneisses  are  fully  discussed  in  subsequent  pages  of  the  present 
volume,  it  will  not  be  needful  to  consider  here  'at  any  length  the  criticisms 
above  quoted.  I  may  merely  say  in  the  first  place  that  the  Penokee  fault 
appears  to  us  entirely  demonstrated  by  the  facts  presented  on  the  maps  and 
in  the  text  of  vol.  iii  of  the  Geology  of  Wisconsin;  so  that  any  question 
as  to  the  existence  of  this  fault  becomes  a  question  as  to  the  presentation 
of  facts  and  not  as  to  the  correctness  of  the  conclusions  drawn  from  them. 
Again,  I  may  say  that  in  case  the  stratiform  arrangement  of  the  gneiss  at 
Penokee  gap  is  a  foliation  (pressure  result)  its  discordance  with  the  sedi- 
mentation plane  of  the  overlying  slaty  series  is  sufficiently  good  evidence 
of  an  unconformity;  and,  finally,  that  the  general  unconformable  position 
of  the  Iron  series  of  this  region  with  regard  to  the  more  southern  rocks 
seems  completely  estabhshed  by  the  facts  presented  in  the  following  pages. 

Irving  (R.  D.)  and  Van  Hise  (C.  E.).  On  Secondary  Enlargements  of  Mineral 
Fragments  in  certain  Eocks.    Bulletin  of  the  U.  S.  Geol.  Survey,  No.  8,  1884,  5,6  pp. 

Among  the  special  rocks  with  which  the  general  conclusions  of  this 
pamphlet  with  regard  to  the  origin  of  quartzite  are  fortified,  are  from  the 
Penokee  region  vitreous  quartzite,  quartzite-schists,  mica-schists,  and  gray- 


78  THE  PENOKEE  IBOl^r-BEARING  SERIES. 

wacke,  the  quartz  fragments  of  these  several  rocks  bemg  shown  to  have 
received  secondary  enlargements  in  optical  contiguity  with  the  original 
grains,  subsequently  to  the  aggregation  of  the  rocks.  Since  the  publica- 
tion of  this  pamphlet,  which  was  the  first  announcement  of  the  existence 
of  this  very  general  and  widespread  mode  of  induration  of  rocks,  our 
experience  has  only  confirmed  the  conclusions  presented,  showing  us  not 
only  these  enlargements  are  to  be  widely  met  with  in  the  Penokee  district, 
but  more  than  that,  that  quartzite  fragmental  rocks  of  all  ages  are  found 
to  be  much  more  rarely  without  these  enlargements  than  with  them. 

188S. 

WiNCHELL  (N.  H.).  The  Crystalline  Rocks  of  the  Northwest.  Address  before 
Section  E.,  Am.  Assoc.  Adv.  Sci.,  at  Philadelphia,  September,  1884.  Proceedings, 
33d  Meeting,  pp.  366-379. 

In  this  address  Prof  Winchell  "calls  the  attention  of  Section  E  to' 
some  of  the  interesting  problems  that  beset  the  geologist  who  undertakes 
to  study  the  crystalline  rocks  of  the  Northwest,  and  especially  that  part  of 
the  Northwest  which  is  included  in  the  state  of  Minnesota,"  and  aims  at  a 
concise  review  of  "  the  broad  stratigraphic  distinctions  of  the  crystalline 
rocks  that  have  lately  been  studied  in  Michigan,  Wisconsin,  and  Minnesota 
by  the  aid  of  the  published  results  of  the  surveys  of  Brooks,  Wright, 
Irving,  Rominger,  Pumpelly,  and  others,"  who  undertook  to  formulate  a 
generalized  statement.  To  this  he  adds  also  "such  published  results  and 
unpublished  field  observations  from  Minnesota  as  may  be  furnished  by  the 
survey  of  that  state,  in  order  that  the  scheme  may  cover  coiTectly  the 
crystalline  rocks  of  the  entire  Northwest." 

The  following  table  indicates  the  six  groups  into  which  Prof  Winchell 
would  di^dde  all  of  the  rocks  of  the  Northwest  which  belong  below  the 
Copper-beaiing  series: 


(iEOLOGlCAL  EXl'LOKATION.S  AND   LlTIOIIATIHiK. 


79 


Groups. 

Knuivalonts  in 
MicbiKan. 

I'lipiivalonts  in 
Wisconsiu. 

Equivalents  in 
Minnesota. 

•  irniiii     1,     Granite     anil 
Syiiiito  with  r.:it)l)n). 

XX 

1  anil  la  at -lilackriver. 

Duhith. 

lirnlo  Mountain. 
Misi|uah  hills. 
Keavor  bay. 

(ironp  II,  Mica-Schist. 

XIX  at  MnniucMr. 
XVII-XIX  at  Menomi- 
nee. 

XX-XXIIatl'<'nokec. 

Litth'.  falls. 
I'ikc  rapids. 
Outlet     of    V(^rmilion 
lake. 

Grou]>  III,  Carbouaceons 
and  Arenaceous    Black 
Slate. 

XIV-X  VII  at  Marquette. 
XV  and  XVI  at  Menom- 
inee. 

VI-XVI  at  Penokee. 

Aniniikie. 
Black  slates. 
Grand  portage. 

Group  IV,  Hydrouiicaand 
Maguosiau  Slate. 

VI-XIV  at  Marquette. 
VI-XI  at  Menominee. 

IV-VI  at  Penokee. 

At  "  The  Mission." 
Vermilion  lake. 
Verndlion  Iron  mines. 

Group  V,   Quartzite  and 
Marble. 

V  at  Marquette. 
II-V  at  Menominee. 

I-III  at  Penokee. 

Ogishke-Muncie  lake. 

Group    VI,   Granite  and 
Gneiss  with  Honhlendic 
Gneiss. 

Laurcntiau. 

Laureutian. 

Laureutian. 

This  table  shows  also  what  members  of  the  different  series  that  have 
been  described  as  occurring  in  Michigan,  Wisconsin  and  Minnesota,  cor- 
respond with  each  one  of  these  groups.  It  does  not  appear  that  Prof  Win- 
chell  would  regard  these  several  groups  as  necessarily  separated  by  time-gaps 
from  one  another,  but  merely  as  certain  lithologically  distinct  horizons,  to 
which  all  of  the  rocks  of  the  lake  Superior  region  may  be  referred.  In  the 
original  tabulation,  of  which  the  above  is  a  partial  copy,  Prof.  Winchell 
gives  also  four  other  columns,  one  of  which  indicates  the  equivalency  of 
these  six  groups  to  the  several  divisions  of  the  pre-Potsdam  rocks  of  New 
England  as  recognized  by  Emmons.  The  other  columns  headed  respect- 
ively Hunt,  Brooks,  and  Irving,  give  the  relations  of  these  six  groups  to  the 
divisions  recognized  by  the  geologists  named.  As  to  the  relation  of  any  of 
these  rocks  to  any  of  the  several  groups  recognized  by  Emmons  and  others 
in  the  East,  we  do  not  propose  now  to  say  anything,  but  merely  to  restrict 
oiirselves  to  a  few  remarks  with  regard  to  equivalencies  indicated  in  the  table 
for  the  lake  Superior  region  itself     To  begin  with,  we  can  by  no  means 


80  .  THE  PENOKEE  lEON-BEAEING  SREIES. 

accept  the  six  groups  of  tlie  first  column  even  as  a  mere  general  strati- 
graphic  succession.  Group  i,  called  "  Granite  and  Syenite  with  Gabbro," 
shoiild  rather  read  gabbro  with  some  reddish  acidic  rocks.  Here  belong 
the  enormous  masses  of  gabbro  which  appear  in  the  Bad  river  country  of 
Wisconsin,  and  over  a  large  area  in  northern  Minnesota  at  the  base  of  the 
Keweenaw  series.  That  these  rocks  belong  rather  to  the  Keweenaw  series 
itself  than  a  separate  group,  is  indicated  both  by  their  lithological  similarity 
to  that  series,  and  the  very  striking  unconformity  which  they  present  with 
regard  to  the  lower  slaty  rocks,  both  in  northern  Minnesota  and  the  Peuokee 
district. 

The  second,  or  mica-schist  group,  is  hardly  a  valid  one  unless  the  term 
mica-schist  be  made  to  include  an  enormous  mass  of  rocks  Avliich  is  not 
ordinarily  covered  by  such  a  name.  It  is  true  that  mica-bearing  schistose 
rocks,  quite  different,  however,  from  those  completely  crystalline  mica- 
schists  which  belong  at  a  much  lower  horizon  in  the  lake  Superior  region, 
are  met  with  in  certain  places  in  the  lake  Superior  region  at  the  summit  of 
what  has  ordinarily  been  called  the  Huronian  series ;  but  these  micaceous 
schistose  rocks  are  after  all  a  mere  phase  and  by  no  means  the  predominant 
one  of  a  great  thickness  of  genuine  fragmental  rocks,  which  merge,  both 
horizontally  and  vertically,  into  those  occurring  farther  below,  called  the 
carbonaceous  and  arenaceous  black  slate  group. 

The  fourth  group,  the  hydromica  and  magnesian  slate  group,  has,  so 
far  as  we  are  aware,  no  distinct  existence;  certainly  none  at  any  such 
stratigraphical  horizon  as  here  .indicated.  Hydromica-slates  and  magnesian 
slates  are  here  and  there  stratified  among  the  Iron-bearing  rocks  of  different 
parts  of  the  lake  Suijerior  region,  but  they  never  constituted,  so  far  as  we 
are  aware,  any  continuous  or  well  marked  horizon. 

The  fifth  group,  which,  however,  has  certainly  no  separate  and  dis- 
tinct existence,  is  fau-ly  well  represented  in  the  Marquette,  Menominee,  and 
Penokee  districts,  though  Prof.  Winchell's  reference  to  this  one  horizon  of 
the  Huronian  series  of  Canada  has,  we  think,  no  basis. 

The  sixth  group  correctly  enough  includes  those  granites  and  gneisses 
which,  with  certain  schists,  also  belong  throughout  the  lake  Superior 
country  beneath  all  the  stratiform  rocks. 


GEOLOGICAL  EXPLORATIONS  AND  LITERATURE.  81 

Turning'  now  to  tlir  cDliimns  slmwinj;-  tlid  e([iiivalent.s  of  tliese  several 
gi'oups  in  Michigan,  Wisconsin,  and  Minnesota,  \v<'  note  in  tlie  first  place 
that  Brooks's  horizon  No.  xx,  which  is  iiuuh'  especially  for  the  granite  south 
of  tlie  Menominee  rivei'  in  northern  Wisconsin,  appears  to  us,  on  the 
contrary,  to  belong-  to  the  lowest  one  of  Winchell's  groups;  in  other  words, 
to  be  nothing  more  or  less  than  a  part  of  the  great  basement  series  upon 
which  the  great  thickness  of  stratiform  rocks  was  originally  spread;  that 
numbers  1  and  1  a  of  the  Black  river  succession,  Wisconsin,  belong  simi- 
larly ^to  this  low  liorizon  ;  that  while  we  would  put  tlie  Dulutli  gabbro  and 
the  Brule  m()untain  red  rocks  at  the  horizon  given  (noting,  however,  the 
total  dissimilarity  of  tliese  red  rocks  to  the  granitic  rocks  just  referred  to), 
the  reddish  rocks  of  Beaver  bay,  on  the  other  hand,  are  at  a  very  much 
higher  horizon,  well  u^j  in  the  Keweenaw  series;  that  numbers  xx  and  xxii 
of  the  Penokee  series  are,  as  they  are  followed  eastward  from  the  gap, 
found  to  consist  mainly  of  but  little  altered  fragmental  rocks,  the  mica- 
schist  being  in  fact  an  alteration  phase  of  a  merely  fragmental  graywacke; 
and  that  the  Aiiimikie  series,  taken  as  a  whole,  appears  to  us  to  have  a 
thickness  several  times  as  great  as  indicated  by  Prof.  Winchell,  and  to  cor- 
respond to  the  whole  thickness  of  those  rocks,  which  on  the  south  shore  of 
lake  Superior  intervene  between  the  gabbro  and  the  Lower  Huronian 
rather  than  to  be  equivalent  to  so  small  a  portion  of  theni. 

Irving-  (R.  D.).  Divisibility  of  the  Arcliean  iii  the  Northwest.  Extract  from 
Address  as  retiring  President  of  the  Wisconsin  Academy  of  Sciences.  Delivered 
Decembet  30,  1884.  Published  iii  the  Am.  Jour.  Sci.,  od  ser.,  vol.  xxix,  1885,  pp. 
237-249. 

Tliis  paper  presents  in  brief  certain  arguments  for  a  belief  in  the 
divisibility  of  all  of  those  rocks  which  in  the  Northwest  lie  beneath  the 
base  of  the  Keweenaw  series  into  two  wholly  distinct  groups,  separated 
from  one  another  by  a  great  unconformity,  the  uppermost  of  the  two  being 
in  its  turn  separated  by  a  great  discordance  from  the  Keweenaw  or  copper- 
bearing  series.  These  unconformities  indicate  the  intervention  between  the 
several  groups  of  periods  during  which  prolonged  denudation  of  land  sur- 
face was  being  carried  on.  These  two  groups  are,  beginning  below:  (1) 
The  great  basement  complex  of  gneiss,  granite,  and  various  schistose  rocks; 

•  MON  XIX 6 


82  THE  PENOKEE  IRON-BEARING  SERIES. 

(2)  the  Iron-bearing  slate  series,  in  large  measure  composed  of  little 
altered  fragmental  rocks.  The  uppermost  of  these  successions,  it  is  main- 
tained, is  plainly  enough  the  equivalent  in  general  of  the  original  Huronian 
sei'ies  of  Canada,  and  should  be  so  called.  To  the  lower  series  we  may 
for  the  present  apply  the  Canadian  term,  Laurentian.  This  lower  group 
may  or  may  not  be  further  divisible  into  subordinate  numbers ;  and  indeed 
there  are  here  and  there  indications  of  subordinate  breaks  in  the  upper 
series,  but  for  the  present  the  only  distinct  and  widely  applicable  classifica- 
tion to  be  made  is  that  above  indicated. 

The  proofs  cited  in  favor  of  the  conclusions  presented  in  this  paper  are 
drawn  in  very  large  measure  indeed  from  the  Penokee  district.  Since  the 
same  facts  are  presented  much  more  fully  in  the  pi-esent  volume,  it  is  not 
desirable  to  repeat  them  in  this  connection  with  any,  detail.  We  merely 
copy  from  the  paper  the  tabulated  statement  that  proofs  of  unconformity  in 
the  Penokee  district  were  found. 

1.  lu  the  iniiuuei'  in  which  the  regularly  succeeding  belts  of  the  higher  series 
traverse  the  courses  of  those  (jf  the  lower. 

2.  In  the  strong  coutrnst  between  the  two  series  as  to  rock  kinds,  the  bedded 
members  of  the  lower  series  having  arrived  at  a  nearly  complete  recrystallization, 
while  th(jse  of  the  higher  are  but  little  altered. 

3.  In  the  highly  folded  and  contorted  condition  of  the  lower  series,  as  contrasted 
with  the  unfoliled  condition  and  simple  stratigraphy  of  the  higher. 

4.  In  the  striking  contrast  between  the  contacts  of  the  granite  with  the  lower 
schists  and  with  the  higher  slates,  the  former  being  invaded  by  it  in  an  intricate 
manner,  the  latter  never  when  the  two  come  together.  (Granite  iu  veins  and  inter- 
secting masses  occurs  among  the  upper  mica-schists  of  the  Penokee  series  (see  map), 
but  this  always  of  a  different  character  from  the  granite  at  the  southern  contact, 
■which  has,  as  yet,  never  been  found  to  intersect  the  slates.) 

5.  In  the  discordant  laminations  of  the  two  sets  of  rocks  when  seen  iu  contact 
or  close  i3roximity. 

6.  In  the  occurrence  iu  the  upper  series,  not  only  at  horizons  above  the  base, 
but  also  at  iioints  on  the  contact  line,  of  abundant  detrital  material  derived  from  the 
lower  series. 

Irving  (R.  D.).  Preliminary  Paper  on  an  Investigation  of  the  Archean  Forma- 
tions of  the  Northwestern  States,  Fifth  Annual  Report  U.  S.  Geol.  Survey,  1885, 
pp.  175-243. 


GKOLOGIOAI.  EXPLOIIATIONH  AND  LITERATUEE.  83 

This  paper,  as  the  title  indicates,  is  merely  preliminary  to  a  general 
study  ot"  the  pre-Kevveeuawan  formations  of  the  lake  Superior  country, 
recently  begun  by  the  writer,  it  g•i^■es  a  general  outline  review  of  what 
was  known  with  regard  to  these  formations  at  the  time  of  the  beginning  of 
the  investigation,  and  also  of  the  new  material  gathered  to  the  date  of  the 
paper  by  the  author  and  his  assistants.  Although  the  Penokee  series  is 
several  times  referred  to  in  this  publication,  there  is  nothing  that  needs 
especially  to  be  quoted  here. 

1886. 

Wright  (Charles  E.).  The  Ajiogebic  Iron  Range.  In  Mineral  Resources  of 
Michigan,  1SS5,  by  Charles  D.  Lawton,  Commissioner  of  Mineral  Statistics,  Lansing, 
18S6,  pp.  131-147. 

The  late  Mr.  Wright  gives  a  brief  account  of  the  history  of  exploration 
in  the  range  and  a  detailed  statement  as  to  the  amount  of  development  at 
each  of  the  mining  properties  at  the  end  of  the  year  1885.  For  the  most 
part  these  details  have  little  geological  interest,  but  the  irregular  character 
of  the  ore-bodies  and  their  association  with  the  soap  rock  and  the  under- 
lying quartzite  are  brought  out.  The  only  general  points  mentioned  are 
given  by  the  following  quotations: 

In  1879  F.  H.  Brotherton,  Esq.,  and  party  located,  for  the  Canal  Company,  very 
closely,  the  Huronian  belt  across  ranges  44,  45,  46,  and  47,  T.  47,  and  the  ore  vein 
within  this  belt;  in  fact  all  the  discoveries  of  iron  ore  made  in  the  above  towns  are 
within  a  hundred  feet  or  so  of  the  line  determined  by  Mr.  Brotherton.   (P.  131.) 

Speaking  of  the  Ashland  mine,  he  says : 

The  quartzite  on  the  foot  wall  side  is  in  places  more  like  a  hard-pressed  sand 
bank,  caused  apparently  from  the  decomposition  of  the  matrix  or  cementing  material 
of  the  quartzite.  This  is  not  to  be  wondered  at,  as  the  water  forcing  its  way  down- 
ward naturally  follows  the  junction  of  the  strata.  Assures,  and  joints,  dissolving  a 
portion  of  the  mineral  ingredients  of  the  rocks  it  traverses  and  again  replacing  it 
with  others.  It  is  highly  probable  that  the  purity  of  some  of  the  soft  hematites 
is  due  to  this  very  iirocess,  as  has  been  noted  in  previous  numbers  of  this  report 
Many  of  the  soft-ore  veins  were  originally,  no  doubt,  very  siliceous,  but  "  alkaline" 
water  filtering  through  them  under  pressure,  especially  if  by  any  means  they  had 
Itecome  broken  or  shattered,  would  in  time  carry  away  the  silica  in  solution  and  leave 
the  iron  oxide  and  other  bases  behind. 


84        "  THE  PENOKEE  IRON-BEARING  SERIES. 

Further,  of  the  Ashland  mine : 

One  interesting-  feature  is  the  presence  of  occasional  rounded  bowlders  of 
quartzite  in  the  ore.  Whether  these  bowlders  will  disappear  in  depth  is  a  problem 
that  may  throw  some  light  on  the  origin  of  these  hematite  veins  (pp.  141-142). 

Ill  his  account  of  the  Noirie  mine  he  further  states : 

As  at  the  Ashland  mine,  there  are  large  rounded  bowlders  of  quartzite  in  the 
ore,  and  the  quartzite  next  to  the  ore  is  frequently  disintegrated  into  common  sand 

(p.  144). 

The  accuracy  with  which  the  iron-bearing  belt  was  located,  by  means 
of  magnetic  attractions,  by  the  Wisconsin  Greological  Survey  between  the 
West  Branch  of  the  Montreal  and  the  Montreal  river,  where  there  are  no 
exposures,  has  been  indicated  by  the  fiuotations,  from  its  reports.  It 
appears  that  Mr.  Brotherton,  in  1879,  located  this  belt  in  the  same  manner 
on  the  Michigan  side  of  the  line,  although  no  report  of  his  work  has,  so  far 
as  I  know,  been  published.  The  suggestion  which  Mr.  Wright  made  as  to 
the  ore  of  the  Ashland  mine  being  enrich'  I  by  percolating  waters  is  inter- 
esting as  being  in  conformity  with  the  conclusions  which  a  detailed  study 
has  developed.  The  appearance  of  rounded  bowlders  of  quartzite  in  the 
ore  at  the  Ashland  and  Norrie  mines  appears  to  indicate  that  liere,  before 
the  beginning  of  the  deposition  of  the  layers  of  the  iron-bearing  formation, 
the  nnderlying  quartzite  was  broken  by  erosion. 

Van  Hise  (C.  li.).  Upon  the  Origin  of  the  Mica-Schists  and  Black  Mica- 
Slates  of  the  Penokeo-Gogebic  Iron-Bearing  Series.  Am.  Jour.  Sci.,  3d  series,  vol. 
XXXI,  1880,  pp.  453-459. 

This  paper  presents  facts  and  arguments  going  to  show  that  certain 
mica-bearing  slates  and  schists,  which  in  the  Bad  river  country  occur  in  the 
upper  horizons  of  the  Penokee  series,  are  merely  alteration  forms  of  the 
plainly  fragmental  rocks  which  make  up  the  greater  part  of  these  upper 
horizons.  The  process  by  which  this  alteration  has  been  brought  about, 
and  by  which  in  the  extreme  cases  the  original  fragmental  texture  of  the 
rocks  has  been  nearly  (_)r  (juite  obliterated,  has  consisted  mainly  in  the 
development  of  muscovite  and  biotite  from  the  feldspar  fragments,  a  sepa- 
ration of  silica  taking  place  at  the  same  time.     Besides  this  main  mode  of 


(iKOI.OGlCAL   KXI'LOUATIONS  AND   LITKItATHUK.  Hi) 

chango,  the  autlior  sliows  tli;it  the  pmccss  included  also  tlic  sccoiidaiy 
enliirf^'enu'iit  of  (|ii:irtz  traiiiuciits,  and  occasionally  of  feldspar  fra<>ni(^nts. 
^riic  facts  which  arc  sunuMarizcd  in  this  paper  arc  j^'ivcn  in  detail  in  sid»se- 
(pient  pages  of  the  present  vohnne,  of  wliich,  in  fact,  the  ])aper  was  merely 
an  a<lvanced  announcement. 

It  should  be  said  that,  although  tiiese  conclusions  of  Prof.  Van  Hise  in 
this  paper  have  stood  the  test  of  all  of  our  later  works,  we  have  ne^•er 
yet  been  able  to  trace  Avith  certainty  to  a  similar  origin  completely  crystal- 
line mica-schists  which  belong  to  the  lower  or  basement  series.  Penokee 
mica-schists,  as  also  others  like  them  from  the  Marquette  district  of  Mich- 
igan and  from  the  Mississippi  valley  in  the  vicinity  of  Little  Falls,  are 
perhaps  rather  mica-slates  than  mica-schists;  at  least  they  are  not  highly 
foliated.  The  mica-schists,  which  are  associated  with  the  ancient  gneisses, 
may  or  may  not  have  had  a  similar  origin,  so  far  as  our  observations 
have  gone. 

Irving  (R.  D.).  Origin  of  the  Ferruginous  Schists  and  Iron  Ores  of  the  Lake 
Superior  Region.     Am.  Jour.  Sci.,  3d  series,  vol.  xxxii,  1880,  i)p.  255-272. 

Like  the  preceding  paper,  this  one  also  is  a  preliminary  statement  of 
results,  the  better  proofs  of  which  results-  are  elaborated  at  length  in 
subsequent  pages  of  the  present  volume.  The  paper  argues  that  the 
original  form  of  the  beds  of  the  iron-bearing  horizons  of  the  lake  Superior 
region  was  that  of  a  series  of  thinl}-  bedded  carbonates  interstratified  with 
carbonaceous  shal)'^  layers,  which  were  also  often  impregnated  by  the 
ferriferous  carbonate;  that  by  a  process  of  silicification  these  carbonate- 
bearing  layers  were  transformed  into  vai'ious  forms  of  ferruginous  rocks 
now  met  with  in  this  region;  that  the  iron  removed  from  the  original  rock 
at  the  time  of  silicification  passed  into  solution  in  the  percolating-  waters, 
to  be  redeposited  in  various  places  as  it  became  further  oxidized,  thus 
making  ore  bodies  and  various  iron-impregnated  rocks;  that  in  other  places, 
however,  instead  of  leaching  it  out  more  or  less  completely,  the  silicifying 
waters  seem  to  have  decomposed  the  iron  carbonate  in  place  in  such  manner 
as  to  produce  the  well  known  actinolitic  magnetite-schists  of  these  districts; 
that  the  bodies  of  rich  ore  have  probably  had  different  origins  in  different 
cases,  some  having  originated  from  a  direct  oxidation    in  place  of  the 


86  THE  PKNOKEE  lEOXBEARING  SERIES. 

origiual  carbonate,  while  others  are  redepositions  of  the  dissolved  iron  in 
new  places,  and  the  origin  of  still  others  is  as  j'et  not  understood;  and 
that,  finally,  the  siiicifying-  process  began  before  the  folding  of  these 
formations,  but  continued  afterwards. 

1887. 

Van  PIisb  (C.  R.).  Note  on  tlio  Enlargeinent  of  HornbleiKles  aud  Augites  in 
Fragmontal  and  Ernptive  Ro(;ks.  Ain.  Jonr.  Sci.,  3d  series,  vol.  xxxiii,  18S7,  pp. 
386-388. 

Prof.  Van  Hise,  having  previously  noted  and  described  second  growths 
on  hornblendes  in  certain  fragmental  rocks  of  northern  Minnesota,  in  this 
paper  describes  similar  enlargements  which  occur  in  certain  eruptive  green- 
stones of  the  Penokee-Gogebic  district.  He  shows  tliat  the  secondary 
hornblende  has  in  these  greenstones  attached  itself  at  times  to  a  nralitic 
hornblende  and  at  times  to  even  an  unaltered  augite.  Hornblende  addi- 
tions to  hornblende  crystals  in  eruptive  rocks  had  already  been  described 
by  Friedrich  Becke. 

Irving  (R.  D.).  Is  there  a  Huronian  Group?  Am.  Jour.  Sci.,  3d  series,  vol. 
XXXIV,  1887,  pp.  204-216,  249-263,  365-374. 

This  paper,  which  was  read  before  the  National  Academy  of  Sciences 
at  Washington,  D.  C,  April  22,  1887,  presents  at  some  considerable  length 
a  series  of  arguments,  obtained  from  all  portions  of  the  lake  Superior 
region,  tending  to  establish  the  genuineness  of  the  Huronian  group  for 
that  region.  The  paper  maintains  the  entire  separateness  of  this  group 
from  the  underlying  basement  complex  of  gneiss,  granite,  and  crystalline 
schists  and  from  the  Keweenaw  series  above.  It  maintains  also  the  title 
of  the  Huronian  to  a  rank  equivalent  with  that  of  Cambrian,  Silurian, 
Devonian,  etc.,  and  argues  that  both  Huronian  and  Keweenaw  should  be 
admitted  to  the  geological  column  with  this  rank,  but  that,  since  these 
groups  carry  no  fossils,  they  cannot  be  directly  con-elated  Avith  pre- 
Cambrian  groups  of  geological  provinces  wholly  distinct  from  that  in 
which  they  are  found.  The  only  correlation  that  can  be  safely  made  is  that 
of  all  of  the  clastic  groups  collectively  Avhich  in  any  one  region  intervene 
between  the  Cambrian  and  the  basement  crystallines  with  any  group  or 


gn  mi) 


(IKOI.OCM'.M,   KXri-OltATlnNS   AM)    LITKIi ATUUK.  87 

j)s  fiillini^'  wltliln  tlic  snur  iiitci\;il  in  utlicr  rcj^'ions.  '\\>  cov  cr  tliis 
genei-al  geolog-iciil  intcrxal,  ii;tiiLrl\-,  tluit  lying  iH'tween  tlic  ( ';iinlii'i:ni  iind 
the  basement  i-rystallines,  t.)  w  liicli  it  is  proposed  that  the  term  Archean 
should  be  resti-icted,  the  paper  advocates  the  use  of  tlie  new  name 
A(fUoi()zoi(\  of  e(|ual  rank  ^itli  I'dlfozoir,  MpsozoIc,  etc. 

B[RiciNi5iNE  (John).  Tlie  Irmi  Ores  Kast  of  tlie  Mississi])!)!  River.  U.  S. 
Geological  Survey,  J.  W.  Powell,  Director.  Mineral  Resources  of  tlie  United  States 
for  1886,  David  T.  Day,  Chief  of  Division  of  Mining  Statistics  and  Technology,  pp. 
39-103.  ■ 

Mr.  John  Birkinbine  contributes  the  article  upon  "The  Iron  Ores  East 
of  the  Mississippi  River"  for  this  volume.  In  it  he  gives  quite  a  full  and 
accurate  account  of  the  development  of  the  Gogebic  range,  from  which  the 
following  somewhat  extended  extracts  are  made  (pp.  67-72): 

In  order  of  importance  as  shippers  the  various  districts  comprising  the  lake 
Superior  region  ranked  in  1886  as  follows:  (1)  Marquette;  (2)  Menominee;  (3)  Gogebic; 
(4)  Vermilion;  and  this  order  will  be  maintained  in  1887,  with  the  possibility  of  the 
Gogebic  and  Menominee  ranges  changing  places,  but  it  is  probable  that  these  two 
districts  will  not  vary  greatly  in  their  outputs  for  1887. 

Geographically  the  Gogebic  iron  range  may  be  described  as  running  nearly 
parallel  with  the  southern  shore  of  lake  Superior,  and  about  15  miles  distant  from  it. 
The  Montreal  river  (which  is  the  boundary  between  the  State  of  Wisconsin  and  the 
upper  peninsula  of  Michigan),  flowing  northward  into  the  lake,  cuts  through  the 
range  nearly  midway  between  the  extremes  of  the  present  exploitations,  about  one- 
half  of  the  ore  strike,  as  now  believed  to  be  determined,  lying  in  Ontonagon  county, 
Michigan,  and  the  other  half  in  Ashland  county,  Wisconsin. 

The  occurrence  of  ores  similar  in  character  to  those  of  the  Gogebic  iron  range  in 
lenses  or  pockets  in  the  Marquette  and  Menominee  ranges  naturally  points  to  like 
deposits  in  this  newer  district,  and  there  seems  good  reason  to  believe  that  the  ores 
lie  in  lenses  of  greater  or  less  width  and  depth  throughout  an  ore-bearing  stratum 
confined  by  the  quartzite  hanging  wall  of  what  is  believed  to  be  the  north  vein,  and 
the  foot  wall  of  what  is  usually  known  as  the  south  vein,  with  a  greater  probability  of 
finding  the  ore  in  this  newer  region,  owing  to  the  apparent  persistence  and  regularity 
of  the  foot  wall.  Jiocal  opinions  favor,  however,  and  not  without  reason,  the  existence 
of  two  veins,  although  the  presence  of  two  apparently  distinct  ore  bodies  is  shown,  so 
far,  in  but  few  instances.  The  belief  in  the  existence  of  two  veins  is  based  upon  their 
positions  relative  to  the  foot  wall,  and  also  to  a  greater  percentage  of  mapganese  in 
the  south  vein. 


88 


THE  PBNOKEE  lEOl^ir-BEAEING  SEEIES. 


The  miues  alreadyopeued  and  worked  sbow  a  high  grade  of  red  liematite  ores, 
most  of  which  are  .strictly  of  the  Bessemer  class,  the  balance  of  the  ore  prejiared  for 
shipment  being  rich  in  iron  and  close  to  the  Bessemer  limit  in  jdiosphorus.  While 
some  ores  high  in  manganese  are  inined,  none  can  be  said  to  be  high  in  phosphorus, 
and  it  is  doubtful  if  the  run  of  any  of  the  developed  properties  would  show  2  parts  of 
phosphorus  in  1,000  of  iron.  It  may  be  asserted  as  a  rule  that  where  the  ore  lies  in 
large  masses  but  little  of  it  will  require  sorting,  and  even  in  mining  the  jffoportion  of 
lean  ore  and  foreign  nuvterial  is  iiisigniticant,  except  near  the  confining  walls  or  where 
"horses"  of  rock  occur.  These  "horses"  are  by  no  means  uncommon,  and  are  ftmnd 
in  most  of  the  mines  now  extensively  oi>ened,  but  they  are  not  a  cause  for  discourage- 
ment; for  already  after  .jiassing  through  a  "horse"  ore  has  been  found  below  it,  or  the 
projection  of  a  "horse"  into  the  ore  body  has  apparently  forced  the  ore  in  front  of  it. 
A  fact  of  apparent  similarity  to  tlie  oldei-  i-egions  is  in  the  grouping  of  the  large  pro- 
ducers along  a.  coniparatixely  limited  strike,  and  it  is  i>robable  that  the  (iogebic 
range  will  show  the  great  jiroportioii  of  its  future  shipments  made  from  a  few  large 
mines.     .     .     . 

The  appearance  of  tlie  ores  from  the  various  mines  and  in  some  cases  from  the 
same  mine  differs  materially  both  as  to  color  and  hardness.  The  colors  are  nearly 
black,  blue  black,  bi'own,  and  almost  brick  red;  the  hardness  varies  from  a  soft,  mass 
of  finely  comminuted  ore  to  compact  lumps,  and  occasionally  grape,  needle,  or  kidney 
forms,  with  l)rilliant  surface.     .     .     . 

From  tlie  Gogebic  mines  the  following  amounts  were  shipped  in  the  first  and 
second  years  of  development : 

Production  of  iron  ores  from  the  leading  Gogehic  mines  in  18S5  and  1886. 


1885. 

1886. 

Colby                                                

Tons. 

84,  302 

15,  419 

6,471 

5,  634 

Tons. 
257,  432 
124,  844 
74,  015 
20,  069 
94,  553 
29, 184 
18,  424 
18,  497 
17, 688 
16,  388 

Norrie                                                              

Aurora                                                                  -  -   -  - 

Kiikagon                                 .-               

Pabst 

1,103 

T*iii*itaii 

Total  for  1886 

671,  094 

d 

Between  the  eastern  and  western  extremes  of  ])ractical  exploration  upon  the 
Gogebic  iron  range  the  distance  is  fully  30  miles,  but  the  properties  held  as  iron  lands 
extend  to  the  east  and  west  of  these  extremes.  The  real  work  of  development  to  date 
is  covered  by  a  distance  of  about  20  miles  along  the  ridge. 


(ik()I,(h;i('al  KXi'LoifATioNs  AXi>  ij  rin;  \Tiri;K.  SiJ 

Tlic  iiiosi  (Mslcrii  scctidii  w  liiclMii;i\  l)c  liiirl y  ciiiisidcird  as  open  is  locally  kli"Wli 
as  till'  Sunday  lake  disd'icl,  liaxiii^;  Wakclicld,  .Michij;iui,  as  its  Imsiiicss  (■enter.  Mere 
sfVfral  mines,  notably  tlie  I'.idtlieiton,  Sunday  lake,  and  Iron  Cliief,  sliipix'd  in  the 
iifi'fiTejj'ate  about  .'U,(>()()  tons  ol'ore  in  lS.S(i  from  underijroiind  workings,  and  as  in  tliis 
\  icinity  the  ridn'e  is  less  delined  and  the  .t;round  al  a  lowei'  le\-el,  liic  ex|)l()itation  has 
been  more  exi)ensive  and  ditlienlt  an<l  tlie  i>rojiress  less  niaiked  than  elsewlKue.  The 
ov(^-beariiiy  roeks  iii)i)arently  strike  M'n-oujili  Sunday  lake.  Followinj;'  \\(\st  3  miles 
the  Black  I'iver  cuts  tbi'ougii  the  ridge,  and  tlie  oic  has  been  found  in  this  gap. 
Three  miles  still  farther,  the  highest  ele\ati(ni  of  the  ridge  is  found  (reporteil  as  1,100 
feet  above  lake  Superior  or  1,700  feet  abo\'c  tide)  at  the  Colby  mine,  which  overlooks 
the  town  of  liessemer,  Michigan,  built  on  the  northern  sIojjc  of  the  ridge.  Just  west 
of  the  Colby  mine  the  strike  of  the  ore-bearing  rocks  crosses  a  valley  about  half  a 
mile  wide,  and  then  follows  the  ridg-e  for  about  .^  nnles  to  where  the  main  branch  of 
the  Montreal  river  cuts  through  it,  and  in  the  west(M'n  half  of  this  section  are  found 
three  mines,  the  Aurora,  the  Norrie,  and  the  Ashland,  which,  next  to  the  Colby,  have 
been  the  ])rincipal  producers,  and  whose  aggregate  output  for  the  year  1886  was 
about 320,000  tons.  In  addition  otheis  of  smaller  capacity  are  operated.  After  rising 
from  the  valley  of  the  Montreal  i'iver  into  Wisconsin  the  ore  mines  are  ou  elevated 
ground  for  about  3  miles,  iu  which  there  are  several  producers. 

After  crossing-  the  west  branch  of  the  Montreal  i'iver  there  are  few  more  produc- 
ing mines,  but  exploitation  has  been  carried  beyond  this  stream  for  3  miles,  and 
farther  west  large  syndicates  o\vn  land  on  which  prosjiecting  is  fairly  active. 

The  Colby  mine,  the  best  known  and  largest  developed  working  iu  the  Gogebic 
iron  range,  is  at  present  operated  under  a  lease  which  has  less  than  two  years  to  run. 
The  time  limit  has  undoubtedly  encouraged  the  large  outputs  of  8sl:,302  tons  in  1885, 
the  first  year  of  actual  working,  and  248,810  tons  in  188C.  The  mine,  being  located  at 
the  most  elevated  point  in  the  region,  is  reached  by  a  switch-back  railway  connection 
which  formerly  ran  directly  into  the  open  workings,  and  the  ore  was  dug  and  loaded 
onto  the  cars  which  carried  it  to  Ashland  for  shipping.  While  the  open  pitwork  of  the 
Colby  and  Aurora  mines  are  the  features  of  the  Gogebic  region,  the  deposit  as  found 
in  the  Norrie  and  Ashland  mines  indicates  what  may  be  considered  as  specially  good 
mines,  and  if  one-half  of  those  already  operating  can  reach  the  output  of  these  the 
district  will  be  an  enormous  producer.  As  underground  workings  must  eventually 
be  generally  adopted,  the  operations  of  these  two  mines  will  prove  a  guide  as  to  the 
future  possibilities  of  others  according  to  the  width  of  the  ore  bodies.  In  them  are  also 
found  the  "horses  "  which  add  to  the  uncertainty  of  the  mining  enterprises.     .     .     . 

TVTost  of  the  mines  that  were  on  the  shipping  list  in  1886  have  been  sinking 
and  opening  up  ground  for  the  season's  work.  No  large  deposits  have  been  found 
during  winter  development  except  possibly  at  the  Anvil  and  Ryan.  A  great  many  ex- 
plorations are  being  cari-ied  on,  but  the  most  productive  portion  of  the  range  seems 


90 


THE  PENOKEE  lEOlSf-BEAEIiSfG  SERIES. 


to  be  in  Michigan,  from  the  Montreal  river  to  the  Colby  open  qnarrj'  (11  miles).  Th6 
greatest  depth  \duch  has  been  attained  is  275  feet  at  the  Ashland  mine.  The  ISTorrie 
mine  comes  next,  being  250  feet  deep. 

The  Ashland  mine  has  not  opened  iip  much  ground  on  the  fourth  level,  so  that 
while  it  cannot  be  affirmed  that  the  vein  widens  as  it  increases  in  depth,  still  the  lens 
is  wider  at  the  third  than  at  the  second  level  and  much  longer.  The  third  level  of  the 
Ashland  showed  a  width  in  the  fall  of  1S86  of  145  feet  when  "soap  rock "  was  struck. 
In  May,  1887,  a  cut  made  through  about  15  feet  of  this  (the  supposed  hanging  wall) 
was  made,  and  ore  again  found  that  analyzed  63  per  cent  of  iron  and  0-011  of  phos- 
phorus. A  cross-cut  35  feet  in  this  ore  at  last  reports  showed  no  signs  of  a  hanging 
wall. 

Analyses  of  some  of  the  ores  of  the  Gogebic  region. 


Iron  King  mine : 

North  vein 

South  vein 

Norrie  mine : 

From  stock  pile 

Aurora  mine : 

From  cars 

Germania  mine  :* 

From  stock  pile  No.  l... 

Numher  2 

Pabst  mine  (check  sample) : 

Searle,  of  Hurley 

Haldeman,  of  Colby 

Joilet  Steel  Works 

Camp,  of  Pittsburg. ..... 


Iron. 


Phos- 
phorus. 


W.  J.  Olcott- 


Superior  mine : 

1 

2.  _ 

3 

4... 

Anvil  mine - 

Eyan  mine : 

1 

2 

3 

Hoppenjar  mine  (magnetic  ore) 

Ashland  mine  (average  for  season's  ship- 
ments, 48  cargoes) 


Per  cent. 
60.85 
55.74 

62.83 

62.93 

59.38 
59.70 

58.47 
58.46 


58.38 


64.83 
65.18 
64.25  ! 
59.30  j 
60.00  j 

61.78  j 
64.15 
57.67  ' 
51.49  I 

j 

64.50 


Per  cent. 

0.027 

.034 

.0474 

.0278 

.058 
.056 

.040 
.044 
.031 
.035 

ft. 037 
I  t.036 
[   .036 

.047 
.040 
.054 
.079 
.035 

.050 
.064 
.044 
.0.59 

.047 


Silica. 


Per  cent. 
5.44 
3.47 

5.18 

3.65 


Man- 


Perceut. 

1.30 

12.28 


4.00 


3.88 


3.73 


3.65 


■  Messrs.  Kerr  &  Olcott,  analysts. 


t  Volumetric. 


OEOLOmCAL  EXPLORATIONS  AND  LITEIiATURE. 


91 


Tlif  occiiriciicf  (if  iiiaiif^aiii'st' ciiii  not  Irorii  incscnl  cxiildilations  he  considered 
as  lu'iiiy  at  all  it-Kular,  hut  its  ai)iK'ai'an<'i'  in  what  is  ('ailed  the  south  vein  is  more 
general  than  in  tlie  ore  of  the  so-ealled  north  vein.  Tlie  in-ojjortions  of  this  metal 
vary  from  a  traee  to  33  per  cent  in  quantities  of  ore,  and  si)ecimens  of  pyrolusite  are 
found.  Alumina  is  found  in  most  of  the  ores,  the  am(mnt  varying  from  0-5  to  5  per 
eent,  while  the  sulphur  is  from  0-()3  to  -13  i)er  cent.  Water  to  the  extent  of  5  per 
cent  exists  in  the  hard  ores  and  to  a  greater  amount  in  the  softer  varieties.  Traces 
of  sulpliur,  magnesia,  and  lime  are  also  determined  by  analysis. 

The  Colby  mine  produces  an  ore  wliieh  carries  more  manganese  tlian  most  of 
those  on  the  Gogebic  range,  the  average  composition  of  the  ore  as  shipped  being  as 
follows : 

Average  composition  of  shipments  of  iron  ore  from  the  CMlby  mine,   Gogebic  range. 


Iron 

Manganese  . 
Phosphorus 


Per  cent. 


58.5 
3.5 
.04 


Some  of  the  ore  taken  from  what  was  termed  the  south  vein  carried  as  much  as 
33  per  cent  of  manganese.  Other  analyses  of  the  ore  from  this  mine  show  the 
following : 

Additional  analyses  of  iron  ore  from  the  Colby  mine,  Gogebic  range. 


Iron 

Silica 

Phosphorus 
Ahimina  .. . 
Mansauese . 


No.  1. 


Per  cent. 

58.67 

5.87 

.049 

1.05 

3.49 


No.  2. 


Per  cent. 

59.59 

6.13 

.05 

.51 

8.45 


Mr.  Birkinbine  naturally  at  this  time  sljared  in  the  current  opinion 
that  there  was  in  this  region  two  well  defined  horizons  at  which  the  ore 
occurred  in  lenses.  Suffice  it  here  to  say  that,  as  explained  at  length  later, 
the  ore-bodies  do  not  occur  in  lenticular  but  in  peculiar  shaped  bodies . 
which  have  been  ascertained  to  have  remarkable  relations  Avith  the  under- 
lying quartzite  and  the  associated  soapstones.  The  only  mention  made  in 
this  report  of  the  association  of  the  ore  and  soap  rock  is  an  occurrence  in 
the  Ashland  mine.     Mr.  Birkinbine  notices  that  the  manganese  is  upon  the 


92  l^HE  PENOKEE  IRON-BEAEmrT  SERIES. 

whole  more  prevalent  in  the  deposits  near  the  foot-wall  quartzite.  This 
peculiarity  is  subsequently  used  in  the  explanation  of  the  origin  of  the 
ores. 

Lawton  (Charles  D.).  Tlie  Gogebic  Iron  Range.  Annual  Report  of  the  Com- 
missioner of  Mineral  Statistics  of  the  State  of  Michigan  for  1886.  Lansing,  1887. 
pp.  125-165. 

This  report  contains  a  full  account  of  the  rapid  development  of  the 
mines  on  this  range  during  the  year  1886.  It  describes  in  detail  each  of  the 
important  ore-producers,  and  also  refers  to  the  more  promising  prospects. 
Alread)'  several  of  the  mines  have  been  developed  sufficiently  to  show  that 
they  contain  large  deposits  of  ore.  While  this  report  is  of  great  economic 
value,  it  adds  comparatively  little  to  the  previous  knowledge  of  the  geology 
of  the  district.  The  fact  that  the  dike-rocks  have  a  tendency  to  undercut 
the  ore-bodies  is  noticed,  but  the  generalization  as  to  their  genetic  relations 
was  not  reached. 


1888. 


Lawton  (Charles  D.).  The  Gogebic  Iron  Range.  Annual  Report  of  the  Com- 
missioner of  Mineral  Statistics  of  the  State  of  Michigan  for  1887.  Lansing,  1888,  pp. 
90-114. 

Mr.  Lawton's  report  for  1888  records  the  collapse  which  followed?  the 
great  speculative  excitement  of  the  previous  year.  Notvsithstanding  the 
abrupt  termination  of  speculation,  the  product  of  iron  ore  from  the  range  is 
as  large  as  in  the  previous  jear.  Detailed  accounts  are  given  of  additional 
developments  in  each  of  the  iron  mines,  and  their  condition.  The  fact  that 
the  dike-rocks  form  the  northern  basement  of  the  iron-ore  deposits  is  noted 
of  quite  a  number  of  the  mines.  In  the  description  of  the  Norrie  mine  it  is 
said,  "the  foot-wall  flattens  out  and  extends  a  great  distance  north,  carrying 
the  ore-body  with  it."  Whether  this  is  due  to  a  fault  in  the  foot- wall  at 
this  point,  or  only  represents  the  irregular  configuration  of  the  surface  of  the 
country  at  the  end  of  the  accumulation  of  the  Quartz-slate  member  is  not 
explained. 

BiRKiNBiNE  (John).  Iron  Ore  Mining  in  1887.  U.  S.  Geological  Sm-vey,  J. 
W.  Powell,  Director.  Mineral  Resources  of  the  United  States  for  1887;  David  T. 
Day,  Chief  of  Division  of  Mining  Statistics  and  Technology,  pp.  30-57. 


GEOLOGICAL  KXl'LOIJATIONR  AND  LITEKATUKE. 


93 


The  report  (HI  "Iron  Ore  Mining'  in  ISM?"  is  again  by  John  Birkin- 
bine.  In  his  account  <<(  tlic  (lcvch)|)nient  of  the  Gogebic  range  for  this  year 
he  adds  some  points  of  interest  to  his  former  nccount  of  the  region,  from 
which  the  foHowing  (piotiitions  iu-e  niiide: 

The  Goftfbic  ranyc  in  i\u-  tliiid  .><'ar  of  its  (knelopinent  outstripped  the  Me- 
nominee district  by  .58,001  long  tons,  the  figures  being: 

LoDg  tons. 

Product  of  the  Gogebic  range  in  1887 1,237  704 

Product  of  the  Menominee  range  in  1887 1,198  743 

This  is  accounted  for  by  tlie  mistaken  policy  of  over  capitalization,  which  trans- 
formed tlie  Gogebic  range  into  a  center  for  stoclv  speculation,  rather  than  for  legiti- 
mate iron-ore  mining  enterprises.  The  result  was  that,  with  the  desire  to  realize  on 
the  money  invested,  developments  of  some  mines  were  made  in  advance  of  actual 
requirements  and  without  studying  judicious  methods.  Each  organization 'strove  to 
get  its  ore  to  market  and  to  be  recognized  as  a  shipping  mine,  and  the  competition 
for  lake  freights  forced  them  to  rates  ruinous  to  the  shippers.  These  rates  also 
enc(mraged  all-rail  shipments,  and  the  Gogebic  range  in  1887  increased  its  output 
63-0  per  cent  over  that  of  188(5,  taking  precedence  of  the  Menominee  range.  But  it  is 
probable  that  in  1888  it  will  go  behind  its  older  rival,  for  the  "bubble"  which  floated 
so  many  mining  companies  into  prominence  has  collapsed  and  some  Gogebic  mines 
have  suspended  operation;  the  leases  of  others  have  reverted  to  the  owners  of  the  fee 
on  account  of  defaults  on  royalties,  and  others  which  have  been  opened  by  imperfect 
methods  must  practically  be  developed  anew.  But  the  Gv>gebic  range  will  continue 
as  a  very  important  factor  in  the  lake  Superior  region,  and  will  be  a  large  producer  of 
ores;  in  fact,  under  management  which  seeks  to  win  ore  cheaply  and  maintain  the 
mines,  the  success  of  the  district  is  more  assured  than  Avhen  the  operations  were 
largely  so  regulated  as  to  bolster  the  stock  shares  above  their  intrinsic  value.  The 
four  large  mines,  which  up  to  the  close  of  1887  had  produced  almost  70  per  cent  of 
the  ore  nuned,  give  promise  of  continuing  to  add  to  the  ore  supply  of  the  country  and 
to  maintain  the  Gogebic  range  as  an  important  center  of  iron-ore  mining. 

The  products  of  these  four  mines  in  1887  are  given  as  follows : 

OtUpiil  of  prominent  mines  in  Ike  Goijebiv  district  in  1SS7. 


Quantity. 


Colby  . . 
Norrie. . 
Ashland 
Aurora  . 


Long  Ions. 
258, 518 
217,  254 
175,  561 
159,  252 


94    ,  THE  PENOKEE  IRON-BEAEING'  SERIES. 

Concerning  the  iron-ore  deposits  of  the  Gogebic  range,  Mr.  Richard  A. 
Parker,  m.  e.,,  indorses  the  opinion  that  there  are  not  two  veins.  He  says  of  the  ore- 
bearing  strata  coafined  by  the  red  slates  and  jasper  hanging  wall  that  "there  are  not 
in  any  sense  two  veins;  in  tweaty  or  more  miles  of  developmeat  there  are  but  three 
well  established  lenses  of  ore  lying  to  the  north  of  the  strong  foot- wall  deposits,  which 
have  been  called  north  veins,  and  their  interrupted  occurrence  is  so  rare,  compared  to 
the  continuity  of  more  southerly  deposits  on  the  Laurentian  schists,  as  to  be  scarcely 
suflcient  to  warrant  the  use  of  the  significant  term  'vein,'  which  was  adopted  and 
widely  advertised  by  those  interested  in  stock  operations.  As  for  manganese  being 
made  the  basis  of  distinction  between  two  veins,  there  may  be  instanced  the  continued 
occurrence  of  quite  a  regular  percentage  of  it  in  the  Kakagon  and  Bessemer  mines, 
while  the  developed  properties  upon  either  side  upon  the  same  strike  (Nimikon  and 
Superior  mines)  are  entirely  free  ft-om  it."    .     .     . 

Mr.  Parker  notes  another  point  of  interest  in  the  frequency  with  which  sheets  of 
talcose  matter,  locally  known  as  "soap  rock,"  penetrate  the  ore  bodies.  At  some 
period  of  development  of  all  the  large  mines  these  sheets  have  been  found,  varying 
from  1  to  25  feet  in  thickness.  One  of  the  cleanest  and  most  easily  observed  sheets  is 
seen  as  the  floor  of  the  open  pit  at  the  Aurora  mine.  It  has  a  pitch  about  the  same 
as  that  of  the  ore  lenses,  but  cuts  across  the  deposit  at  right  angles  to  the  dip,  ending 
when  it  reaches  the  foot-wall  quartzite.  It  is  soft,  of  smooth,  even  grain,  and  com- 
paratively free  from  iron  or  iron  stain.  Where  the  ore  came  in  contact  with  it  the 
former  was  decomposed  for  a  foot  or  so,  and  the  analysis  showed  that  it  contataed  a 
higher  percentage  of  phosphorus  than  usual  (pp.  35-38). 

Mr.  Birkinbine  by  this  time  clearly  sees  that  there  is  not  in  any  sense 
two  continuous  veins  of  ore  in  the  region,  as  is  shown  by  his  quotation  . 
from  Mr.  Richard  A.  Parker,  In  this  the  southerly  deposits  are  explained 
as  occurring  on  the  Laurentian  schists.  This  is  in  no  case  correct,  the  most 
southerly  ore  deposits  always  being  found  upon  a  fragmental  quartzite 
which  belongs  to  a  group  of  rocks  of  which  the  iron-bearing  member  is  one 
formation.  What  Mr.  Parker  says  sbout  the  soapstone  in  the  Aurora  mine 
terminating  when  it  reaches  the  foot- wall. quartzite  seems  to  me  improbable. 
He  does  not  give  any  evidence  that  it  does  not  cut  the  foot- wall  quartzite, 
and,  considering  that  it  is  now  known  that  these  soapstones  are  dike-rocks 
and  in  many  cases  do  not  cut  the  foot-wall  quartzite,  it  seems  probable  that 
the  same  thing  occurs  here. 

Birkinbine  (John).    The  Resources  of  the  Lake  Superior  Region.     Transac- 
tions American  Institute  of  Mining  Engineers,  vol.  xvi,  1888,  pp.  168-203, 


GEOLOGICAL  EXPLORATIONS  AND  LITERATURE.  95 

This  additional  account  by  Mr.  Birkinbine  of  tlie  development  of  the 
range  is  accurate,  and  from  it  the  following-  quotations  are  made: 

If  tho  develoi)iii(Mit  n\'  t  he  Vcrinilinii  inmuic  district  is  startling,  that  of  the 
Gojiebic  iron  laiige  in  Wisconsin  ;in(l  MicliigaJi  is  even  ni(M'e  so;  for,  although  the 
existence  of  visible  outcrops  of  ore  was  long  known,  am]  considerable  amounts  of 
money  were  exi)ended  by  some  of  our  large  iroji  comi)anies,  no  actual  exploitation  can 
be  considered  as  having  been  made  until  the  year  1885,  when  railroad  connections 
were  completed  to  Ashlaiul  and  dock  facilities  provided.  During  1884  1,022  gross 
tons  were  sent  from  what  is  now  the  Gogebic  region,  but  in  1885  this  amount  was 
increased  enormously,  and  the  shipments  amounted  to  119,766  gross  tons;  and  in 
1886,  756,281  gross  tons  were  sent  to  market.     .     .     . 

The  development  along  the  apparent  strike  of  the  ore  covers  nearly  twenty 
miles  in  length,  and  active  exploration  is  in  progress  for  as  great  a  distance  both 
east  and  west  of  this  territory,  and  also  upon  a  parallel  ridge  12  miles  south,  the 
latter  being  for  magnetic  ore.  The  center  of  the  present  developed  iron-ore  properties 
is  near  to  the  Montreal  river,  which  forms  the  boundary  between  Wisconsin  and 
Michigan ;  the  largest  producers  up  to  the  present  time  are,  however,  chiefly  in  the 
State  of  Michigan. 

The  ores  in  the  Gogebic  range  differ  from  those  of  the  Vermilion  range  in  being 
softer,  and  therefore  more  easily  mined,  jdelding  less  iron  and  also  less  phosphorus, 
but  carrying  a  greater  percentage  of  manganese,  and  more  moisture.  The  dip  of  the 
Vermilion  ore  is  nearly  vertical ;  that  of  the  Gogebic  ores  approximately  70°. 

The  ores  of  the  Gogebic  range  evidently  lie  in  a  series  of  lenses,  often  connected 
or  in  echelon,  and  the  region  has  attained  considerable  notoriety  from  the  fact  that 
ore  indications  or  actual  deposits  have  been  found  u2:)on  nearly  every  property  along 
the  apparent  strike  of  the  vein  matter. 

The  general  geology  of  the  district  is  explained  by  Mr.  J.  Parke  Channing,  of 
Bessemer.  He  says:  "The  chief  characteristic  of  the  Gogebic  range,  and  that  which 
makes  it  so  easy  to  explore,  is  the  regularity  and  persistency  of  the  formation  and 
the  strongly  marked  character  of  the  footwall,  which  dips  from  45  to  70  degrees  to 
the  north,  being  flattest  near  Sunday  lake.  The  sinking  of  a  shaft  on  the  North 
Aurora,  which  it  is  thought  will  reach  the  continuation  on  the  Aurora  vein  at  a  depth 
of  about  1,200  feet,  will  illustrate  the  faith  in  its  persistency. 

"Their  magnitude  (aside  from  the  occurrences  of  horses  and  fluctuations  in  their 
width)  has  been  aft'ected  in  at  least  one  instance  by  a  dike  which  cuts  clear  across  the 
vein.  In  a  few  cases  dikes  of  this  description  have  been  sunk  through,  and  the  ore 
found  under  them.  Faults  of  the  entire  formation  have  been  suspected  in  one  or  two 
localities,  but  are  not  directly  proven."    .     .     . 

The  Gogebic  ores  carry,  on  an  average,  about  60  per  cent  of  iron  when  dry,  some 
of  the  mining  being  above  that  figure.     Carelessness  in  mining,  particularly  in  open 


96  THE  PENOKEE  lEOIf-BEAEING  SERIES. 

pits,  is  almost  invariably  followed. by  a  decline  in  the  iron-contents.  Althougli  the 
ore  from  a  few  mines  is  almost  too  high  in  phosphorns,  the  average  product  of  the 
district  is  of  Bessemer  grade.  The  ore  <'arries  about  13  per  cent  of  moisture  in  the 
winter,  sometimes  running  as  high  as  15  per  cent,  while  in  the  summer  it  is  from  4  to 
5  per  cent  less,  averaging  7  to  10  per  cent  (pp.  184-187). 

Irving  (R.  D.).  On  the  Classification  of  the  Early  Cambrian  and  pre-Gambrian 
Formations:  A  Brief  Discussion  of  Principles,  Illustrated  by  Examples  Drawn 
Mainly  from  the  Lake  Superior  Region.  U.  S.  Geological  Survey,  J.  W.  Powell, 
Director.     Seventh  Annual  Report,  pp.  365-454. 

Ill  this  paper  Prof.  Irving  gives  a  full  account  of  the  "Unconformities 
of  the  Penokee-Gogebic  region  of  noi-thern  Wisconsin  and  Michigan." 
The  following  is  the  first  paragraph  of  the  discussion : 

No  other  so  striking  example  of  unconformity  between  a  series  of  liighly  tilted 
but  unfolded  strata  above  the  break,  and  a  deeply  folded  series  below,  as  that  afforded 
by  the  Peuokee  region  is  known  to  the  writer.  Indeed,  there  are  in  this  region  two 
notable  stratigraphical  breaks:  one  between  the  iron-bearing  series  and  the  folded 
gueissic  formation  to  the  south  of  it;  another  between  the  unfolded  but  inclined 
iron-bearing  series  and  the  equally  higlily  inclined  Keweenaw  series  to  the  north. 
These  breaks  and  the  terranes  which  they  separate  are  the  counterparts  of  those  just 
described  as  obtaining  north  of  lake  Superior  (p.  423). 

The  proof  of  the  positions  here  taken  are  not  given,  as  the  whole 
question  is  gone  over  in  a  subsequent  chapter  of  this  monograph. 

Wtnchell  (N.  H.).  The  Gogebic  Iron  Region.  In  the  Geological  and  Natural 
History  Survey  of  Minnesota.  Sixteenth  Annual  Report,  for  tlie  year  1887.  St.  Paul, 
1888,  pp.  54-60. 

A  cursory  examination  was  made  of  some  of  the  mines  in  the  Gogebic  iron 
region,  in  order  to  be  able  to  compare  the  features  of  the  rocks  and  the  manner  of 
occurrence  of  the  ore  with  the  iron-bearing  rocks  of  the  Vermilion  iron  range,  and 
some  rock-samples  Avere  collected  for  future  lyicroscopic  comparisons  (p.  54^. 

At  the  Colby  mine  .  .  .  there  is  apparent  no  hanging  wall  or  footwall  except 
the  rock  of  the  country,  and  that  is  a  thin-bedded  siliceous  rock  which  itself  is  almost 
ore  in  some  places,  because  of  the  high  degree  of  ferruginization.  This  siliceous 
material  is  jasperoidal  and  distinctly  bedded  exactly  like  the  bedding  of  stsdimenta- 
tion.  The  base  of  it  all  is  apparently  a  fine  "chalcedonic"  silica,  the  same  as  that  of 
the  jaspilite,  though  in  some,  or  many,  of  the  beds  it  is  a  softer  material,  whicli  may 
be  earthy.     .     .     ,     The  south  wall  of  the  south  Colby  mine  ccmsists  of  a  crumbling, 


(!EOI;Or.l('AL   KXIM-OKATTONS  AND  LITERATTTRE.  97 

coarsely  gra  I  Hilar,  siliceous  saiidsl  one,  (lie  jjraiiis  hcinfisubaiiguliir  after  disintegration. 
In  some  plafcs  it  is  lirni  and  correctly  hears  the  name  of  (|nartzit(^  It  is  stained, 
locally,  with  niucU  or  little  iron,  and  witli  manganese.  Tlu;  "  sllicilicatiou"  process 
seems  here  certainly  to  have  produced  fragmental  silica,  and  sHt)se(piently,  being 
interrupted,  to  have  been  followed  by  the  process  of  ferruginization,  which  stained 
this  sandstone  with  iron  and  manganese,  sometimes  almost  constituting  it  an  iron  ore. 
It  is  uut  possible  to  say  this  ferruginization  is  a  secoiulary,  or  rather  a  third,  step, 
later  than  the  general  ferriiginizing  i)rocess,  and  that  this  (luartzitc  has  aeipiired  the 
iron  by  reason  of  the  accidental  contiguity  since,  for  the  <piartzite  graduates  into  the 
i-ock  of  the  mine,  the  accpxired  substances  (iron  and  manganese)  being  the  same  as  in 
the  real  ore.  The  general  circumstances  of  the  situation  will  not  allow  of  such  a 
separation. 

About  250  feet  farther  north  is  an  opening  known  as  the  North  Colby.  This  is 
a  long  deei>  pit,  where  the  iron  has  been  worked  out  superficially,  and  timbered  shafts 
are  being  prepared  for  deener  mining. 

On  the  spur  track  from  the  Colby  to  the  Valley  mine  is  a  short  cut  in  siliceous 
greenish  and  yellowish  slate,  now  largely  ferruginous.  This  is  sometimes  earthy,  like 
some  seen  at  the  Colby  mine,  but  it  is  very  siliceous  with  "chalcedonic"  silica,  some 
of  it  being  almost  Avholly  white,  although  the  prevalence  of  hydro-mica  and  perhaps 
of  other  flue  grained  mineral  particles  intimately  disseminated  through  the  siliceous 
parts  gives  a  greenish  color  to  even  the  hardest  and  most  quartzose  parts  of  this  rock. 
This  dips  so  as  to  conform  to  the  rock  in  the  Colby  mine,  and  probably  lies  under  the 
foot  wall  of  the  South  Colby.  The  full  thickness  of  the  bedding  here  involved, 
making  allowance  for  the  oblique  direction  in  which  the  road  goes  across  it,  is  about 
250  feet  (pp.  54-55). 

At  the  Valley  mine  .  .  .  in  a  ditch  beside  the  railroad  track  at  Bessemer,  is  a 
small  exposure  of  tlie  bottom  conglomerate  of  the  Cupriferous.  It  contains  numerous 
pebbles  of  quartz  and  of  fine  porphyry  like  the  copper -bearing  conglomerate  at  the 
Calumet  and  Hecla  mine.  The  dip  can  not  be  made  out,  as  the  exposure  is  small  and  the 
pebbles  are  rather  evenly  distributed  throughout  the  whole.  This  conglomerate  must 
lie  in  unconformity  on  the  Gogebic  iron-bearing  rocks,  and  if  it  be  the  equivalent  of 
the  conglomerate  at  Negaunee  (Cascade),  as  it  appears  to  be,  it  indicates  the  greater 
age  of  the  Hurouian  quartzite  (the  New  York  Potsdam  sandstone)  than  the  Cupriierous 
formation,  i.  e.,  than  the  greater  ])art  of  the  Cupriferous,  since  the  Cupriferous  strikes 
east  and  west  near  Bessemer,  forming  a  conspicuous  range  of  hills, of  eruptive  rock 
but  about  a  mile  farther  north,  the  dip  being  such  as  to  cause  it  to  overlie  this  con- 
glomerate. 

The  Aurora  mine,  at  Ironwood,  has  a  sandstone  or  a  soft  quartzite,  rather 
coarse,  identical  with  that  on  the  south  wall  of  the  South  Colby  mine,  for  its  south  or 
foot  wall. 

HON  XIX 7 


98  THE  PENOKEE  lEOJSrBEARING  SERIES. 

At  the  Aurora  mine  au  interesting  observation  was  made  oii  the  "granite" 
which  forms  a  low  hill  range  a  short  distance  south  of  the  mine,  and  its  manner  of 
contact  with  the  rocks  lying  just  north. 

The  low  granite  range,  Avhich  has  been  mapped  as  Laurentian  by  the  Wisconsin 
geologists,  rises  about  50  feet  above  the  mine  and  lies  south  from  the  mine  about  600 
feet.  The  section  of  strata  intervening  between  the  mine  and  the  granite  range  is 
made  up  about  as  follows,  in  southward  (and  descending)  order: 

1.  Iron  ore,  soft  hematite,  100  to  150  feet. 

2.  Sandstone  (sometimes  a  quartzite),  about  15  feet  seen. 

3.  Gray  and  greenish  slates  and  quartzites,  in  beds  from  half  an  inch  to  4  and 
6  inches ;  distinctly  sedimentary,  580  feet. 

i.  Gray  quartzite  like  that  of  No.  3. 

5.  Granite,  hornblendic  and  massive. 

The  sandstone  (No.  1)  forms  the  south  wall  of  the  Aurora  mine.  The  quartzites 
and  slates  (No.  2)  are  not  all  exposed  at  the  mine,  but  at  the  Colby  mine  a  section  of 
250  tVet,  iu  a  connected  exposure,  can  be  seen  along  the  spur  track  west  of  the  niine. 
About  100  feet  of  similar  strata  are  visible  at  the  Aurora  mine.  It  is  partly  assumed, 
therefore,  that  the  whole  interval  of  580  feet  consists  of  the  same  rock  as  seen  at 
the  northern  and  southern  limits.  It  seems  to  be  a  part  of  the  Animike  slates  and 
quartzites  (pp.  55-56). 

The  hornblende-granite  contains  bowlder  forms  of  different  rock  from  the  mass 
of  the  granite,  some  of  them  being  of  some  dark-coh)red  greenstone-like  rock  and 
others  of  some  earlier  granitic  I'ock.  The  great  mass  of  the  granite  is  mainly  liomo- 
geneous,  and  these  bowlder  forms  appear  most  distinctly  on  the  weathered  surface  of 
the  bluff.  When  these  bowlder  masses  were  not  originally  of  greenstone  they  are 
apparent  by  a  blotched  aspect  which  the  granite  presents,  the  blotches  being  caused 
by  some  patches  of  rounded  outline,  much  liner  grained  tlian  the  i.est,  or  by  a  marked 
difference  iu  the  relative  amounts  of  feldspar  and  ipiartz  compared  to  the  same  min- 
erals in  the  most  of  the  granite.  The  bowlder  forms  are,  when  distinguishable,  from 
two  or  thi'ce  inches  in  diameter  to  twelve  incites.  Their  longer  axes  lie  prevailingly 
in  the  same  direction,  but  sometimes  they  have  lost  their  lirst  shape  and  have  been 
drawn  out  into  jjoints  in  one  or  more  directions  or  have  been  a  little  distorted  by 
unequal  i^ressure.' 

This  granite  also  has  a  uniform  rift  or  grain,  brought  out  by  the  constant  or 

prevalent  elongation  of  the  hornblende  crystals  in  the  direction  about  east  and  west. 

The  bowlders  have  their  longer  axes  in  the  same  ixjsition.     It  ajipears,  therefore,  that 

originally  this  granite  was  some  conglomerate,  which  has  been  plasticor  fluid-like. 

At  the  most  southerly  of  the  three  low  bluffs  of  this  granite,  each  of  which  faces 

'  Similar  changes  from  conglomerates  to  gneiss  aro  mentioned  by  Dr.  E.  Hitchcock  in  Vermont, 
in  Am.  Jour,  Sci.,  2d  ser.,  vol.  xxxi,  p.  372. 


GEOLOGICAL  lOXl'LOltATIONB  AND  LITERATURE.  99 

toward  (lie  noidi.  (lie  jrniiiilc  ciiii  hv.  seen,  on  tli.'  north  side,  to  lie  upon,  i»r  at  least 
al<HlJ;■si(U^  oi;  a  jjia.v  (iiiartzitc.  Tliorc  is  an  abrupt  and  simple  contact  line,  with  a 
sudden  transition  hctwe.en  tlicni. 

About  liO  feet  t:<ivther  north  is  tiu^  sc(roTid  low  north ward-tVu;ing  blufl"  of  granite. 
It  here  look.s  as  if  the  granite  had  been  a  molten  rock  and  had  Howed  uucouformably 
over  the  (puirtzitc,  the  dii)  of  which  can  not  here  l>e  ascertained. 

The  third  low  outcroj)  of  tlie  f^ianite  is  about  10  feet  lower  than  the  last  and 
about  30  feet  farther  north.  In  the  north  face  of  this  little  bluff,  not  more  than'6  feet 
hiyh,  is  observable  the  contact  seen  in  Fi«-.  2  of  Pig.  S.  Here  the  grainte  is  uncon- 
formable on  and  embraces  i)icces  and  tongues  and  slabs  of  the  gray  quartzite  and 
quartzite  slates.  This  (luartzite  slate  is  line  grained,  banded  by  sedimentation,  and 
light  greenish  weathering.  It  is  greenish  gray  within.  At  one  point  in  this  little 
bluff  is  a  small  area  of  granite  surrounded  by  crumpled  and  broken  portions  of  the 
slate.  So  at  least  it  appears  on  the  face  of  the  bluff,  but  this  isolated  granite  area  is 
only  superficial  and  doubtless  was,  and  perhaps  is  still,  united  with  the  main  granite 
mass. 

These  three  little  bluffs,  running,  so  far  as  they  have  apparent  extension,  in  the 
same  direction,  do  not  conform  in  their  trend  with  the  direction  of  strike  of  the  bedded 
rocks  that  intervene  betweeu  them  aud  the  Aurora  mine.  They  vary  from  it  about 
20°,  as  illustrated  in  Fig.  1  of  Fig.  8. 

The  interpretation  of  these  facts  and  their  bearing  on  the  stratigraphic  problems 
that  relate  to  the  horizon  of  the  iron  ore  of  the  Gogebic  range  seem  to  warrant  tlie 
following  conclusions : 

(1)  The  granite  acts  the  role  of  an  eruptive  rock,  but  was  originally  a  conglomer- 
ate. It  was  so  far  molten  or  plastic  that  it  flowed  over  the  adjoining  sedimentary 
strata,  but  not  so  completely  fused  as  tg  render  the  resultant  grauite  entirely  homo- 
geneous. 

(2)  The  accompanying  beds  of  sedimentary  rock  being  a  perfect  lithologic  repre- 
sentative of  the  quartzites  and  slates  of  the  lower  part  of  the  Auimike,  this  con- 
glomerate can  be  parallelized,  stratigraphically,  with  the  Ogishke  conglomerate  of 
Minnesota,  in  which  have  been  seen  (Fifteenth  Annual  Report  aud  later  in  this 
report)  similar  semifuf.ed  conditions,  producing  porphyries,  syenite,  and  porphyritic 
conglomerates. 

(3)  The  horizon  of  the  ore  of  the  Gogebic  range  is  probably  that  of  the  Animike 

rocks. 

(4)  The  granite  is' not  of  Laurentian  age,  but  is  younger  in  its  present  condition 
than  the  Animike  slates,  though  (jriginally  a  conglomerate  older  than  those  slates 

(pp.  56-59). 

WiNCHELL  (Alex.).  The  Gogebic  Iron  Belt.  In  the  Geological  and  Natural 
History  Survey  of  Miuuesota;  Sixteenth  Annual  Report,  for  the  year  1887;  St.  Paul, 
1888,  pp.  185-195. 


100  THE  PENOKEE  IRON-BEAEING  SERIES. 

At  the  south  vein  of  the  Colby  mine  the  ore  is  limited  on  the  south  by  a  com- 
pact hematitic  slate,  which  is  light-colored  and  sandstone-like  in  places.  South  of 
the  mine  is  an  outcrop  of  siliceous  argillite,  which  is  interbedded  with  quartzite- 
scliist. 

By  the  railroad  near  Bessemer,  coming  from  the  Valley  mine,  is  an  outcrop  of 
conglomerate,  with  slight  dip  north.  It  is  a  small  exiiosiu-e,  but  in  place,  com])osed 
mostly  of  pebbles  of  dark  red  sandstone,  hematite,  granulite,  and  diabase.  It 
appears  like  a  conglomerate  at  the  base  of  the  Keweenian  (p.  186). 

The  section  at  the  Aurora  mine,  in  descending  order,  is:  Broken  and  mixed  ore. 
Main  deposit  of  ore  dipping  north  at  an  angle  of  about  65o.  Quartzite  forming  the 
foot  wall  in  the  Colby  and  Aurora  mines.  Siliceous  argillite  south  of  Colby  mine — 
seen  250  feet;  this  perhaps  occupies  part  of  the  concealed  space  south  of  the  Aurora. 
Quartzite  and  syenite  gneiss  interbedded  in  the  hill  633  feet  south  of  the  north  face 
of  the  quartzite  in  the  Aurora;  thickness,  595  feet.  Syenite  gneiss  (502)  on  the  hill 
south  of  the  Aurora. 

This  syenite  is  a  heavy  outcro]).  It  contains  some  fragments,  mostly  of  _green- 
stone,  and  only  partly  rounded.  It  reminds  me  of  the  Seagull  and  Saganaga  regions, 
Minnesota.  The  rock  weathers  light  colored.  It  is  granular  and  porous,  varying  to 
compact.  The  feldspar  is  very  pale  pinki.sh,  and  the  hornblende  is  grayish  greenish. . 
It  presents  all  the  characters  of  true  syenite,  showing  no  trace  of  bedding  within  the 
area  subject  to  observation.  But  it  is  evidently  a  fragmental  rock,  since  it  contains 
many  rolled  fragments.  It  furnishes  us  ocular  evidence  that  real  syenite,  with  all  its 
proper  crystalline  characters,  m^y  be  a  rock  of  sedimentary  origin. 

Looking  around,  we  discover  other  evidence  of  its  close  affinity  with  ])roducts 
of  sedimentary  action.  Close  by,  it  overlies  a  true,  fine-grained  quartzite  (503).  The 
contact  is  apparent  in  several  places.  Four  rods  south  of  this  syenite,  a  considerable 
mass  of  quartzite  is  imbedded  in  syenite.  Down  the  hill  a  few  rods  farther,  we  find 
a  vertical  ledge  of  gneiss  including  warped  and  broken  sheets  of  a  siliceous  schist 
(pp.  186-187). 

The  succession  in  the  vicinity  of  Penokee  gap  is  given.  Here  at  the 
south  is  found  hornblende-schist  and  similar  rocks  988  feet  in  thickness, 
which  dip  to  the  south,  and  then  the  Penokee  series  with  3,480  feet  of 
strata  dipping  to  the  north,  including,  from  south  to  north,  viti'eous  quartzite, 
siliceous  slates,  magnetitic  slates,  and  argillites. 

The  firmly  accepted  conclusion  of  the  Wisconsin  geologists  in  reference  to  the 
equivalency  of  the  magnetitic  and  carbonaceous  slates  of  the  Penokee  gap  makes 
them  a  continuation  of  the  hematitic  schists  of  the  Gogebic  range.  That  is,  they  hold 
the  formation  in  both  regions  to  be  Huronian.  That  the  Gogebic  iron-bearing  strata 
are  not  Huronian,  I  feel  prepared  to  afi&rm.    And  I  can  not  resist  serious  doubts  of 


(;E()L()(il('AL  KXI'LOKATIONS  AND  LITERATURE.  101 

tlioir  (Miuivalt'iwo  with  the  Pciiokcc  strata,  whicli  stroiio-ly  impross  me  as  holding 
characters  strikingly  siniiliar  to  those  of  the  Ilnronian  slates  of  lake  irnron  and  the 
Aniniike  slates  of  (Jnnllint  lake  and  Thnnder  bay.  This  resemblance  impressed  me 
from  the  besinninfi';  bn(  I  feci  rclnctant  to  controvert  the  judgment  of  the  Wisconsin 
geologists. 

But  while  I  hold  the  decision  in  abeyiince,  I  take  the  lilterty  to  offer  a  few  points 
for  consideration : 

(1)  We  discover  the  strong  litbological  resemblance  referred  to. 

(2)  The  litbological  characters  are  unlike  those  of  the  ore-bearing  strata  of  the 
Gogebic,  Manjuette,  and  Vermilion  regions. 

(3)  The  ore  also  is  magnetic  instead  of  hematitic. 

(4)  It  is  diffused  through  the  laminated  sheets  of  the  formation,  as  at  Guuflint 
lake,  and  not  segregated  in  lodes,  as  in  the  other  regions  mentioned. 

(5)  A  higher  system  of  black  slates,  apparently  unconformable  07i  the  hematitic 
schists,  appears  to  exist  in  the  Gogebic  and  Marquette  regions,  as  it  certainly  does  in 
the  eastward  prolongation  of  the  Vermilion  schists. 

(6)  At  the  distance  of  15  to  18  miles  in  a  direct  line  SSB.  of  the  Gogebic  range, 
is  a  well  known  line  of  magnetic  attractions,  such  as  are  exerted  by  the  magnetitie 
schists  of  Penokee  gap.  » 

(7)  These  lines  of  attraction,  though  as  far  as  I  know  they  lie  too  far  south,  may 
nevertheless,  when  more  accurately  located,  be  found  in  the  strike  of  the  Penokee 
schists,  especially  if'the  great  exposure  a  mile  north  of  Penokee  station  affords  a 
reliable  indication  of  the  strike;  for  that  is  S.  67°  E.  But  this  is  probably  disturbed 
somewhat  by  the  great  fault. 

Should  the  Penokee  slates  be  identified  with  the  Animike  (true  Huronian)  then 
the  Marquette  or  Kewatin  system  will  be  found  underlying,  and  the  juxtaposition  of 
the  Penokee  slates  with  the  (sup]iosed)  Laurentian  schists  on  the  south,  may  be  due 
to  au  overslide  accompanying  the  formation  of  the  great  fault.  .  If  the  dislocation 
resulted  from  a  horizontal  movement,  instead  of  an  upthrow,  then  the  strata  on  the 
east  side  must  have  slijjped  southward  over  900  feet;  and,  if  the  movement  was  con- 
fined to  the  Penokee  slates,  they  may  thus  have  concealed  a  thin  representation  of 
the  Kewatin  which,  on  the  identification  assumed,  is  at  present,  wanting  at  this 
point  (pp.  194-195). 

Our  own  observations  as  to  the  occurrences  of  the  rocks  south  of  the 
Aurora  mine  differ  radically  from  those  of  the  Professors  Winchell. 
The  granite  south  of  the  Aurora,  described  by  N.  H.  Winchell  as  consist- 
ing of  three  parallel  knobs  interstratified  with  quartzite,  was  found  to  be 
granite  cut  by  fine  grained  dike-rocks.  Winchell's  quartzite-slate  is  then 
a  dike  which  cuts  the  granite.     These  dikes  are  separated  from  the  gran- 


102  THE  PENOKEB  lEON-BEAEING  SEEIES. 

ite  by  perfectly  sharp  lines,  and  their  intrusive  character  is  unmistakable. 
The  granite  is  described  as  at  once  an  eruptive  rock  and  a  conglomerate. 
This  description  is  doubtless  due  to  the  fact  that  in  the  granite  are  dark 
colored  obscure  roundish  areas  which  have  a  pseudo-fragmental  appear- 
ance. These  areas,  composed  largely  of  dark-colored  minerals,  are  very 
common  in  the  ancient  granites  of  the  Northwest  and  may  be  either  actual 
fragments  of  the  more  ancient  crystalline  schists  which  have  been  caught 
in  the  granite  at  the  time  of  its  eruption  and  only  partially  absorbed,  or 
they  may  represent  segregations.  The  north  face  of  the  granite  ledge  is 
brecciated  to  a  certain  degree  and  its  joints  are  filled  with  cherty  and  sili- 
ceous slates  belonging  to  the  base  of  the  Penokee  series.  The  granite 
surface  is  plainly  one  of  erosion,  and  there  is  absolutely  no  evidence  that 
the  granite  has  been  plastic  subsequent  to  the  formation  of  the  slate. 
The  phenomena  are  precisely  those  that  are  always  present  when  a  frac- 
tured and  jointed  granite  constitutes  the  surface  upon  which  a  fragmental 
slate  formation  begins  to  be  deposite.d. 

If  the  above  observations  are  correct,  Prof.  N.  H.  Winchell's  conclu- 
sions that  the  granite  acts  the  role  of  an  eruptive  rock,  but  was  originally 
a  conglomerate  which  flowed  over  the  sedimentary  strata,  and  that  the 
granite  is  not  of  Laurentian  age  but  is  younger  than  the  Animikie  slates, 

are  erroneous. 

Van  Hisb  (C.  E.).  The  Iron  Ores  of  the  Penokee-Gogebic  Series  of  Michigan 
and  Wisconsin.    Am.  Jour.  Sci.,  3d  series,  1889,  vol.  xxxvii,  pp.  32-48. 

This  article  is  a  condensed  advance  account  of  what  is  contained  in 
the  present  volume  with  reference  to  the  position  of  the  ore-bodies  in  the 
Iron-bearing  member  and  their  genesis.  As  the  subject  is  treated  fully 
in  a  subsequent  chapter,  an  abstract  of  this  paper  will  not  here  be    given. 


CHAPTER   TI 


By  C.  11.  Van  Hise. 


THE  SOUTHERN  COMPLEX. 

General.      Geographical   Distribution.      The    Westeru   granite.      The  Western    green    schist.     The 
Central  granite.     The  Eastern  green  schist.     The  Eastern  granite.     Summary. 

General — The  rocks  south  of  the  Penokee  series  (PI.  IT)  are  exceed- 
ingly complex,  both  as  to  theii-  lithological  character  and  structural  relations. 
They  comprise,  first,  unmistakable  eruptives,  fresh  and  in  vai-ious  stages  of 
alteration,  including  diabases,  syenites,  gneissoid  granites,  granites;  and, 
second,  many  different  vai'ieties  of  gneisses  and  schists.  There  are  large 
areas  which  contain  only  massive  rocks,  and  other  large  areas  which  con- 
tain only  schistose  rocks,  except  for  infrequent  cutting  basic  eiiiptives;  but 
between  the  different  areas  are  zones  in  which  are  found  mingled  massive 
and  schistose  kinds  and  apparent  gradations  between  the  two.  The  folia- 
tions of  the  schists  vary  widely  in  dip  and  strike.  More  often  than  other- 
wise they  are  in  rough  conformity  with  the  members  of  the  overlying 
Penokee  series;  but  this  is  true  only  in  a  very  general  way,  the  strike 
frequently  being  almost  or  quite  at  riglit  angles  to  the  strikes  of  those  rocks. 
Further,  the  strikes  vary  widely  within  short  distances,  presenting  a  strong 
contrast  in  this  particular  to  the  rocks  to  the  north.  If  this  variation  in 
strike  is  noticeable,  the  variation  in  dip  is  still  more  remarkable,  inclinations 
in  opposite  directions  frequently  occurring  within  a  short  distance  of  each 
other.  These  abrupt  changes  in  strike  and  dip  clearly  indicate  that  the 
series  is  one  which  has  been  closely  crumpled. 

This  report  is  not  primarily  designed  to  cover  the  complex  basement 
series.     What  follows' is  therefore  of  an  incomplete  and  somewhat  general 

103 


104  THE  SOUTHERN  COMPLEX. 

character.  It  is  chiefly  a  Hthological  treatment,  and  even  as  such  is  but  an 
outline,  the  only  reason  for  entering  into  the  subject  at  all  being  to  give  a 
basis  for  comparison  with  the  rocks  of  the  Penokee  series.  Beginning  at 
the  west,  each  of  the  areas  is  taken  up  in  order  and  the  rocks  contained 
briefly  described. 

Geographical  distribution  (PI.  II). — In  Sec.  20,  T.  43  N.,  R.  7  W., 
Wisconsin,  as  discovered  by  Mr.  Charles  E.  Wright,  late  state  geologist  of 
Michigan,  the  rocks  of  the  Copper-bearing  series  and  a  gneissoid  granite 
belonging  to  the  complex  system  under  consideration,  are  found  upon 
opposite  banks  of  the  Numakagon  river.  To  the  west  of  this  point  no 
rocks,  aside  from  these,  are  known  for  a  considerable  distance;  so  that  it 
may  be  considered  that  here  is  the  westernmost  point  at  which  exposures 
belonging  to  the  Penokee  series  proper  will  be  foimd.  The  next  exposures 
in  the  Southern  Comjjlex  east  of  this  locality  are  in  Sees.  21  and  22,  T.  44 
N.,  R.  5  W.,  Wisconsin,  this  rock  being  a  gneissoid  granite.  From  here 
eastward  to  Bad  river,  in  Sees.  14  and  23,  T.  44  N.,  R.  3  W.,  Wisconsin, 
occasional  outcrops  of  gneissoid  granites  are  found.  The  Western  granite 
then  is  one  which  extends  east  and  west  for  more  than  25  miles,  and  north 
of  which  is  found  the  Penokee  series.  It  may  be  that  within  this  area 
rocks  other  than  the  gneissoid  g-ranites  occur ;  for  the  known  exposures  are 
comparatively  few,  and  if  within  it  softer  schistose  rocks  were  contained,  it 
is  probable  that  they  wovild  not  be  exposed. 

At  Bad  river,  veiy  close  to  the  Penokee  series,  is  a  schistose  rock. 
The  fine-grained  crystalline  schists  which  here  first  appear  continue  without 
a  break,  so  far  as  known,  to  the  West  Branch  of  the  Montreal  i-iver,  in  the 
northwest  part  of  Sec.  3.5,  T.  46  N.,  R.  2  E.,  Wisconsin,  another  stretch  of 
more  than  25  miles.  As  in  the  gneissoid  granite  area  to  the  westward,  the 
exposures  are  very  few,  but  in  at  least  two  places,  at  the  Potato  river  and 
the  West  Branch  of  the  Montreal,  the  basement  hxyer  of  the  Penokee  series 
is  seen  in  contact  with  the  schists  to  the  south.  How  far  the  green  schists 
of  this  area  extend  south  of  the  Iron-bearing  series  is  not  known,  but  the 
traverses  have  in  places  extended  as  far  as  two  miles.  At  Penokee  gap  and 
eastward  for  some  distance,  a  little  way  south  of  the  Iron-bearing  series,  the 
granites  and  schists  are  found  in  the  immediate  vicinity  of  each  other,  the  line 


TUK  SOUTIIEKN  COMPLEX.  105 

separating-  them  ^-i-adually  swinging-  away  from  the  I'enokee  rocks,  leaA'ing 
a  wedge  of  gi-ecii  scliist  between  the  latter  and  th((  gneissoid  granites.  Tlie 
eastern  end  of  tiiis  green  selii.st  area  also  nms  to  a  sliar])  ])oint,  the  granite 
being  found  south  of  it  live  or  six  miles  west  of  the  point  at  which  it  reaches 
the  Penokee  rocks.  The  distribution  of  the  granite  east  and  west  of  the 
schists  suggests  that  the  two  areas  are  connected.  If  this  is  the  case,  the 
Western  green  schist  is  an  isolated  area  bounded  on  the  south  by  tlie 
granite  and  on  the  north  by  the  Penokee  series. 

The  second  granitic  area  extends  immediately  south  of  the  Penokee 
series,  from  Sec.  35,  T.  46  N.,  R.  2  E.,  Wisconsin,  to  about  the  middle  of 
T.  47  N.,  R.  46  W.,  Michigan.  At  the  latter  point  the  schists  again  appear, 
and  like  those  to  the  westward,  widen  out  in  a  wedge-shaped  area,  leaving 
the  granite  as  the  southernmost  laiown  rock  for  quite  a  distance  south  of 
the  second  schist  area.  This  granite  area  has  many  exposures  in  such  close 
proximity  to  those  belonging  to  the  Iron-bearing  series  for  the  distance 
mentioned  as  to  leave  no  doubt  that  it  is  here  the  basement  upon  which  the 
former  rests.  While  it  is  called  a  granite  area,  it  contains  large  masses  of 
rock  of  a  more  basic  character,  including  both  syenites  and  altered  gabbros, 
the  relations  of  which  to  the  granites  will  be  referred  to  subsequently. 

East  of  this  Central  granite  follows  another  schist  area,  running  from 
near  the  middle  of  T.  47  N.,  R.  46  W.,  Michigan,  to  within  a  mile  and  a 
half  of  the  east  side  of  T.  47  N.,  R.  43  W.,  Michigan.  The  schists  are  in 
places  nearly  three  miles  wide,  but  throughout  the  entire  distance,  with  the 
exception  of  two  or  three  miles  in  the  east  half  of  T.  46  N.,  R.  44  W.,  Mich- 
ig-an,  g-ranitic  I'ocks  are  known  to  lie  to  the  south. 

East  of  this  Eastern  green  schist  again  appear  granites  and  gneissoid 
granites  which  south  of  the  Penokee  rocks  extend  to  Gogebic  lake.  What 
has  been  said  about  the  connection  of  the  Central  granite  and  Western 
granite  applies  even  more  strongly  to  the  Central  and  Eastern  granites, 
which  would  perhaps  have  been  connected  if  the  country  had  been  traversed 
a  little  farther  to  the  south.  The  boundaries  between  the  Eastern  g-reen 
schist  and  the  adjacent  granites  are  of  a  most  irregular  nature,  and  quite  often 
in  the  same  exposure  both  granitic  and  schistose  rocks  are  found.  The 
significauce  of  these  facts  will  be  discussed  later. 


106  THE   PENOKEB   IRON-BEARING   SERIES.  _ 

The  Western  granite.— The  granites  and  gneissoid  granites  of  the 
western  area  are  classed  together,  as  they  differ  but  little  from  each  other. 
The  gneissoid  granites  vary  in  character  from  those  so  coarsely  schistose 
as  to  be  with  difficulty  distinguished  from  the  granites  to  those  which  are 
extremely  contorted  and  quite  finely  foliated.  Although  the  area  is  large,  the 
rocks  included  within  it  are  petrographically  of  essentially  the  same  char- 
acter. The  massive  phases  are  relatively  infrequent,  and  are  medium 
grained  granites.  Those  phases  which  are  most  foliated  are  somewhat  finer 
grained.  "The  rocks  of  the  area  vary  from  almost  white  to  quite  a  deep 
red,  depending  upon  the  coloration  of  the  feldspar,  which  mineral  is  the 
preponderating  constituent,  the  iron-bearing  silicates  and  quartz  being 
usually  in  subordinate  quantities,  but  not  infrequently  plentiful  enough  to 
give  the  rocks  a  strongly  mottled  appearance  or  even  a  dark  gray  color. 
The  normal  rocks  of  the  area  (PL  xiv,  Fig.  1),  when  examined  under  the 
microscope,  are  seen  to  be  composed  very  largely  of  alkaline  feldspars, 
including  the  species  orthoclase,  microcline,  and  acid  plagioclase.  The 
feldspar  is  usually  in  indi^^duals  of  nearly  uniform  size  which  show  more 
or  less  perfect  idiomorphic  outlines.  With  most  individuals  the  average 
greatest  dimensions  are  less  than  J  mm.,  although  occasionally  individuals 
are  found  which  run  up  to  4  mm.  Frequentl)^  the  feldspars  interlock  with 
each  other,  in  which  case  the  crystal  outlines  are  more  or  less  broken, 
although  the  regular  forms  are  to  some  extent  maintained.  The  only 
important  iron-bearing  silicates  present  are  biotite  and  chlorite.  These 
minerals  occur  at  times  included  within  the  feldspars,  but  more  frequently 
are  found  in  the  spaces  between  them.  The  chlorite  often  occurs  in  well 
defined  blades  having  a  rectangular  extinction,  and  appears  at  times  to  be 
secondary  to  biotite,  although  often  it  is  in  part  secondary  to  feldspar. 
Occasionally  a  little  muscovite  is  found  included  within  the  feldspar. 
Quartz  has  been  the  last  mineral  to  crystallize  and  has  filled  up  the  spaces 
left  by  those  previously  mentioned.  It  occurs  in  small  rovmdish  or  irregular 
areas  between  the  feldspars,  giving  the  rock  a  completely  interlocking 
crystalline  character.  In  one  or  two  cases  the  quartz  and  adjacent  feldsjmr 
have  a  pegmatitic  structure.  This  may  signify  that  the  quartz  and  feldspar 
in  the  final  stages  of  crystallization  were  forming  simultaneously,  or  that 


THE  SOUTHERN  COMl'LEX.  107 

this  is  secondary  saturatino-  quartz.  Tf  tlu'  lattor  is  truo  it  becomes  not 
impossible,  altlinii<.li  hardly  probable,  that  tluf  independent  (piartz  in  the 
interstices  of  the  feldspar  is  a  secondary  material,  in  which  case  the  original 
rock  would  have  been  a  syenite. 

All  of  the  minerals  are  more  or  less  altered.  A  kaolinitic  decom- 
position has  widely  affected  the  feldspar.  Less  frequently  chlorite  and 
mica  have  developed  within  it,  and  the  chlorite  has  further  partly  changed 
to  epidote.  Jiut  the  most  interesting  decomposition  shown  by  the  feldspar 
is  an  alteration  into  quartz  and  biotite.  This  alteration  has  occurred  to 
some  extent  in  quite  a  number  of  the  rocks,  and  is  very  clearly  shown  by 
the  gneiss  a  short  distance  west  of  the  south  quarter  post  of  Sec.  23,  T.  44 
N.,  R.  5  W.,  Wisconsin  (PL  xiv,  Fig.  2)  on  Marengo  river.  Here  the  large 
individuals  of  feldspar  have  each  decomposed  in  a  great  measure  to  the 
more  basic  mineral  biotite,  the  excess  of  silica  apparently  separating  as 
finely  crystalline  quartz,  so  that  a  single  feldspar  contains  many  score  folia 
of  biotite  and  grains  of  quartz.  In  certain  pai'ts  of  the  section  this  decompo- 
sition has  gone  on  until  little  or  no  feldspar  remains,  the  result  being  a 
finely  crystalline  interlocking  aggregate  of  quartz  and  biotite  in  place  of  a 
single  grain  of  feldspar.  In  other  words,  a  somewhat  coarsely  crystalline 
strong'ly  feldspathic  rock — the  normal  phase  of  granite — has  changed  to  a 
finely  crystalline  gneissoid  biotitic  qfuartz  rock.  It  is  interesting  to  note  in 
this  connection  that  these  are  the  identical  changes  which  have,  in  the  upper 
belt  of  the  Penokee  series,  changed  a  feldspathic  fragmental  rock  to  a 
crystalline  mica-schist. 

The  Western  green  schist. — Tlie  exposures  within  this  area  are  com- 
paratively few  in  number,  particularly  in  its  western  half  The  rocks  are 
quite  finely  laminated,  dark  colored,  fine  grained,  crystalline  schists. 
They  are,  then,  in  strong  contrast  to  the  coarse  grained  granites  and  gneis- 
soid granites  to  the  westward.  In  different  parts  of  the  area  they  have 
very  different  characters,  so  that  they  can  not  be  described  together. 

At  Bad  river  the  schistose  rocks  are  technically  all  gneisses.  They 
have  a  strike  which  is  nearly  conformable  to  the  strike  of  the  overling 
Penokee  series ;  but  their  dip  is  south  instead  of  north,  and  hence  they 


108  THE  PENOKEB  IRON-BE AEING  SERIES. 

are  not  in  conformity  with  that  series.^  The  chief  constituents  of  the 
gneisses  are  feldspar,  quartz,  hornblende,  biotite  and  quite  frequently 
mag-netite,  epidote  being  an  accessory.  In  certain  of  the  specimens  bio- 
tite is  absent  and  in  others  hornblende.  The  relations  of  the  minerals  are 
those  ordinarily  found  in  typical  crystalline  schists,  which  give  little  or  no 
indication  of  their  original  condition.  Some  of  the  giieisses,  however,  show 
that  the  biotite  has  in  a  large  measure  been  derived  from  feldspar.  The 
alteration  of  each  grain  of  the  latter  mineral  to  many  folia  of  mica,  with 
the  simultaneous  separation  of  quartz,  presents  exactly  the  appearance  sub- 
sequently described  (chapter  vi) .  as  characteristic  of  the  fragmental  feld- 
spar rocks  of  the  Upper  slate  belt,  which  there  carried  to  the  extreme  has 
resulted  in  forming  from  a  fragmental  rock  a  completely  crystalline  mica- 
schist.  There  is  this  difference,  however :  In  the  Upper  slate  the  crys- 
talline schist  was  traced  back  into  an  unmistakable  fragmental  rock,  while 
here  the  origin  of  the  rock  is  unknown.  In  these  gneisses  much  of  the 
quartz  and  feldspar  has  a  ge^ieral  roundish  appearance,  which  suggests  a 
fragmental  origin.  The  hornblende-gneisses,  upon  the  other  hand,  associ- 
ated with  these  biotite-gneisses  contain  comparatively  little  or  no  quartz,  a 
good  deal  of  hornblende,  and  plentiful  magnetite.  The  last  are,  then,  rocks 
which  are  presumably  schistose  eruptives." 

Between  Bad  river  and  Potato  river  the  country  south  of  ttie  Penokee 
series  is  low,  swampy,  and  almost  without  exposure,  so  that  little  is  known 
of  the  character  of  the  rocks.  One  exposure  of  an  obscure  chlorite-gneiss 
is  found  in  Sec.  5,  T.  44  N.,  R.  1  W.,  Wisconsin,  the  onl}^ 'noticeable  thing 
about  which  is  the  occurrence  of  a  considerable  quantity  of  tourmaline,  a 
rather  uncommon  accessory  in  the  crystalline  schists  of  this  district. 

At  Potato  river,  in  contact  with  and  just  south  of  the  Penokee  rocks, 
large  exposures  of  the  underlying  schistose  rocks  are  found.  They  are 
here  fine  grained;  some  of  them  are  nearly  massive  and  others  highly 
schistose.  They  are  seen  to  contain,  microscopically,  many  small  grains  or 
crystals  of  feldspar,  roundish  areas  of  chlorite,  and  sometimes. large  areas 

'  For  iletaileil  descriptions  of  the  exposures  here  see  Geol.  Wis.,  1880,  vol.  in,  pp.  93-96,  aud 
p.  224. 

2  A.  Gelkie:  Recent  work  of  the  Geological  Survey  in  the  Northwest  Highlands  of  Scotland. 
Quart.  Jour.  Geol.  Soc.  Loudon,  vol.  xliv,  1888,  pp.  387-399. 


Tin-:  souTiiKiiX  ('oimpi.fa'.  109 

of  calcite.  They  are  of  ;i  mottlcil  dark  ynnMi  color,  and  ard  more  hig'hly 
altered  near  (lie  jiiucliou  with  the  overlying'  rocks  than  t'artlier  to  tlie  soutli- 
ward.  They  can  hardly  he  said  to  lia\'e  any  true  strike  and  dip,  hut  the 
fibers  of  the  schist  abut  almost  perpendicularly  a<j-ainst  the  slates  of  the 
Penokee  series,  which  here  as  usual  dip  northward.  In  thin  sections  these 
rocks  are  nmch  altered,  but  they  can  perhaps  best  be  named  chloritic  and 
hornblendic  gneisses.  A  groundmass  is  always  present,  which  consists  of 
finely  crystalline  quartz,  partly  decomposed  tabular  crystals  of  feldspar, 
and  areas  of  chlorite,  with  kaolin,  epidote,  sericite,  and  hornblende.  Con- 
tained in  this  groundmass  are  larger  complex  and  simple  areas  of  quartz, 
many  crystals  of  feldspar,  including  both  orthoclase  and  plagioclase.  Some 
of  the  latter  are  of  large  size.  They  often  have  roundish  outlines,  and 
include  near  their  extei'iors  chlorite  and  magnetite.  ^Magnetite  in  numerous 
crystals  of  small  size,  and  calcite  in  large  areas  are  contained  as  late  formed 
secondary  minerals,  particularly  in  that  part  of  the  exposure  near  the  over- 
lying senes.  The  appearance  of  these  rocks  is  such  as  to  suggest  highly 
altered  silicified  porphyritic  basic  eriqjtives  ;  but  if  they  were  porphyrites, 
the  series  of  alterations  through  which  they  have  gone  has  been  of  a  very 
complicated  character. 

The  exposures  between  these  last  described  and  the  West  Branch  of  the 
Montreal,  although  sparse,  are  more  numerous  than  anywhere  else  in  the 
area.  They  are  essentially  all  of  one  :  class  of  I'ock — crystalline  horn- 
blende-gneisses, although  in  one  or  two  specimens  the  feldspar  is  scant  and 
these  rocks  might  technically  be  called  schists.  Macroscopically  the  rocks 
are  all  dark  green,  fine  grained,  compact,  yet  finely  schistose  rocks.  Their 
schistose  structure  is  so  close  that  in  the  field  no  proper  strike  or  dip  is  in 
general  obtainable.  When  examined  in  thin  section  they  are  seen  to  be  as 
completely  crystalline  schists  as  any  rock  can  possibly  be.  Their  back- 
grounds are  generally  in  about  equal  quantity  finely  crystalline  quartz  and 
feldspar,  although  each  in  certain  sections  becomes  pi'eponderant.  The  two 
minerals  intricately  interlock,  both  with  each  other  and  with  themselves. 
In  a  majority  of  the  sections  in  the  background  are  larger  grains  of  quartz 
and  feldspar  which  have  a  roundish  appearance.  This  feature  is  particu- 
larly characteristic  of  the  quartz,  although  in  each  case  the  exteriors  of  the 


no  THE  PENOKEE  IEOJT-BEAEIjSTG  SEKIES. 

grains  are  minutely  angular.  A  general  oval  form  is  also  very  marked  with 
some  of  the  feldspars  (PL  xiv,  Fig.  H).  The  larger  feldspars  in  the  coarser 
phases  of  the  rock  are  about  1"""  in  greatest  length.  They  are  nearly  all 
striated  and  have  the  appearance  of  this  mineral  in  ordinary  basic  eruptives. 
Included  in  the  feldspars  are  blades  of  hornblende  and  grains  of  quartz, 
the  latter  in  some  cases  averaging  not  more  than  |^mm  in  diameter. 
Manifestly  either  the  feldspar  has  crystallized  subsequently  to  the  quartz 
and  hornblende,  or,  and  this  is  perhaps  more  probable,  the  decomposition 
of  the  feldspar  has  formed  the  hornblende  and  quartz.  The  appearance  of 
the  hornblende  individuals  both  within  and  without  the  feldspars  is  such 
as  to  suggest  that  they  are  now  in  the  process  of  growth  (Plate  xiv,  Fig. 
4).  In  general  the  feldspar  is  badly  altered,  so  that  it  not  only  includes 
hornblende  and  quartz,  but  a  gray  decomposition  product  taken  to  be 
kaolin.  When  the  alteration  of  a  large  feldspar  has  proceeded  far,  the 
area  may  consist  of  many  detached  particles  of  feldspar,  included  in  which 
are  large  hornblendes  and  numerous  quartzes.  Unless  examined  closely 
this  peculiar  relation  would  not  be  noticed  and  the  area  would  be  concluded 
to  be  an  intricately  interlocking  one  in  which  the  particles  of  feldspar  are 
independent,  as  they  are  recognized  oidy  as  a  skeleton  of  a  single  feldspar 
when  closely  examined  in  polarized  light.  The  hornblende  is  of  the  pale 
green  variety ;  it  has  normal  pleochroism  and  extinction.  The  individuals 
do  not  have  well  defined  outlines,  but  fray  out  in  every  direction  in  ragged 
stringers  (PI.  xiv,  Fig.  4).  This  is  especially  noticeable  in  sections  cut 
parallel  to  the  vertical  axes,  but  is  also  distinctly  seen  in  transverse  sec- 
tions. If  this  mineral  was  the  first  to  crystallize,  as  must  be  the  case  if  all 
those  individuals  now  present  are  original  crystallizations,  the  exceedingly 
ragged  forms  are  inexplicable.  If,  upon  the  other  hand,  the'hornblende  is 
a  product  which  has  formed  stibseq^uently  to  the  feldspar,  these  are  pre- 
cisely the  forms  which  would  be  expected.  It  thus  seems  probable  that 
these  rocks  are  much  altered  ones  which  originally  contained  much  more 
feldspar  than  at  present ;  perhaps  those  containing  hardly  any  feldspar  being 
as  feldspathic  as  those  of  the  gneiss  which  contains  a  large  amount  of  this 
mineral.  In  some  of  the  sections  biotite  is  as  abundant  as  the  hornblende. 
It  is  intimately  associated  with  that  mineral  and  bears  the  same  relation  to 


Tni-:  SOUTIIKHN  COMI'LKX.  Ill 

the  lV4dspar  tliut  the  iKirnblcudcj  docs.  Magnetite  in  small  areas  and 
crystals  is  plentiful  in  some  of  the  sections.  It  is  scattered  quite  unifonnly 
throufjhout  all  the  minerals  as  though  it  were  the  earliest  present.  The 
secondary  minerals — epidote,  sericite  and  chlorite — are  at  times  in  quantity. 
The  character  of  these  hornblende-schists  and  gneisses  gives  no  certain 
clew  as  to  their  origin.  With  almost  equal  plausibility,  taking  the  rocks  of 
this  area  alone,  a  case  could  be  made  out  for  their  derivation  from  frag- 
mental  sediments  or  from  eruptive  rocks. 

At  the  West  Branch  of  the  Montreal  a  somewhat  peculiar  rock  occurs. 
In  thin  section  chlorite  in  an  almost  solid  mass  composes  a  large  part  of  the 
sections..  Calcite  and  quartz,  with  a  little  magnetite,  are  included  in  this 
chlorite  as  subordinate  constituents.  Actinolite  is  the  only  remaining  min- 
eral of  importance.  When  cut  perpendicular  to  the  vertical  axis  or  approxi- 
mately so,  it  shows  crystal  outlines,  in  which  case  its  prismatic  cleavage  is 
nicely  developed.  Wlien  cut  parallel  to  the  vertical  axis  each  individual  of 
actinolite  terminates  in  many  long  needle-like  points,  which  penetrate  deeply 
into  the  chlorite.  Sometimes  the  actinolite  blades  are  clustered  into  sheath- 
like forms.  The  peifect  freshness  of  this  mineral  and  its  deep  penetration 
of  the  chlorite  indicate  that  the  actinolite  is  of  a  secondary  character-. 

The  Central  granite. — The  granitic  area  east  and  west  of  the  Montreal 
river  is  a  large  one,  but  it  has  not  been  sufficiently  traversed  to  find  a  great 
number  of  exposures.  The  rocks  are  always  coarse  grained  and  are  some- 
times massive,  although  more  often  they  have  the  structure  of  granitoid 
gneisses  or  coarse  gneisses  with  a  well  developed  foliation.  The  rocks  here 
included  vary  greatly  in  their  chemical  composition,  running  from  granites 
to  gabbros.  The  three  chief  types  of  rock  are  the  granites  (including  one 
microgranite),  the  syenites,  and  the  gabbros. 

The  granites  have  their  large  development  east  of  the  Montreal  river. 
They  vary  in  exposure  from  massive  granites  to  granitoid  gneisses ;  in  tex- 
ture they  are  always  moderately  coarse  and  evenly  granular;  in  color  they 
vary  from  almost  white  to  deep  flesh  color.  Feldspar  is  usually  the  pre- 
ponderating constituent,  although  both  quartz  and  the  iron-bearing  silicates 
are  often  abundant.  (PI.  xv,  Fig.  1.)  Under  the  microscope  these  rocks 
are  found  to  be  in  almost  every  particular  like  those  of  the  Western  gi-anite 


112  THE  PENOKEE  IKON-BEAEmG  SEEIES. 

already  described,  except  that  hornblende  takes  the  place  of  biotite,  so  that 
this  mineral  and  chlorite  are  the  chief  constituents  aside  from  the  quartz  and 
feldspar.  The  hornblende  of  the  ordinary  variety  is  found  in  large  well 
developed  blades  and  crystals.  It  is  at  times  included  within  the  feldspar, 
but  in  general  it  is  between  different  individuals  of  that  mineral,  although 
even  here  the  relations  are  such  as  to  show  that  the  feldspar  has  accommo- 
dated itself  to  the  hornblende.  This  must  mean  that  if  the  hornblende  is 
an  orighial  mineral  it  was  the  first  one  to  crystallize.  Its  pleochroism  is:  C, 
dark  bluish  green ;  h,  dark  yellowish  green ;  a,  light  yellow  ;  absorption,  C 
about  equal  to  tr  >  a.  The  chlorite  of  the  rocks,  where  it  is  found  instead 
of  the  hornblende,  bears  the  same  relation  to  the  feldspar  that  the  horn- 
blende does.  In  some  of  the  sections  the  chlorite  is  in  part  secondary  to 
hornblende  and  it  may  all  be  of  this  derivation.  • 

Constituting  an  exception  to  the  granites  just  described  is  an  exposure 
of  microgranite  found  in  the  NE.  i  of  Sec.  27,  T.  47  N.,  R.  47  W.,  Michigan. 
Macroscopically  it  has  an  aphanitic  background  which  contains  simple  and 
complex  areas  of  varying  sizes,  consisting  of  coarse  individuals  of  feldspar, 
a  dark  colored  mineral,  and  quartz.  In  other  words  the  background  in- 
cludes many  areas  which  have  a  ti'ue  granitic  texture.  These  complex  areas 
of  greatly  varying  sizes  are  included  in  the  most  irregular  fashion.  More 
abundant  than  these  are  single  porphyritic  crystals.  In. thin  section  the 
background  is  very  fine  grained,  and  apparently  consists  of  intricately  inter- 
locking quartz  and  feldspar.  As  indicated  by  the  hand  specimen  it  contains 
numerous  roundish  areas  consisting  of  large  individuals  of  quartz  and  feld- 
spar, with  quite  a  quantity  of  chlorite,  each  mineral  occurring  separately 
and  together  in  complex,  intricately  interlocking  areas.  Quite  a  number  of 
the  single  individuals  of  quartz  show  plainly  an  interrupted  growth,  which 
gives  each  grain  an  appearance  like  that  of  enlarged  quartzes  met  with  in 
fragmental  rocks.  Here  this  doubtless  means  that  the  cores  belong  to  a 
first  generation  and  the  enlargements  to  a  second  generation  of  crystals. 
The  porphyritic  feldspars  include  orthoclase,  microcline,  and  plagioclase. 
This  rock  difiPers  from  any  which  I  remember  to  have  seen.  In  most  cases 
in  which  crystallization  has  thus  occurred  in  two  generations  only  detached 
porphyritic  crystals  are  contained  in  a  finer  grained,  matrix.     Here,  how- 


THE  SUUTHEHN  COMPLEX.  113 

ever,  lar<?e  and  small  cDiuplcx  arcns  Imxini;'  the  nppearanceboth  iiiacroscop- 
ically  1111(1  iiiicroscopically  ol'  ;iii  onliuarN'  ^riiiiitc  an;  coiitaiiK'd  in  a  matrix  of 
exceedingly  tiiicly  rrystallizcd  material.  II'  this  diffcrcucc;  in  tlicciiaracter  of 
tlie  coarse'aud  lilu^  parts  is  taken  to  iiieau  tliat  tlic  coarser  part  crystallized  at- 
deptli  and  tin-  latter  after  a  cluuige  of  ('onditioii  and  near  the  snrface,  the  c.oii- 
elnsion  would  be  that  the  crystallization  had  proceeded  inueli  further  than 
usual  iu  porphyries  before  the  change  in  condition  ()ccurre<l.  And,  if  this  is 
so,  the  specimen  throws  some  light  U[)on  tlie  manner  of  crystallization  of 
granites.  In  the  develojiment  of  the  minerals  in  massive  igneous  rocks,  it  is 
usually  assumed  that  the  crystallization  of  the  different  species  is  in  the  main 
successive,  although  to  some  extent  simultaneous.  It  would  appear  that  in 
this  rock,  while  this  niay  be  to  a  small  degree  true,  it  is  certain  that  before 
the  change  of  condition  occurred  all  of  the  minerals  found  in  a  granite  had 
begun  to  develop,  and  in  such  a  fashion  as  to  make  small  masses  of  perfect 
granite.  These  masses  of  granite,  associated  with  simple  individuals  of 
quartz  and  feldspar,  were  separated  from  each  otlier  by  the  liquid  magma 
in  which  they  were  contained.  After  the  change  of  conditions  the  magma 
rapidly  crystallized,  thus  preserving  the  individuals'  and  clusters  of  individ- 
uals which  had  before  formed.  If  the  growth  had  contiiuied  without  a 
change  of  conditions,  the  simple  and  complex  areas  would  probably  have 
formed  larger  and  larger  masses  of  granite  until  the  whole  space  was  occu- 
pied. This  implies  that  in  the  outward  growth, of  each  mass  all  of  the  min- 
erals which  go  to  make  up  a  granite  separate  from  a  magma  in  which  these 
same  minerals  have  before  developed  in  such  relations  to  each  other  as  to 
have  for  short  distances  the  typical  structure  of  a  deep  seated  crystalline 
rock.  This  manner  of  growth  differs  radically  from  that  first  mentioned  as 
the  ordinary  conception  of  the  crystallization  of  igneous  rocks. 

The  exposures  west  of  the  Montreal  river,  and  a  few  of  those  a  short 
distance  east  of  it,  are  of  a  more  basic  character  than  the  granites.  The 
rocks  here  included  are  syenites  and  gabbros,  the  latter  being  much  altered. 
The  granites  east  of  the  Montreal  differ  in  acidity.  In  content  of  quartz 
they  vary  from  a  normal  rock  to  one  in  whicli  the  quartz  is  distinctly 
fjubordinate  to  the  other  constituents,  and  which  might  with  equal  pro- 
priety be  classed  as  a  granite  or  a  syenite.  The  syenites  west  of  the  Mon- 
MON  XIX — -6 


114  THE  PENOKEE  lEON-BEAEIFG  SEEIES. 

treal  river  are  even  more  Ijasic  than  this  rock  (PI.  xv,  Fig.  2),  containing 
little  or  no  quartz  Tliey  liave  a  well  developed,  coarse  foliation,  and  were 
regarded  in  the  field  as  coarse  gneisses  rather  than  syenite-schists.  The  speci- 
mens vary  from  mottled  red  and  black  to  black,  depending  upon  the  quantity 
of  the  iron-bearing  silicates  present  and  the  color  of  the  feldspar.  "When 
examined  in  thin  section,  they  are  found  to  be  very  much  coarser  grained 
than  would  have  been  suspected  from  the  hand  specimens.  They  contain 
as  a  background  large — upon  an  average,  2  nun.  in  diameter — closely  fit- 
ting and  interlocked  individuals  of  alkaline  feldspar  comprising  the  three 
species,  orthoclase,  raicrocline,  and  plagioclase.  So  far  as  seen,  the  schist- 
ose strut;ture  of  the  rock  does  not  in  any  measure  affect  these  feldspars. 
In  the  centers  of  some  of  the  largei"  orthoclases,  in  irregular  areas,  the  mi- 
crocline  twinning  is  developed.  This  peculiar  appearance  indicates  either 
that  one  of  them  may  alter  into  the  other  or  that  in  their  growth  one  has 
enveloped  the  other.  In  most  of  the  syenites  the  abundant  mineral,  aside 
from  the  (juartz,  is  hornblende  accompanied  with  a  considerable  quantity 
of  biotite  and  chlorite.  The  hornblende  is  in  well  defined  blades  and 
crystals  and  the  Ijiotite  in  its  usual  broad  leaflets.  Sometimes  transverse 
sections  of  hornblende  show  almost  perfect  crystals  which  exhibit  the 
prismatic  faces  ijr  these  combined  with  the  orthopinacoids.  If  these  min- 
erals are  original  crystallizations,  they  must  have  formed  before  the  feld- 
spars. If  they  are  secondary,  they  must  have  developed  within  the  feld- 
spars. The  strongly  marked  schistose  structure  characteristic  of  the  rock 
is  wlR)lly  due  to  the  arrangement  of  the  hornblende  and  Ijiotite  luinerals, 
the  larger  blades  usually  having  their  greater  lengths  in  a  common 
direction.  In  places  they  are  massed  together  in  large  areas  so  as  to 
wholly  exclude  the  feldspar.  At  times  a  single  Ijlade  of  biotite  or  horn- 
blende cuts  through  several  individuals  of  feldspar.  The  hornblende  of 
these  rocks  is  somewhat  abnormal  in  its  optical  character.  It  has  strong 
relief  and  gives  very  brilliant  interference  colors.  The  angles  c  :  C  run  as 
high  as  21°.  Its  pleochroism  is:  C,  bluish-green;  h,  yellowish-green;  a, 
pale  honey  yellow;  It>C>  «•  Differing  from  these  hornblende-syenites  is  a 
mica-pyroxeue-syenite,  the  pyroxene  iu  this  case  replacing  for  the  most 
part  the  hornblende.     This  rock  is  like  the  other  syenites  in  all  other 


THE  SOUTHERN  COMPLEX.  115 

points,  and  the  relati  1)1  IS  nrtlH'  iiiic;i  ;iii(l  pyroxene  tn  tlie  f'eklspar  are  the 
same  as  those  of  llic  liiiriil)l(Ui(le  antl  biotite  in  the  niicii-lioi-iihlende-sye- 
nites.  This  rock  also  carries  some  liornblende,  hut  in  siihonUnate  qun-ntity. 
In  some  cases  the  biotite  seems  to  be  forming'  as  a  secomlar)^  pnnhict  from 
it.  The  pyi'o.\ene  is  coh)rless,  stronj>'ly  (h»ubly  refracting-,  ;ui(l  from  its  oj)- 
tical  properties  is  taken  to  be  malacohte.  It  is  (generally  in  ronnihsh  gran- 
ides  in  cross-section,  and  in  hhides  several  times  h)ng'er  than  broad  in  lon- 
gitudinal section.  Many  of  the  biotite  blades  are  nmch  distorted.  All  of 
the  syenites  contain  very  numerous,  large,  well  formiul  crystals  of  apatite, 
which  are  included  in  all  of  the  other  minerals. 

Not  very  fixr  in  basicity  from  these  syenites  are  coarse,  altered  gabbros, 
winch  in  large  exposures  are  closely  associated  with  them.  In  the  held 
and  in  hand  specimen  they  do  not  dififer  greatly  in  appeai-ance  from  some 
of  the  syenites  just  described.  Their  feldspars  have  a  reddish  cast,  as  in 
them,  and  their  structure  is  much  the  same.  However,  when  they  are 
examined  in  thin  section  they  are  seen  at  once  to  be  altered  Ijasic  eruptive 
rocks  of  the  ordinary  type.  The  chief  original  constituents  are  plagio- 
clase,  magnetite,  anddiallage,  which  have  relations  characteristic  in  a  rcjck  in- 
termediate in  structure  between  a  diabase  and  a  gabbro.  The  plagioclase  has 
altered  very  extensively  to  chlorite  and  kaolin,  and  is  in  many  places  replaced 
to  some  extent  by  saturating  quartz.  The  magnetite  occurs  in  well  defined 
areas  and  crystals,  which  give  no  evidence  of  alteration,  it  being  the  only  min- 
eral within  the  rock  which  is  unaffected.  The  pyj-oxene  has  very  largely 
decomposed,  the  resultant  jjroducts  being  hornblende,  biotite,  and  chlorite, 
more  largely  the  first.  It  is  a  very  noticeable  thing  that  the  secondary  horn- 
blende is  not  paramorphic  but  several  or  many  blades  have  formed  which 
are  in  crystallographic  position  independent  of  the  original  augites,  and 
much  of  this  hornblende  in  its  forms  is  like  the  hornblende  contained  in  the 
syenites  just  described.  As  a  result  of  the  comjdete  decomposition  of  the 
diallage,  hornblende  and  biotite  are  formed  with  their  axes  arranged  quite 
often  in  a  common  direction.  In  this  case  these  minerals,  in  their  appear- 
ance and  relations,  are  much  like  those  of  the  same  minerals  in  the  associ- 
ated syenites.  It  is  further  noticeable  that  these  rocks  also  contain  numer- 
ous large,  well  formed  crystals  of  apatite.     Considering  all  of  the  foregoing 


116  THE  PEWOKEE   lEOX-BEAEING  SEEIES. 

facts,  it  is  quite  probable  that  the  gabbros,  syenites,  syenite-scliists,  and 
possibly  the  granites  are  parts  of  the  same  continuous  rock-mass. 

The  Eastern  cjrecn  schist. — The  shape  of  this  area  will  be  seen  by 
examining  PI.  ii.  The  exposures  contained  are  very  numerous,  and  the 
rocks  within  a  certain  narrow  range  have  great  variety.  While  quite  large 
subareas  contain  only  a  single  phase  of  rock,  they  grade  into  other  areas 
containing  rocks  of  a  different  character,  and  even  a  single  exposure  fre- 
quently contains  quite  dissimilar  rocks.  This  confusion  is  so  great  that  no 
attempt  will  be  made  to  subdivide  the  area  into  smaller  ones,  each  contain- 
ing a  detinite  phase  of  rock,  nor  will  any  attempt  be  made  to  explain  the 
field  relations  of  the  different  })hases.  The  rocks  here  included  may  all  be 
technically  classed  under  one  general  term — gneiss — although  perhaps  some 
of  them  might  better  be  called  syenitic  schists  and  most  are  fine  grained 
green,  crystalline  schistsi  In  these  gneisses,  aside  from  the  quartz  and  feld- 
spar, any  one  of  the  minerals  hornblende,  biotite,  chlorite,  sericite,  or 
ejiidote  may  be  the  additional  chief  constituent,  ^diile  often  two  or  more 
are  equally  abundant.  Consequently  the  area  contains  hornblende-gneiss, 
biotite-gneiss,  sericite-gneiss,  chlorite-gneiss,  epidote-gneiss,  and  various 
intermediate  phases.  The  rocks  vary  greatly  in  coarseness,  running  from 
those  that  are  so  exceedingly  fine  grained  that  it  is  with  extreme  difficulty 
that  the  minerals  are  determined,  to  rocks  which  approach  a  granitoid 
gneiss.  It  is  noticeable  that  the  coarser  grained  phases  are  mostly  found 
near  the  granitoid  gneiss  areas.  In  fact  the  line  forming  the  boundary 
between  this  schist  area  and  the  surrounding  granite  and  gneissoid  granite 
areas  is  arbitrarily  drawn.  As  the  granite  area  is  neared  the  schists  become 
coarser  and  coarser,  grading  into  a  coarse  gneiss.  The  change  is  so  gradual 
at  the  east  end  of  the  area  that  tlie  boundary  between  the  gneisses  and 
granites  would  probably  not  be  put  by  another  observer  in  the  same  place 
as  mapped.  The  principle  followed  in  our  mapping  is  to  include  within  the 
schist  area  "all  rocks  which  in  hand  specimen  do  not  have  a  strong  granitic 
appearance.  While  there  is  this  change,  it  is  not  a  passage  which  is  made 
out  in  any  case  by  actually  tracing  a  finely  schistose  rock-mass  in  contin- 
uous exposure  into  a  granite,  but  the  various  phases  are  presented  by 
numerous  detached  exposures.     xVside  from  this  apparent  passage,  the  fiiae 


TIIK  S()IT[Ii:itN  CO.MI'lilOX. 


117 


grained  scliists  adjaccut  to  tlic  ;4raiiit(' area  not  iitilr('(|ii('iitl\- contain  massive 
granite.     This  uranitc  is  somctinics  t'ouiid  as  a  boss  upon  wliicli  tlio  sirliist . 
rests,  liut  more  ot'tcii  <'onstitutos  one  part  ol'an  ('Xjxisin'c,  the  sdiists  cut  )»y 
it  coniposiiiij;'  the  other  Dart  (Fig".  3).      lu  sonic  phiccs  the  niannei'  in  whicli 


FiQ.  3.— Schist  cut  by  massive  gi-.inite,  NAV.  ?„  Sec,  4,  T.  10  N.,  li.  2  E.,  "W'isennsin. 

the  granites  cnt  the  schists  is  sucli  as  to  leave  no  (h)ul)t  of  the  eruptive 
character  of  the  latter  (Fig.  4). 

Schist. 

'  "■'-  ':^  --■-- 


Fio.  4.— Schist  cut  by  granite,  NB.  cor.  Sec.  28,  T.  47  N.,  E.  40  W.,  Micliigan. 

Macroscopically  the  gneisses  of  the  area  are  light  gray  to  very  dark 
green.  They  vary  in  coarseness  from  aphanitic  to  the  texture  of  a  gneissoicl 
granite.  In  all  of  the  rocks  a  schistose  structure  is  very  strongly  developed. 
In  the  coarser  gneisses  the  foliation  is  marked  in  liand  specimen  and  in 
exposure,  but  in  the  finer  grained  varieties  it  is  often  onlj^  exhibited  by  a 
readier  cleavage  in  one  direction.  The  different  phases  described  under 
the  microscope  differ  from  each  otlier  but  little  macroscopically.  tSome  of 
the  syenite-schists  are  an  exception  to  this  statement  in  that  they  approach 
in  structure  to  a  basic  eruptive. 

One  of  the  most  characteristic  and  abundant  phases  of  schist,  as  seen 
in  thin  section  (PI.  xv,  Fig.  3),  is  that  which  has  a  white  granular  background 
composed  of  quartz  and  feldspar.  The  proportions  of  these  two  minerals 
vary  widely  in  the  different  specimens,  running  from  those  in  which  quartz 
is  preponderant  to  those  in  which  the  feldspar  almost  entirely  excludes  that 
mineral.     The  individuals  of  each  of  these  minerals  in  this  backscround  are 


118  THE  PENOKEE- IRON  BEARIISrC=^  SERIES. 

approximately  of  uniform,  size  in  each  section,  although  varying  widely  in 
different  sections  ;  in  general  they  have  roundish  or  oval  forms.  The  parti- 
cles of  the  two  minerals  fit  each  other  very  closely,  but  do  not  intricately 
interlock.  The  eifect  of  this  even  grained  background  composed  of  round- 
isli  ])articles  is  to  suggest  a  fragmental  origin,  but  nowhere  is  there  any 
conclusi^'e  evidence  that  any  of  the  material  is  of  fragmental  character.  If 
the  grains  represent  fragments  they  could  not  have  thus  perfectly  fitted  each 
other  as  originally  deposited,  and  nowhere  is  there  any  })roof  of  enlarge- 
ment in  either  the  quartz  or  feldspars,  and  there  is  no  space  between  them 
for  a  fine  grained  interstitial  material ;  so  this  granular  structure  can  not  be 
taken  as  proof  of  fragmental  origin. 

The  feldspar  is  very  largely  of  the  species  orthoclase,  although  micro- 
dine  and  plagioclase  are  often  abundantly  present.  It  has  usuall}'  largely 
altered,  the  resultant  products  being  kaolin,  sericite,  chlorite,  epidote,  and 
at  times  biotite  and  hornblende.  This  alteration  is  sometimes  certainlj^, 
and  probably  often,  attended  with  the  simultaneous  separation  of  quartz. 
Tliis  decomposition  in  the  cases  when  it  has  extended  far  enough  results  in 
producing  in  the  place  of  a  feldspar  a  complex  interlocking  mass  of  finely 
crystalline  material.  In  a  few  cases  in  which  the  gneiss  was  originally 
strongly  feldspathic  the  alterations  have  caused  it  to  closely  resemble  some 
of  the  felds})athic  graywackes  of  the  Upper  slate  member  of  the  Penokee 
series  (chapter  vi).  As  mentioned  in  the  case  of  a  similar  occurrence  in 
the  Western  granite  there  is,  however,  the  great  difference  that  no  e^'idence 
can  here  be  found  that  the  original  feldspathic  rock  was'  of  a  fragmental 
character. 

The  abimdant  iron-bearing  minerals  of  the  gneiss  are  hornblende,  bio- 
tite, and  elilorite.  Less  frequently  epidote,  sericite,  and  calcite  occur.  In  a 
single  section  any  one  of  these  minerals  may  be  predominant;  two  or  more, 
or  all  of  them  may  occur  together.  There  seems  to  be  absolutely  no  regu- 
larity as  to  their  occurrence,  either  in  the  field  or  in  the  sections  themselves. 
Hornblende  is,  however,  the  preponderant  mineral  in  many  sections,  although 
chlorite  and  biotite  are  hardly  less  frequently  so.  In  the  coarser  grained 
gneisses  (PI.  xv.  Fig.  4)  the  hornblende  often  occurs  in  large,  well  defined 
crystals,  which  in  transverse  sections  show  either  the  planes  of  the  prism 


Tllk  SOTTllKKN  CO.MI'IJ'A.  119 

dl'  the  |il;iiit's  111'  the  pi'isiii  (•(iniliiiicd  with  the  iii;ici'o|iiii;ic(ii(ls.  The  tci'- 
niiu;il  |)l;iiH's  iirc  luit  oftoii  well  (IcNrlopcd.  'I'lic  |)l('()clii'i)isiii  oi' the  Ikmu- 
hlcndc  is  Ncry  uiiitonii;  c  <lark  j^Toenisli  hliu^,  ll  dark  yrccu,  a  Ikuicn'  \el- 
l(t\v.  Tlio  absorption  is:  Xt>  C>U;  <•  :  C  varies  from  1,'}"  to  If)'^.  Tlic  liorii- 
Idciiilc  individuals  always  include  many  grains  of  tlie  other  minerals 
present,  usually  more  (juartz  and  feldspar  than  of  chlorite  and  1)iotite.  Tt 
a])parently  shows  by  its  inclusions,  combined  witli  its  idiomorpliic  forms, 
that  it  was  the  last  mineral  to  crystallize.  Its  growth  within  these  gneisses 
may  be  compared  to  the  growth  of  crystals  of  garnet  and  staxn-olite  in 
staurolitic  and  garnetiferous  mica-schists,  which  frequently  include  large 
quantities  of  foreign  materials.  In  its  develo])ment,  if  a  late  mineral,  it 
took  within  itself  such  material  as  it  could  not  force  aside.  More  often  in 
the  gneisses  the  hornblende  is  in  small  blades  free  from  inclusions,  located 
between  the  particles  of  quartz  and  feldspar  or  else  penetrating  them 

The  biotite  and  chlorite  occur  in  well  developed  folia,  and  in  small 
fibers  and  irregular  areas.  Each  of  these  minerals  is  in  turn  in  quite  a 
number  of  the  exposures  the  chief  constituent  aside  from  the  quartz  and 
feldspar.  Each  frequently  contains  numerous  sinaller  particles  of  what  is 
faken  to  be  oxide  of  iron,  which  are  arranged  within  their  parts  in  the 
same  regular  manner.  A  portion  of  the  chlorite  and  l^iotite  is  certainly 
secondary  to  feldspar  and  hornblende.  This  is  particularly  ti-ue  of  the 
chlorite.  However,  there  is  no  evidence  that  their  abundant,  well  defined 
blades  are  secondary.  The  epidote,  so  plentifully  present  at  times,  is  found 
alike  in  the  feldspar,  chlorite,  hornblende,  and  biotite,  although  most  com- 
mon in  the  first  two.  It  occurs  in  numerous  small  granules  and  large 
irregular  areas.  A  large  part  of  it  is  certainly  derived  from  feldspar,  chlo- 
rite often  being  an  intermediate  stage  in  its  formation.  It  also  has  formed 
from  hornblende.  In  some  of  the  rocks  the  epi<h^te  is  with  quartz  the  chief 
constituent  and  the  rock  consequent!}'  is  an  epidosite.  There  is  here  no 
evidence  of  the  derivation  of  the  epidote  from  feldspar,  hornblende,  and 
chlorite,  but  it  may  be  true  that  the  i-ocks  in  which  the  origin  of  the 
epidote  is  plain  is  an  intermediate  phase  in  the  formation  of  the  epidosite. 
The  other  accessories  most  frequent!}'  present  in  the  gneisses  are  pyrite  and 
iron  oxide;  tourmaline  is  rarely  found. 


120  THE  PENOKEE  lEON-BBAllING  SERIES. 

In  general  the  hornblende,  biotite,  and  chlorite  are  largely  in  the 
sjjaces  bet'iveen  the  quartz  and  feldspar,  but  they  also  are  extensi^•ely 
included  Avithin  them,  a  single  indiAadual  of  one  of  these  minerals  perhaps 
penetrating  several  particles  of  quartz  or  feldspar,  or  both.  Usuall}-  the 
chlorite,  biotite,  and  hornblende  are  arranged  with  their  axes  more  or  less 
regularh-  in  a  common  direction.  Sometimes  this  arrangen^ent  is  but  imper- 
fect, while  in  other  cases  the  parallelism  is  remarkable,  the  fibers  of  the 
mauA'  individuals  Ipng  almost  exactly  parallel  Avith  each  other.  When 
this  parallelism  is  so  marked  it  is  sometimes  the  case  that  the  quartz  and 
feldspar  particles  are  elongated  in  the  same  direction.  This  elongation  of 
all  of  the  minerals  in  such  a  ease  doubtless  indicates  that  the  rocks  have 
been  subject  to  great  dynamic  forces. 

Within  these  evenly  granular  gneisses  are  quite  often  larger  grains  of 
quartz  and  feldspar,  which  have  A'ery  distinct  oval  outlines.  This  is  partic- 
ularh'  true  Avith  reference  to  the  large  quartz  grains  (PI.  xv.  Fig.  3),  although 
Avhen  examined  closel)^  they  are  seen  to  be  minutel}'  angular.  As  in  the 
roundish  grains  of  the  quartzose  background,  no  proof  has  been  found  that 
the  quartzes  are  of  a  fragmental  character. 

By  a  gradual  lessening  of  the  quartz  in  the  hornblende-gneisses  they 
pass  into  a  rock  in  which  the  chief  constituents  are  hornblende  and  an  alka- 
line feldspar.  These  rocks  might  perhaps  better  be  named  syenite-schists. 
In  the  cases  of  some  of  theni  the  hornblende  is  very  abundant ;  the  feld- 
spar closely  interlocks,  and  they  contain  man)'  areas  of  black  iron  oxide, 
which  is  doubtless  menaccanite,  as  it  is  generall}'  surrounded  by  a  bril- 
liantly polarizing  reactionary  i-ing  supposed  to  be  titanite.  These  feldspar- 
hornblende-menaccanite  rocks  in  their  mineralogical  character  are  identical 
with  the  syenites. 

They  have  not,  it  is  true,  in  any  case  been  traced  in  continuous 
exposure  to  a  rock  Avhicli  has  the  genuine  syenitic  texture,  but  considering 
the  fact  that  it  has  been  proved  that  this  class  of  rocks  is  in  certain  cases 
derived  from  basic  eruptives,  it  becomes  possible  that  these  syenitic  schists 
are  of  tins  origin. 

A  second  phase  of  hornblende-gneiss  found  abundantly  in  this  area  is 
exactly  like  the  hornblende-gneisses  already  described  and  illustrated  (PI. 


Till':  SOIITllKItN  ('()!MIM-K.\.  121 

XIV,  Fii^-s.  .■)  ;ui(l  t)  Ix'twt'cii  the  Potato  i-ivci-  and  tlic  West  I'nuicli  of  tlie 
Montreal  (pp.  109-111). 

Tliev  will,  then,  not  l)e  described  in  detail  ]n're  ag'tiin.  It  will  he 
reniend)ered  that  they  contain  nuniorous  lar^v  plagioclase  feldspars,  within 
which  are  blades  of  hornblende  and  ])arti('les  of  ([uartz.  In  case  the  feld- 
spars are  larg-e  and  the  alteration  has  proceeded  far,  within  a  single  individ- 
ual may  be  found  many  grains  of  cpiartz  and  blades  of  hornblende.  It  will 
also  be  remembered  that  the  hornblendes  have  peculiar  jagged  terminations, 
which  are  shown  in  a  marked  degree  when  the  sections  a,i-e  longitudinal,  but 
are  also  observable  in  transverse  sections.  It  was  noted  that  they  might, 
with  almost  equal  ))lausibility,  be  regarded  as  of  eruptive  or  sediinentary 
origin.  In  the  Eastern  green  schist  these  rocks  have,  however,  been  traced 
step  by  step  into  those  which  are  almost  certainly  of  an  eruptive  origin. 
This  rock  in  hand  specimen  in  one  case  is  massive,  and  has  strongly  the 
appearance  of  a  massive  basic  eruptive.  Within  the  same  exposure  are 
other  phases  of  rock  which  have  the  distinct  foliation  of  the  horidilende- 
gneisses  of  the  region.  These  rocks  and  others  within  the  locality  under 
consideration  have  almost  no  quartz,  the  background  being  composed  of 
large  areas  of  feldspar,  wdiich  intricatel}'  interlock.  Between  these  feld- 
spars and  in  them  are  many  hornblendes  of  varying  sizes,  individuals  fre- 
quently beiiig  large.  They  have  the  same  jagged  outlines  which  are  char- 
acteristic of  the  hornblende  already  alluded  to.  The  secti(ins  also  contain 
much  comparatively  fresh  magnetite.  There  can  be  little  doubt  that  this 
massive  rock  is  a  moditied  eruptive,  or  that  the  schistose  one  from  the  same 
exposure  is  of  the  same  character.  As  there  is  every  phase  of  gradation 
betw'een  this  massive  rock  and  those  schistose  ones  in  which  there  is  a  large 
amount  of  quartz  present,  it  is  exceedingly  probable  that  all  of  this  class 
of  hornblende-schist  is  eruptive,  although,  as  before,  I  would  guard  myself 
by  saying  that  we  have  no  absolute  proof  of  this. 

Between  the  two  main  kinds  of  rock  above  described  there  are  all  inter- 
mediate phases. 

Besides  these  phases  of  rock  there  are  many  subordinate  varieties.  It  is 
worthy  of  remark  that  in  one  of  the  fine  grained  schists  which  is  found  in 
large  exposures  a  background  of  very  finely  crystalline  quartz  and  chlorite 


122  THE  PEKOICBE  iRON-BEARlNG  SERIES. 

contains  minute  tabular  crystals  of  feldspar.  In  this  matrix  are  found  large, 
much  altered  crystals  of  the  same  mineral.  Menaccanite  and  leucoxene  are 
also  ahundant  in  small  granules.  These  rocks  would  be  regarded  ordinarily 
as  modified  porphyrites,  although  in  the  field  and  in  hand  specimen  there  is 
the  strongest  possible  foliation.  In  character  they  are  allied  to  some  of  the 
various  finely  crystalline  quartzose  gneisses  which  stand  intermediate  be- 
tween the  two  main  phases  described. 

The  Eastern  fjranite. — In  all  essential  respects  the  massive  rocks  here 
contained  are  like  those  found  in  the  granite  areas  to  the  westward.  •  The 
phases  here  included  run  from  typical  syenites  to  typical  quartzose  granites. 
Usually  in  them,  as  in  the  previous  areas,  an  alkaline  feldspar,  occurring 
largely  in  idiomorphic  forms,  is  the  chief  constituent.  In  a  few  of  the  ex- 
posures quartz  is  as  abundant  as  the  feldspar.  In  some  cases  in  this  area 
the  decomposition  of  the  feldspar  has  gone  far.  Aside  from  the  qxiartz  and 
feldspar,  hornlilende  and  chlorite,  as  in  the  Central  granite,  are  the  only 
important  minerals,  and  very  frequently  the  chlorite,  as  in  it,  has  resulted 
from  the  alteration  of  the  hornblende  or  the  decomposition  of  the  feldspar. 
The  typical  syenite  exposures  are  more  common  than  in  the  other  two  granite 
areas. 

Included  in  each  of  the  foregoing  areas  are  here  and  there  exposures  of 
fresh  diabases.  These  rocks  in  every  particular  are  like  the  diabases  so 
abundantly  contained  as  dikes  in  the  Penokee  series,  and  no  doubt  were 
contemporaneous  if  not  actuall)"  continuous  with  them.  The  description  of 
these  rocks  is  given  in  Chapter  vii. 

Summary. — The  kinds  of  rock  mentioned  in  the  Southern  Complex  are 
not  necessarily  all  which  may  there  exist,  for  the  traverses  were  not  fre- 
quent enough  to  find  more  than  a  fraction  of  the  exposures.  The  narrow 
range  in  the  lithological  character  of  the  granite  is  very  noticeable.  Three 
large  areas  of  rock,  in  which  the  easternmost  exposures  are  fully  90  miles  from 
the  westernmost  ones,  are  alike  in  every  important  particular.  This  likeness 
of  character  suggests,  as  does  also  their  distribution,  as  seen  in  PI.  ii,  that  these 
areas  are  really  connected.  It  is  also  suggestive  that  a  like  condition  of 
affairs  prevailed  for  a  very  considerable  distance  at  the  time  of  the  forma- 
tion of  these  rocks. 


s 


TUK  .SOI'TIIKHN  COMPLEX.  123 

A  siuuiiiary  of  the  more  iniportunt  cliaractcrs  ol' tli(^  jii-iiiiitcs  and  ^Tiiui- 
toiil  liucisscs  is  licrc  tiivcii.  iVn,  alkaliiu!  feldspar  is  al\va\'s  the  (.'liiuf  con- 
stituent, while  in  the  nuijority  of  cases  it  coin])oses  three-fourths  or  more  of 
the  rocks.  Moreover,  a  very  larj^'e  proportion  of  this  feldsjjar  is  of  the  s})ecies 
orthoclase,  althouf^'h  niicrocline  and  acid  plaj^'ioclase  are  always  present  and 
often  pleutifully.  'i'here  is  a  very  inarked  tendency  in  these  felds})ars 
to  idioniorphic  forms.  In  some  cases  in  which  the  felds[)ar  is  predomi- 
nant this  tendency  is  so  strong-  that  the  rock  mig-ht  be  (lescriV)ed  as  ])anidio- 
morphic.  This  characteristic  of  the  granites  and  sA'enites  is  shown  both  in 
hand  specimen  and  in  thin  section.  Quartz  varies  in  (|ua,ntitv  to  an  amount 
about  equal  to  that  of  the  feldspar.  When  the  cpiartz  is  not  ])lentiful  it  is 
in  part  so  ari-anged  with  reference  to  the  feldspar  as  to  suggest  that  it  might 
be  of  secondary  origin.  This  suggestion  is  reinforced  by  the  fact  that  occa- 
sionally the  feldspars  near  their  exteriors  contain  saturating  quartz.  This 
pegmatitic  structure  may  be  due,  however,  to  the  simultaneous  crystalliza- 
tion of  quartz  in  the  final  stages  of  the  formation  of  the  feldspar. 

Further,  it  does  not  appear  ])robable  that  all  the  quartz  found  in  these 
rocks  is  a  secondary  product,  for  in  the  most  acid  of  them  the  feldspar  and 
quartz  interlock  in  the  irregular  fashion  characteristic  of  ordinary  granites. 
In  the  massive  granites  the  onh'  other  important  constituents  are  horn- 
blende, biotite,  and  chlorite.  The  hornblende  quite  often  occurs  in  well 
defined  crystals.  The  chlorite  frequently  imitates  the  forms  of  the  horn- 
blende. The  biotite  is  in  well  developed,  sharpl}"  outlined  blades.  These 
minerals  are  ordinarily  between  the  feldspars,  but  bear  such  relation  to 
them  as  to  suggest  that  the  feldspars  have  adapted  their  forms  to  these 
minerals  rather  than  the  reverse.  Not  infrequent!)^  each  of  them  is  included 
in  large  blades  or  crystals  in  the  feldspar.  It  follows  from  these  relations 
that  the  hornblende  and  biotite  were  earlier  than  the  feldspar,  or  else  that 
these  minerals  have  developed  subsequently  to  the  consolidation  of  the  rock. 

The  most  important  fact  developed  by  the  study  of  the  Southern 
Complex  is  the  apparent  gradual  chang-e  between  the  massive  rocks  and  the 
schistose  ones.  These  gradations  have  already  been  alluded  to.  It  will,  then, 
here  only  be  necessary  to  remember  that  the  lines  separating  the  granites  and 
the  gneissoid  granites  from  the  fine  grained  gneisses  and  schists  ai'e  more 


124  THE  PENOKEB  IRON-BEARING  SERIES. 

or  less  arbitrary.  In  the  field  the  massive  granite,  gneissoid  granite,  grani-  • 
toid  gneiss,  coarse  gneiss,  and  fine  grained_  gneiss  are  sometiiities  found  in 
order  in  passing  froni  a  granite  into  a  scliist  area.  This  change  has  not 
been  found  in  continuous  exposure,  but  in  detached  ones.  More  freqi;ently 
a  different  relation  is  found,  the  fine  grained  crystalline  schists  being  cut  by 
massive  granites  in  such  a  manner  as  to  leave  no  doubt  of  the  eruptive 
nature  of  the  latter.  In  })assing  from  a  schist  to  a  granite  area,  there  first 
apjjears  cutting  the  schists  rare  small  veins  and  stringers  of  granite;  then 
the  g-ranite  is  found  in  dike-like  forms  or  in  masses  and  bosses  within  the 
schist.  Next,  the  granite  becomes  predominant,  and  finally  the  schists 
altogether  disappear  as  we  get  wholly  within  a  granite  or  gneissoid  granite 
area.^  The  old  interpretation  placed  on  such  apparent  transitions  from 
finel}^  schistose  to  massive  rocks  has  been  that  metamorphic  agencies  have 
transformed  the  crystalline  schists  into  the  massive  granites.  It  has  been 
taken  for  granted  that  the  strongl)^  schistose,  finely  laminated  phases  must 
be  of  sedimentar}'  origin.  This  being  the  case,  their  gradation  into  massive 
rocks  was  taken  as  proof  of  the  derivation  of  the  latter  from  the  former  by 
metamorphosing  agencies;  that  is,  luoisture,  heat,  and  pressure  have  re- 
crystallized  tlie  rock,  giving  it  a  coarsely  crystalline  granitic  structure  in 
place  of  the  finely  t;rystalline  schistose  one.  This  interpretation  is  based 
upon  the  assumption  that  the  strongly  foliated  schistose  rocks  are  of  sedi- 
mentary origin.  If  this  is  unprovable  the  conclusion  is  valueless.  At  the 
present  time  this  derivation  of  massive  syenites  and  granites  from  schistose 
rocks  by  metamorphosing  agencies  is  greatly  discredited.  It  is  almost 
universally  believed  that  massive  granites,  syenites,  and  gabbros  are  of 
eruptive  origin.  If  this  is  assumed,  there  seems  to  be  no  escape  from  the 
conclusion  that  a  large  proportion  of  the  crystalline  schists  and  gneisses  of 
the  area  were  originall)'  eruptives. 

1  These  relations  are  similar  to  those  described  by  Dr.  Audrew  C.  Lawsou,  of  the  Cauadiau  Geo- 
logical Survey,  as  occmTiny  between  the  Kocwatin  ;iiid  CVmtcliiohiug  series,  and  the  associated  I  au- 
reiitian  granites  and  gneisses.  Report  on  tlie  Geology  of  the  Rainy  Lake  Region,  Andrew  C.  Lawsou, 
Auuiuil  Report  of  tlie  Geological  iiud  Natural  History  Survey  of  Canada  for  1887,  Alfred  R.  C.  Selwyn, 
director;  part  F,  pp.  1-182.  In  this  paper  Dr.  Lawsou  maiutaius  that  the  granite  which  is  assoi- 
ciated  with  and  cuts  the  schist  has  been  produced  by  the  fusion  of  the  schists,  thus  changing  them 
to  truly  igneous  rocks. 


THE  SOUTFIKRN  COMPLEX.  125 

Still  further,  it  has  l)een  seen  tli;it   the  syenites  in  the  same  exposure 
vary  into  syenite-schists;    il    luis   Ix'cu   seen    that    tlie  granites   vary   into 
granitoid  gneisses;  and  tlic  nature  i>t'  the,  mineral  changes  which  explain 
this   variation   has   been  given.      The  coarser  grained    syenite-schists   are 
found,  by  examining  them  in  thin  section,  to  hav(!  a  background  composed 
of  closely  interlocking  feldspar  precisely  like  the  massive  syenites.     The 
foliation  of  these  rocks  is  due  to  the  biotite  and  hornblende.     The  large 
blades  of  these  minerals,  arranged  with  their  axes  in  a  common  direction, 
are  within  the  feldspars,  many  blades  often  being  included  withhi  a  single 
individual  of  that  mineral.     The  relation  is  such  in  many  cases  as  to  make 
it  almost  certain  that  these  minerals  have  developed  within  the  feldspar  as 
secondary  products.     The  pressure  to  which  the  rocks  were  doubtless  sub- 
ject during  the  time  of  the  growth  of  these  minerals  i)robably  influenced 
their  arrangement.     The  alteration  of  a  massive  feldspathic  rock  has  pro- 
duced a  foliated  micaceous  or  hornblendic  one.     When  this  alteration  of 
the  acid  feldspar  to  the  more  basic  minerals,  Ijiotite  and  hornblende,  has  been 
accompanied  by  the  simultaneous  separation  of  (quartz,  the  result  is  a  crys- 
talline gneiss.     Phases  of  strongly  foliated  rocks  are  found,   from   those 
which  contain  no  quartz — that  is,  a  syenite-schist — to  those  in  which  quartz 
is  as  abundant  as  the  feldspar,   when  the  rock  is  a  gneiss.     In  the  fine 
grained  syenites  and  schists,  in  which  the  feldspar  occurs  in  small  roundish 
grains,  there  must  have  been  a  recrystallization  of  the  feldspar  itself  if  these 
rocks  are  transformed  massive   eru})tives,   which  is  by  no  means  certam. 
The  question  as  to  whether  any  of  the  crystalline  schists  and  gneisses 
of  the  region  are  of  fragmental  origin  can  not  be  answered.     The  roundish, 
evenly  granular  appearance  of  the  quartz  and  feldspar  in  many  of  them 
strono-ly  suggests  upon  casual  examination  a  clastic  origin.     However,  the 
more  closely  they  are  examined  the  more  clearly  it  is  apparent  that  this  is 
not  sufficient  evidence  of  a  fragmental  character.     It  has  been  noted  that  in 
the  gneisses  of  this  class  the  individuals  fit   each  other  perfectly.     The 
grains  as  fragmental  ones  could  not  possibly  have  thus  been  deposited. 
If  fragmental,  either  they  must  have  recrystallized  or  else  have  been  enlarged 
until  they  interlocked.     Each  of  these  processes  and  botli  combined  are 
now  known  to  transform  fragmental  rocks  into  crystalline  ones;  but  in 


126  THE  PENOKEE  lEON-BEAEIFGr  SERIES. 

order  to  prove  that  such  a  transformation  has  occurred,  it  is  necessary  to 
show  that  the  rock  in  which  they  have  been  found  to  occur  was  a  frag- 
mental  one.  The  rounded  cores  in  a  quartzite  are  sufficient  to  show  this, 
but  in  the  case  of  a  recrystaUization  of  feldspar  it  is  necessary  actually  to 
trace  the  crystalline  rock  back  to  its  fragmental  state.  The  modified  frag- 
mental  rocks  of  these  kinds  in  the  Penokee  series  reveal  themselves  as 
such  with  little  difficulty.  Upon  succeeding  pages  it  is  remarked  that 
whenever  quartz  is  present  in  them  as  a  fragmental  constituent,  even  if 
associated  with  quartz  not  of  this  origin,  it  is  easily  discovered.  Its  original 
outlines  are  so  strongly  marked  by  particles  of  gas,  iron  oxide,  and  other 
inclusions  that  all  subsequent  changes  seem  inadequate  to  obliterate  them. 
While  this  is  the  case  with  these  known  fragmental  rocks,  the  complete 
absence  of  anything  which  suggests  that  there  have  been  cores  in  any  case 
in  the  sections  of  the  crystalline  schists  loses  largely  its  force  as  negative 
evidence  against  a  fragmental  origin;  because  the  fragmental  rocks  men- 
tioned are  nowhere  foliated,  while  the  schists  are  strongly  foliated.  Frag- 
mental rocks  in  other  districts  which  have  been  subject  to  dynamic  forces, 
and  have  therefore  become  foliated,  have  rapidly  lost  evidence  of  their 
original  fragmental  character. 

To  conclude,  if  the  massive,  coarsely  crystalline  rocks  are  assumed  to 
be  of  eruptive  oi-igin,  it  uecessarilj^  follows  that  a  large  number  of  the 
schistose  ones  are  certainly  derived  from  them ;  also,  it  has  not  been  possi- 
ble to  trace  any  of  these  crystalline  schists  back  to  a  fragmental  rock. 


CHAPTER  III. 


By  R.  D.  Ikving  and  (J.  R.  Van  Hisb. 


THE  CHERTY  LIMESTONE. 

Relation  of  tho  liuiostouo  aud  olicrt.  Guogniiiliical  distributiou.  Possible  former  greater  coutinuity. 
Thickness.  Petrographiciil  character  of  the  liiuestone.  Petrographical  character  of  the  chert. 
Change  to  tlie  overlying  Quartz-slate.  Tabulation  of  i)etrographical  observations.  Prominent 
exposures.     Origin  of  the  limestone  and  chert.     Summary. 

Relation  of  the  limestone  and  chert. — In  the  third  volume  of  the  Greol- 
ogy  of  Wisconsin  (pp.  104,  106-108),  and  on  Pis.  xxiii  to  xxvi  of  the  atlas 
accompanying  that  volume,  the  lowest  member  of  the  Iron-bearing-  series  of 
the  Penokee  district  is  represented  as  being  a  tremolitic  crystalline  limestone, 
with  an  average  thickness  of  90  feet,  above  which  is  said  to  succeed  a 
peculiar  white  t^uartzite,  with  an '  average  thickness  of  40  feet.  These 
two  layers,  though  occasionally  wanting  altogether,  as  indicated  on  the 
maps  referred  to,  are  represented  as  having  a  considerable  longitudinal 
extent  and  as  being  separate  from  one  another ;  that  is  to  say,  all  of  the 
white  quartz  is  taken  as  lying  above  all  of  the  limestone.^  That  this 
is  the  case  was  concluded  particularly  from  the  exposures  seen  at  Penokee 
gap,  where  such  a  succession  is  very  manifestly  the  true  one,  as  indicated 
on  PI.  XXVI  of  the  Wisconsin  atlas.  Our  later  investigations,  and  more  par- 
ticularly our  experience  on  the  Michigan  side  of  the  Montreal  river,  have 
shown  us,  however,  that  in  fact  these  two  rocks  form  portions  t)f  (jiie  and 
the  same  layer;  that  is  to  say,  that  the  white  quartz,  (jr  chert,  as  we  now 
think  it  should  more  properly  be  called,  occurs  as  a  rule  interstratified  and 
thinly  interlaminated  with  the  limestone.     Taken  together,   these  two  ma- 

' However,    a  doubt  was  expressed  (Geol.  of  Wis.,  vol.   m,  1880,   p.  110)  as  to  whether  the 

limestone  exposures  in  the  western  part  of  the  Penokee  district  might  not  represent  both  limestone 

and  cljert  as  seen  at  Penokee  gap. 

X27 


128  THE  PEiSrOKEE  lEON-BEAEIJiTG  SEEIES. 

terials  constitute  a  well  marked  belt,  which  is  sharply  separated,  not  only 
from  the  granites  and  schists  to  the  south  of  it,  but  also  from  the  fragmen- 
tal  slates  immediately  above,  from  which  it  ditfers  in  being  almost  free 
from  an  interjnixture  of  fragmental  material. 

Geofjraphkal  distribution. — It  is  difficult  to  define  very  accurately  the 
geographical  distribution  of  the  limestone  member.  Certainly  in  a  number 
of  places  it  is  entirely  wanting,  since  the  strongly  marked  slate  belonging 
immediately  above  it  is  found  directly  in  contact  with  the  granites  and 
schists  which  belong  below  it.  When  the  atlas  sheets  of  the  Geology  of 
Wisconsin  were  drawn,  this  belt  was  represented  as  having  a  greater 
degree  of  continuity  than  is  indicated  on  tlie  maps  that  accompany  the 
present  volume,  exposures  distant  from  one  another  having  been  relied  on 
to  indicate  a  continuity,  so  that,  substantially,  this  member  was  repre- 
sented as  continuous  except  where  it  was  certainly  known  to  be  wanting. 
Our  later  experience  having  taught  us  that  this  member  may  thin  out  and 
disappear  quite  suddenly,  and  that  it  is  lacking  more  often  than  was  for- 
merly supposed,  we  have  pursued  just  the  opposite  course  in  mapping  it; 
that  is  to  say,  we  have  mapped  it  as  occuring  only  in  those  places  where 
exposures  demonstrate  its  existence. 

The  detailed  maps  of  Pis,  v,  vi,  viii,  x,  xii,  and  xiii  of  the  present 
volume,  which  show  the  exact  positions  of  all  of  the  exposures  belonging 
to  the  various  belts  of  the  Penokee  series,  will  serve  to  indicate  to  the 
reader  the  exact  facts  which  we  have  had  at  command  in  deciding  as  to 
the  degree  of  continuity  possessed  by  this  member.  This  member  then 
is  indicated  on  our  maps  as  appearing  in  six  detached  portions  in  the 
distance  between  Atkins  lake,  T.  44  N.,  R.  5  W.,  Wisconsin,  and  the  Little 
Presque  Isle  river,  T.  47  N.,  R.  44  W.,  Michigan.  Beginning  with  the  west- 
ernmost of  these  areas,  it  is  to  be  said  that,  so  far  as  our  knowledge  goes, 
it  may  extend  some  miles  farther  west  than  the  maps  indicate,  since  we  are 
without  exposures  to  prove  either  its  continuity  or  its  absence.  The  eastern 
end  of  this  westernmost  area,  however,  is  sharply  defined  by  actual  expos- 
ures, as  indicated  on  PI.  v,  the  granite  and  siliceous  slate  member  of  the 
iron  series  coming  directly  into  contact  with  one  another  along  the  course 
of  the  Marengo  river,  in  the  SW.  \  of  Sec.  15,  andNW.  \  Sec.  23,' T.  44  N., 


SOUTHERN   COMPLEX. 


Eruptives 


>J^    Exposures  of  eruptives 
Souih  limit 


-North  limn 
South  limit 


;  9Mt  Exposures  of  ferruginous  schist. 
Limits  of  surface  distribution 


SI.  Exposures 
Q.  Exposures 
South   limit 


of  slaty  phases 
rf  quartzitic  phases 


=  Exposures   showing  sTnhe  and  dip.    '^  Exposures  showing  noslructure       Li 

Scale  of  Map  I  inch  =  l  mile  Sc 


Exposuresof  limestone  and  chert  /iW5^   Exposures  of  greenstones  ^/<}:--  Exposures  of  granite 

-  South  limit 


nes  of  exploration  by  U  3  Gaol  Survey    —  —  Lines  of  exploration  by  Wis  Geo!   Survey 
ale  of  Sections!  inch  =  1320  feet 


DETAILED    GEOLOGY  OF  THE    PE;N0KEE   DISTRICT.  SHEET  I. 


THE  (;ili:iiTV    LIMESTONE.  126 

R.  5  W.,  Wisconsin.  Farther  east  wc  do  not  meet  with  any  exposures  of  the 
hniestone  until  Sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin,  southeastward  from 
Hhidder  hike,  is  reached.  In  tlie  interval  the  only  known  rock  is  a  <rreat 
mass  of  gabbro.  Whether  any  of  tlie  members  of  the  iron  series  are 
here  continuous  is  an  open  (piestion.  The  limestone  exposure  just  referred 
to,  which  exposure,  however,  indicates  an  unusual  thickness  for  the  belt, 
is  the  only  fact  we  have  for  the  area  indicated  on  the  general  map  as 
extending  through  Sees.  23  and  24  of  T.  44  N.,  R.  4  W.,  Wisconsin,  and  Sec. 
ID  of  T.  44  N.,  R.  3  W.,  Wisconsin.  So  far  as  exposures  go,  however,  there 
is  nothing  to  indicate  that  this  area  can  not  be  continuous  with  that  long  one 
which  begins  about  a  mile  west  of  Bad  river,  in  Sec.  15,  T.  44  N.,  R. 
3  W.,  Wisconsin.  The  eastward  continuation  of  the  latter  area  is  indicated 
by  several  exposures  in  R.  2  W.,  and  it  is  possible  that  there  is  a  connection 
with  the  area  which  is  represented  as  occurring  in  the  vicinity  of  Tylers 
fork,  in  the  southerly  part  of  T.  45  N.,  R.  1  W.,  Wisconsin.  There  is, 
however,  certainly  a  break  in  the  continuity  before  the  Potato  river  is 
reached,  for  here  the  Quartz-slate  member  of  the  iron-bearing  series  lies 
with  a  marked  unconformity  directly  upon  a  xjhlpritic  schist  of  the  older 
series.  To  the  east  of  Potato  river  only  small  exposures  of  the  member  are 
met  with — one  in  the  middle  of  T.  45  N.,  R.  1  E.,  Wisconsin,  and  two  in  T. 
47  N.,  R.  46  W.,  Michigan — vintil  a  point  some  2  miles  east  of  Sunday  lake 
in  Michifjan  is  reached^.  Through  some  of  this  distance  the  Limestone 
member  must  certainly  be  absent,  since  the  exposures  of  the  Quartz-slate 
member  and  of  the  lower  rocks  come  into  direct  contact  with  one  another, 
as  on  the  West  Branch  of  the  Montreal,  T.  46  N.,  R.  2  E.,  Wisconsin,  or  so 
close  to  one  another  as  to  leave  no  room  for  the  limestone  belt.  The  eastern- 
most of  these  limestone  areas,  that  in  the  eastern  part  of  T.  47  N.,  R.  45  W., 
Michigan,  and  western  part  of  T.  47  N.,  R.  44  W.,  Michigan,  is  one  of  the 
most  extensive  and  best  exposed  in  the  series. 

Possible  former  greater  continuity. — The  occurrence  of  cherty  fragments 
in  the  lower  portion  of  the  Quai-tz-slate  member  of  the  iron  series  in  places 
where  the  limestone  with  this  chert  are .  themselves  entirely  wanting  is 
strongly  suggestive  of  a  former  greater  continuity  for  the  Limestone  member 
than  it  now  possesses.     This  occurrence  of  cherty  fragments  is  frequent 

MON.  XIX 9 


130  THE  PElSrOKEE  lEON-BEAEING  SEEIES. 

at  the  base  of  the  slate.  It  is  shown  below  that  whatever  may  be 
the  origin  of  the  chert  of  the  limestone  belt,  it  had  plainly  in  the  main 
reached  its  present  condition  before  the  beginning-  of  those  processes  of 
mechanical  sedimentation  by  which  the  Quartz-slate  member  was  accumu- 
lated, since  fragments  of  the  chert  are  commonly  included  within  the  slate. 
It  is  more  than  probable  that  the  forces  of  erosion  which  accumulated  the 
fragmental  slate  may  liave  in  places  completely  swept  away  the  underlying 
limestone  and  chert. 

Thickness. — The  thicknesses  of  this  member,  noted  at  different  points, 
vary  between  somewhat  wide  limits.  At  Penokee  ga]?  the  entire  surface 
width  of  the  chert  and  limestone  together  is  125  feet,  which  corresponds  to 
a  thickness  of  about  113  feet.  On  the  other  hand,  in  the  SE.  ^  of  Sec.  18, 
T.  47  N.,  R.  44  W.,  Michigan,  there  is  a  continuous-  surface  exposure  of 
limestone  and  chert  of  such  width  that  the  thickness  of  the  member  in  this 
place  must  be  at  least  300  feet.  This  is  the  greatest  thickness  that  has  any- 
where been  observed,  but  there  are  indications  that  in  some  places  the 
member  may  at  times  be  much  thinner  than  is  indicated  by  the  Penokee 
gap  section,  as  would  be  expected,  since  it  is  at  times  wanting  altogether. 

Petrographical  character  of  the  limestone. — Externally  the  limestones  of 
this  belt  vary  very  considerably  in  appearances,  the  variation  being  due 
mainly  to  a  varying  coarseness  of  grain,  which  is  at  times  so  small  that  the 
rock  is  not  far  different  in  appearance  from  any  compact  earthy  limestone 
of  the  fossiliferous  series.  On  the  other  hand,  it  at  times  presents  a  very 
distinct  and  somewhat  coarsely  crystalline  aspect,  though  the  more  com- 
pact kinds  predominate. 

The  color  generally  ranges  from  white  to  gray,  iron-stained  portions 
being  rather  unusual.  Thin  blades  of  white  tremolite  may  occasionally  be 
seen  on  the  surface  of  a  specimen.  Analyses  of  the  soluble  portions  of 
these  limestones  indicate  that  they  are  always  strongly  magnes^ian ;  indeed, 
as  shown  by  the  following  analyses  by  Mr.  W.  F.  Hillebrand,  of  the  U.  S. 
Geological  Survey,  they  have  so  much  magnesia  as  to  justify  the  applica- 
tion to  them  of  the  name  dolomite.  No.  1  is  from  near  Sunday  lake, 
(specimen  9406)  SE.  \  Sec.  18,  T.  47  N.,  R.  44  W.,  Michigan;  No.  2 
(specimen  9677)  is  from  the  NW.  \  Sec.  22,  T.  44  N.,  R.  6  W.,  Wisconsin. 


U.  S.  GEOLOGICAL    SURVEY- 


MONOGRAPH    XIX    PL  VI 


KEWEENAW     SERIES 


)»W  Exposures  of  erupiives 
South  limit 


PENOKEE   SERIES. 


Upper-Slate  Member  Iron-bearintf  Member. 

=  Z    R,i  «,"?"'f*  "'P^*"^"  =  JOW  Exposures  of  ferrug.nous  sch.st 

iZn        ?.     "^  ^^"^^  "posures  L.m>ts  of  surface  d.str.bution 

:  WW    Q        Quartt.te  exposures 
E  SW  Gw     GraywacKe  exposures 


Quartz-Slate  Member 

SI.Exposures  of  slafy  phases 
0  txposures  of  (^uartiiTic  phases. 


Cheny  Limestone  Member 

^  Exposuresof  limestone  and  chert 
South  limit. 


Eruptives. 

y/tfi  Exposures  of  greensfom 


SOUTHERN   COMPLEX. 

^=^     W  Exposures  of  hornblende  schist. 
■?>:'   Exposures  of  granite 


xposures   sho».ng  sinke  and  d.p  W  Exposures  sho«,ng  no  structure         Lines  of  exploration  by  US  Seol  Survey 

Lines  ofexplorationby  Wis  Geol,  Survey  Scale  of  Map  I  inch -I  mile. 

Scale  of  Sections  I  inch  =  1320  feet 


DETAILED   GEOLOGY  OF  THE    PENOKEE   DISTRICT,  SHEET   2. 


THE  CIIKUTV  lilMESTONK. 
Analysen  of  Hmcstonen, 


Vol 


SlOa 

TiO, 

Al.O;,      

F0;O:, 

FcO 

MuO 

CaO 

MrO 

HjO 

CO, 

PcO. 

SO, 

CI  

FeSj 

Organic  matter. 


No.  1. 


3-07 


OOil 

0-86 

0'15 

20 -72 

19-95 

0'30 

45-31 


Trace. 


99-45 


No.  2. 


0-63 


0-03 

0-75 

0-08 

30-94 

20-68 

0-27 

46-27 


Trace. 


99-85 


Dolomite  and  calcite  are  not  readily  distinguished  in  the  thin  section. 
So  far,  however,  as  appearances  go,  the  carbonate  entering  into  the  compo- 
sition of  these  rocks  seems  to  be  always  the  same,  and,  judging  from  the 
content  of  magnesium  carbonate,  it  is  rather  to  be  counted  as  dolomite  than 
as  a  mixture  of  dolomite  and  calcite  or  as  a  magnesian  calcite,  though  the 
assertion  that  calcite  is  always  absent  is  not  ventured.  Under  the  micro- 
scope the  dolomite  individuals,  which  constitute  the  larger  part  of  each 
section,  present  the  usual  appearance  met  with  in  sections  of  crystalline 
limestone,  being  without  crystal  outlines,  but  fitting  together  closely  along 
irregula*  curved  lines.  The  interlocking  of  the  grains  is  however,  usually 
somewhat  greater  than  in  the  case  of  statuary  marble. 

In  addition  to  the  carbonate,  most  of  the  sections  show  more  or  less 
tremolite,  which  varies  greatly  in  quantity.  Occasionally  only  a  few 
minute  flakes  are  discoverable,  while  in  other  cases  broad  single  blades  of 
radiating  clusters  make  up  a  large  portion  of  the  thin  section.  The  tremo- 
lite blades  often  penetrate  the  carbonate  in  every  direction,  and  in  the  case 
of  the  larger  blades  the  dolomite  appears  to  be  included  within  the  tremo- 
lite. In  yet  other  cases  the  tremolite  in  aggregated  blades  appears  to 
make  up  the  whole  of  certain  narrow  belts  in  the  thin  section.     The  tremo- 


132  THE  PENOKEB  IROISr-BEAEING  SERIES. 

lite  is  known  to  be  such  by  its  usual  microscopical  characters,  which  need 
not  be  here  enumerated.  Fig.  1  of  PL  xvi  is  from  a  section  of  one  of  these 
limestones  magnified  sixty  diamaters,  in  which  there  is  shown  one  of  the 
larger  tremolite  blades,  through  whi(;h  in  places  are  dimly  seen  the  out- 
lines of  the  dolomite  individuals. 

Many  of  the  sections  of  these  limestones  show  more  or  less  of  a  sili- 
ceous ingredient,  which  is  found  in  varying  quantities  up  to  an  amount 
which  very  largely  predominates  over  that  of  the  cax'bonate.  In  fact,  these 
siliceous  varieties  furnish  us  with  a  complete  gradation  into  the  chert  rock, 
which,  as  already  indicated,  at  times  excludes  the  limestone  completely. 
Now  and  then  there  are  apparent  in  the  thin  sections  of  these  siliceous 
limestones  a  few  grains  of  quartz,  whose  fragmental  nature  is  demonstrated 
by  their  rounded  contours  and  by  the  secondary  enlargements  they  have 
occasionally  received.  But  these  fragmental  particles  are  relatively  sparse 
and  unimportant,  the  most  of  the  silica  having  plainly  solidified  in  situ.  Of 
the  latter  silica  there  may  be  distinguished  two  varieties,  which,  however, 
grade  into  one  another.  The  first  of  these  presents  itself  in  the  shape  of 
an  interlocking  mass  of  quartz  individuals  of  finer  or  coarser  grain.  As 
these  become  finer  and  finer,  there  is  found  intermingled  with  them  more 
or  less  of  a  fine  spotty  and  chalcedonic  silica  with  the  characteristic  aggre- 
gate polarization  and  radiating  structure  ;  and  finally  they  pass  into  kinds 
containing  a  good  deal  of  a  completely  amorphous  opaline  material.  In 
Fig.  2  of  PI.  XVI  the  thin  section  of  one  of  the  more  siliceous  varieties  of 
limestone  is  represented  as  seen  in  polarized  light  and  magnified  sixty 
diameters.  In  the  middle  band  of  this  figure  the  dolomite  predominates 
greatly  over  the  silica.  On  the  upper  right-hand  side  of  the  figure  is  a 
band  comjDosed  of  the  finely  crystalline  silica,  and  at  the  lower  left-hand 
corner  is  an  area  of  the  more  coarsely  crystalline  quartz. 

Petrographical  character  of  the  chert. — -As  already  indicated,  the  cherty 
material  of  this  limestone  belt  is  often  in  layers  of  considerable  thickness, 
at  times  apparently  making  up  the  greater  part  of  the  whole  belt.  At 
Penokee  gap,  for  instance,  it  has  a  thickness  of  some  45  feet.  In  other 
cases  it  is  scattered  through  or  is  interstratified  with  the  limestone  in  narrow 
seams.     As  in  the  case  of  the  siliceous  material  just  described  a-s  occurring 


THE  UIIEltTY  LIMESTONE.  133 

at  times  closely  intermiiifrlod  \\'itli  the  dolomite,  so  also  in  the  case  of  this 
chert  rock  the  silica  presents  itself  in  completely  crystalline,  half  crystal- 
line, and  amorphons  conditions,  these  several  phases  occurring  at  times  in- 
termingled with  one  anothin-.     In  some  cases,  however,  as  for  instance  in 
the  rock  at  Penokee  gap,  all  of  the  silica  is  completely  individualized,  and 
the   individuals  furnished  more   or  less  thoroughly  with  crystal  outlines. 
As  seen  microscopically,  this  peculiar  quartz  rock  is  perfectly  white,  stud- 
ded with  minute  crystal  facets,  and  of  a  saccharoidal  texture,  being  often 
so  crumbly  as  to  be  readily  mistaken  for  a  fine  grained  sandstone.     This 
imiH-ession  is  confirmed  by  the  crystal  facets,  which  one  takes  at  once  to  be 
in  the  nature  of  the.  enlargements  of  quartz  fragments  commonly  met  with 
in  sandstone,  but  the  examination  of  the  thin  section  fails  to  substantiate 
the  impression,  since  the  outlines  of  the  original  grains  are  not  perceptible. 
On  the  whole,   then,  on  account  of  the  gradation  varieties  between  this 
peculiar  rock  and  those  phases  in  which  there  is  more  or  less  chalcedonic 
and  amorphous  silica,  we  conclude  that  here  also  the  whole  of  the  silica 
has  separated  out  in  situ. 

The  accessory  constituents  in  this  chert  are  few  and  unimportant  in 
quantity.  They  include  sericite,  brown  iron  oxide,  and  occasionally  mag- 
netite and  dolomite.  The  usual  snow  white  color  of  the  rock  is  due  to  the 
general  absence  or  sparseness  of  the  iron  oxide  ingredient.  Only  rarely  is 
the  iron  oxide  present  in  sufficient  quantity  thoroughly  to  redden  the  rock. 
In  these  rare  cases,  however,  the  chert  resembles  a  jasper. 

In  PI.  XVI,  Fig.  3,  we  have  represented  one  of  the  finer  grained  phases 
of  chert  as  seen  in  polarized  light.  The  concietionary  and  semichalcedonic 
arrangement  of  the  silica  is  plain,  the  finer  grained  portions  tending  to  form 
the  centers  of  areas  the  outer  portions  of  which  and  the  interspaces  between 
which  are  composed  of  the  coarser  grained  quartz.  PI.  xvi,  Fig.  4,  shows 
a  section  of  the  chert  in  which  a  band  of  the  coarser  grained  quartz  is  seen 
adjacent  to  one  of  finer,  but  still  completely  crystalline,  material.  Our  photo- 
graphs of  these  sections  of  the  chert  and  limestone  containino-  the  finest 
grained  and  amorphous  silica  have  not  proved  sufficiently  successful  for  re- 
production, largely  on  account  of  the  confused  appearance  of  the  objects 
themselves. 


134  THE  PENOKBE  lEON-BEAEING  SEEIES. 

One  of  the  most  notable  peculiarities  of  this  cherty  rock  is  its  tendency 
to  assume  a  brecciated  form,  in  which  angular  pieces  of  the  chert,  ranging 
from  microscopic  sizes  to  fragments  two  or  three  inches  across,  are  buried 
in  a  chert  of  a  character  wholly  similar  to  that  of  the  fragments  or  differing 
from  them  only  in  carrying  a  small  quantity  of  other  ingredients,  such  as 
magnetite  and  chlorite.  These  brecciated  phases  occur  at  times  wholly 
within  the  horizon  of  the  Cherty  limestone  member  itself,  and  in  other  cases 
are  very  near  to  its  junction  to  the  overlying  slate,  when  it  is  not  always 
easy  to  draw  the  line  between  the  two  members. 

Change  to  the  overlying  Quartz-slate. — As  already  indicated  and  subse- 
quently further  explained,  the  limestone  or  basal  member  of  the  sedi- 
mentary series  is  directly  overlain  by  a  very  considerable  thickness  of  com- 
pletely fragmental  rocks,  whose  main  constituent  is  quartz,  which  mineral 
is,  however,  accompanied  by  a  large  proportion  of, feldspar  fragments  and 
by  various  alteration  derivatives  from  the  feldspars.  The  change  from  the 
limestone  member  to  this  quartz-slate  is  very  sharp,  the  fragmental  rock 
often  carrying  at  its  base  pieces  from  the  cherty  material  belonging  directly 
beneath  it.  As  this  detritus  in  the  quartz-slate  is  precisely  like  the  material 
of  the  cherty  limestone  as  it  now  exists,  there  must  have  been  a  considerable 
lapse  of  time  between  the  deposition  of  the  two  formations. 

Tabulation  of  petrographical  observations. — Although,  in  the  present  de- 
velopment of  the  knowledge  of  petrography,  it  is  rather  unusual  to  include 
a  description  of  individual  sections  in  a  volume  like  this,  it  is  given  in  this 
and  succeeding  chapters  because  the  essential  unitj^  and  continuity  of  each 
formation,  as  well  as  the  contrast  between  it  and  the  following  one,  thus 
appear  with  a  clearness  which  can  be  enforced  in  no  other  way. 

This  detailed  observation  is  desirable  for  the  further  reason  that,  as 
explained  in  the  Preface,  the  Penokee  district  is  dealt  with  in  greater  detail 
than  is  intended  with  any  subsequent  iron-bearing  district,  the  reason  for 
the  great  elaboration  being  that  tliis  is  the  first  of  the  iron-producing 
areas  of  lake  Superior  in  which  the  geology  has  been  fully  worked  out. 

The  numbers  of  specimens  and  slides  are  usually  those  of  the  collec- 
tion of  the  lake  Superior  division.  Specimens  with  Wr.  after  the  numbers 
are  from  the  collection  of  the  late  Mr.  Chas.  E.  Wright.      Specimens  with 


KEWEENAW     SERIES 


PENOKEE    SERIES. 


SOUTHERN   COMPLEX. 


Upper  Slate  Member  Iron-bearing  Member  Quartz-Slate  Member,  Cherty  limestone  Member  Eruptives 

=    »MC  Exposures  of  ferruginous  schist.  SI. Exposures  of  slaty  phases       ^Exposuresaf  hmestoneand  ch«rt  «*  Exposiires  of  greenstones  jJJJi  i^'^.^^^P^'^-'.^,^^^^^^ 

'       -       '        '--     '      -•^'    -  Q  Exposures  0* .-..- ->- =;-....k  i.«r*  t-  =•     *p 

South   limit 


)SiW  Exposures  of  erupt ives  =    ^WfiJ  M  S  Mtca-slote  exposuiea     —    "w.  i..|ju3ij.c»  u.  .^. ,  L.g,M«ua  a^...*..  ^,. ,.„f^w^_.>,w  >., ,  , 1 —  -  .  u  jg^  ^  „  fxdosi 

-South  limit  ^    ^aw  Bl  SI  Black  slate  exposures Limits  of  surface  distribution  Q  Exposures  of  quartzitic  phases South  limrt  "^       of  granite 


^=    iWK  Q        QuaHzile  exposures 
^=    Afr'Gvv     (jraywache  exposures 

^Exposures   showrng  sTnke  and  dip     >!?»  Exposures  showing  no  structure       Li  ne  s  of  exploration  by  U  S  Geol  Survey Lines  of  exploration  by  Wis  Geol   Survey 

Scale  of  Map  I  inch=l  mile     Scale  of  Section  I  inch  =  1320  feet 

DETAILED    GEOLOGY  OF  THE    PENOKEE   DISTRICT,  SHEET  3. 


Tril';  (MIKKTV  LIMESTOXE.  135 

Wis.  after  tlic  imnil)ci-s  ;irc  iVdiii  tlic  collection  of  the  Wisconsin  Geological 
Survc}-.  Locations  arc  f^ivcii  from  southeast  corner  of  the  section,  in  steps 
of  2,000  per  mile. 

1.  Treiiiolitic  dolomito.  Speciiucn  9078  (slide  3105).  From  1850  N.,  1C75  W., 
Sec.  22,  T.  44  N.,  K.  5  \V.,  Wis. 

A  iiuitbrmly  granular,  grayish  white  limestone;  in  appearance  very  close  to 
marble. 

The  thill  section  is  composed  chiefly  of  interlocking  grains  of  dolomite,  which 
often  show  the  characteristic  cleavage  and  twinning.  Mingled  with  the  dolomite  are 
quite  large  l)lades  of  tremolite,  which  in  many  places  are  more  or  less  decomposed 
(Pl.xvi,  Fig.  1). 

2.  Tremolitic  dolomite.  Specimen  189  Wr.  From  0  N.,  1900  W.,  Sec.  15,  T.  44 
N.,  E.  5  \V.,  Wis. 

The  mass  of  this  rock  is  like  1,  except  that  it  is  of  a  dark  gray  color.  Cutting 
through  it  are  veins  of  tremolite  in  very  coarse,  radiating  blades. 

The  greater  part  of  the  section  is  like  that  of  1,  but  a  portion  of  it  is  cut  from 
the  vein  of  tremolite,  which  material  occurs  iu  radiating  clusters  of  quite  large  sized 
brilliantly  jpolariziug  blades. 

3.  Quartz  rock.  Specimen  9670  (slide  31G3).  From  1900  N.,  1300  W.,  Sec.  22, 
T.  44  N.,  E.  5  W.,  Wis. 

A  finely  granular,  very  friable,  suow-white  quartz  rock,  which  in  the  sunlight 
exhibits  immmerable  glittering  crystal  facets. 

The  thin  section  is  comj»osed  of  a  minutely  crystalline  mass  of  quartz.  The 
quartz  individuals  are  in  large  part  crystal  outlined,  and  appear  to  fit  against  each 
other  x)erfectly,  face  to  face,  with  little  or  no  interlocking.  There  are  usually  no 
vacant  spaces  between  the  crystals,  but  the  fragility  and  ijseudo-arenaceous  texture  of 
the  rock  are  explained  by  its  peculiar  make-up.  No  amorphous  silica  is  present,  nor 
is  there  any  evidence  that  the  crystals  have  been  produced  by  the  enlargement  of 
quartz  fragments,  as  in  ordinary  quartzites. 

4.  Tremolitic  dolomite.  Specimen  9654  (slide  3161).  Prom  1400  N.,  1000  W., 
Sec.  24,  T.  44  N.,  E.  4  W.,  Wis. 

This  rock  differs  from  1  in  that  a  portion  of  it  is  of  a  decided  greenish  color. 
The  section  differs  from  that  of  1  in  that  the  tremolite  is  almost  as  abundant  a? 
the  dolomite. 

5.  Dolomite.     Specimen  9653  (slide  31G0).     Prom  1300  N.,  1000  W.,  Sec.  24,  1 
44  R.,  E.  4  W.,  Wis. 

The  rock  is  fine  grained,  uniformly  granular,  and  of  a  gray  color. 
The  thin  section  is  composed  wholly  of  a  finely  and  evenly  granular  aggregatioi 
of  dolomite  individuals. 

6.  Tremolitic  dolomite.  Specimens  9531  (slide  3138),  1421  Wis.  (slide  251):  Fron 
1440  N.,  1200  W.,  Sec.  14,  T.  44  N.,  E.  3  W.,  Wis. 


136  THE  PENOKEE  mON-BEAEING  SEEIES. 

A  finely  granular,  uniformly  textured,  light  gray  rock,  containing  quite  numer- 
ous small  grains  and  crystals  of  pyrite.  The  rock  contains:  calcium  carbonate,  50-52 ; 
magnesium  carbonate,  33'41 ;  iron,  1'19;  insoluble  Ingredients,  13-85;  undetermined 
1-03=100.1 

The  sections  are  composed  almost  wholly  of  small,  closely  fitting  particles  of 
dolomite.  Scattered  through  them  are  quite  a  good  many  blades  of  tremolite.  Quartz 
and  pyrite  are  sparse  accessories.  A  small  portion  of  the  dolomite  exhibits  the  char- 
acteristic cleavage  and  twin  lamellation. 

7.  Sericitic  quartz  rock.     Specimen  9533  (slide  3139).     From  1460  N.,  1200  W., 
Sec.  14,  T.  44  N.,  R.  3  W.,  Wisconsin. 

A  rock  much  like  3,  but  of  a  coarser  grain. 

The  thin  section  is  composed  of  fine  grains  of  quartz,  which  include  many  minute 
flakes  of  sericite.  The  individuals  of  quartz  are  coarser  than  in  3,  and  while  occa- 
sionally showing  the  crystal  outlines,  the  greater  number  of  particles  form  junctions 
which  are  more  or  less  irregularly  curved.  The  sericite  is  equally  distributed 
throughout  the  section,  being  included  in  each  of  the  quartz  grains.  Often  a  single 
flake  of  sericite  is  seen  to  penetrate  two  or  more  grains  of  the  quartz.  No  evidence 
of  enlargement  of  the  quartz  can  be  detected. 

8.  Quartz  rock.     Specimens  4526  (slide  1112),  4520  (slide  1110).     From  1460  N., 
1200  W.,  Sec.  14,  T.  44  N.,  11.  3  W.,  Wisconsin. 

The  thin  section  differs  from  that  of  7  in  being  coarser  grained,  in  not  having 
the  quartz  particles  very  thoroughly  interlocked,  and  in  containing  relatively  little 
sericite.    Chlorite  also  occurs  as  a  sparse  accessory. 

9.  Sericitic  qirartz  rock.     Specimen  4534  (slide  1465).     From  the  extreme  NE.  :i 
of  Sec.  16,  T.  44  N.,  E.  2  W.,  Wisconsin. 

The  thin  section  resembles  closely  that.. of  7,  the  only  difference  of  importance 
being  the  occurrence  in  it  of  magnetite  in  a  few  small  crystals. 

10.  Flinty  cherts.  Specimens  7511  (slide  2056).  From  1925  N.,  1^34  W. ;  9430 
(slide  3129),  from  1900  N.,  1900  W.;  9434  (slide  3131),  from  1923  N.,  1940  W.,  Sec.  14, 
T.  47  N.,  E.  45  W.,  Michigan. 

A  fine  grained  to  aphanitic,  light  gray  or  pinkish  gray  chert  with  a  couchoidal 

,  fracture. 

The  sections  are  composed  essentially  of  a  minutely  crystalline  silica,  the  parti- 
cles of  which,  however,  vary  a  good  deal  in  size ;  some  portions  looking  as  though 
the  rock  might  be  in  part  amorphous.  In  other  parts' one  sees  that  there  is  a  vague 
concentric  arrangement,  the  amorphous  and  more  minutely  crystalline  silica  tending 
to  lie  in  the  middle  portions  of  certain  areas  whose  exterior  portions  are  made  iip  of 
more  coarsely  crystalline  particles.  This  is  an  arrangement  which  approaches  to  the 
texture  of  true  chalcedony,  the  resemblance  to  which  mineral  is  also  very  evident  in 


'  Wisconsin  Geol.  Survey,  vol.  iii,  p.  107. 


T46N. 


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TllK  CllKKTV  LIMESTONE.  l;37 

the  hand  s])eciiii<'ii  of  tlics<'  rocks.     In  i)laoes  a  very  small  (|uaiitity  of  hiown  iron 
oxide  is  ijrcscnt  in   niinnto  iiarticlcs.     (IM.  XVI,  EIk'-  •^■) 

11.  Ohwt.  Spcic.inicns  !»4li;!  (slide  .'{(Mi.'i),  !I424  (slide  .•iO<J4).  From  1740  N.,  1075 
W.,  Sw!.  14,  T.  47  N.,  H.  4.-)  W.,  Miclii^an. 

A  line  j;rained,  pinlcisli  saccliaroidal  chert. 

The  tliin  sections  are  like  those  last  described,  except  that  the  .silica  is  more 
coarsely  (crystalline.     (IM.  xvi,  Fig-.  4.) 

12.  Dolomites.  S])ecimcn.s  7480  (slide  mi',).  From  .550  N.,  0  W.;  9405  (.slide 
3049).     From  450  N.,  330  W.,  Sec.  18,  T.  47  N.,  K.  44  W.,  Michigan. 

Kocks  almost  exactly  like  0. 

In  thin  section,  as  in'  0,  dolomite  is  the  only  mineral  of  importance,  and  the 
general  appearance  is  the  same  as  in  that  slide.    A  little  tremolite  is  present. 

13.  Cherts  and  dolomites.  Specimens  7485  (slide  1934).  From  550  N.,  300  W.; 
9407  (slide  3124).     From  450  N.,  330  W.,  Sec.  18,  T.  47  N.,  E.  44  W.,  Michigan. 

Consist  of  interstratifled  flue  grained,  gray  limestone  and  milky  white  flinty 
chert. 

The  sections  include  portions  of  both  chert  and  limestone.  The  dolomite  is  like 
that  of  12;  the  chert  like  that  of  slides  2056  and  3131  in  10.  In  section  3124  the  line 
of  separation  between  the  chert  and  limestone  is  seen  to  be  perfectly  sharji.  (PI.  xvi, 
Fig.  2.) 

14.  Chloritic  dolomite.  Specimen  9404  (slide  3270).  From  600  N.,  40  W.,  Sec. 
18,  T.  47  N.,  K.  44  W.,  Michigan. 

This  rock  differs  from  most  of  the  previously  described  limestones  in  being  of 
a  pale  green  color. 

The  thin  section  is  composed  chiefly  of  finely  crystalline  dolomite,  mingled 
with  which  is  a  subordinate  quantity  of  pale  green  nonpolarizing  chlorite. 

15.  Dolomite.  Specimen  9408  (slide  3050).  From  525  IS.,  1925  W.,  Sec  17,  T. 
47  N.,  R.  44  W.,  Michigan. 

A  coarser  grained  rock  than  any  of  the  preceding  limestones;  of  a  bright 
flesh  color,  with  pale  green  spots  and  bands. 

The  thin  section  differs  from  3138  in  6  only  in  being  coarser  grained,  and  in 
containing  a  minute  quantity  of  ferrite,  to  the  presence  of  which  mineral  the  pinkish 
color  of  the  rock  is  due. 

16.  Chloritic  dolomite.  Specimen  9387  (slide  3041).  From  350  N.,  1550  W., 
Sec.  17,  T.  47  N.,  E.  44  W.,  Michigan. 

This  rock  differs  from  6  only  in  containing  streaks  of  a  greenish  material  and 
in  being  of  a  pinkish  gray  color. 

The  thin  section  is  like  that  of  14  except  that  some  ferrite  is  contained. 


138  THE  PEEOKEE  lEON-BEARING  SERIES. 

17.  Flinty  .chert.  Specimens  9413  (slide  3054),  9414  (slide  3055),  9415  (slide 
3056),  all  from  250  N.,  1350  W.,  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 

These  specimens  closely  resemble  9430  and  9434  in  10.  Immediately  overlying 
them  comes  the  fragmental  slate  that  forms  the  first  member  of  the  overlying  series. 

The  sections  of  these  specimens  show  in  part  numerous  quartz  fragments  buried 
in  a  matrix  of  minutely  crystalline  silica,  each  quartz  fragment  having  about  it  a  halo 
of  added  material.  Other  portions  are  without  any  fragmental  quartz,  being  made 
up  of  minutely  crystalline  silica  in  all  respects  like  that  of  2056  and  3131  in  10.  In 
still  other  places,  however,  are  areas  of  very  coarse  grained  quartz,  the  individuals  of 
which  interlock  intricately  and  present  an  appearance  of  true  vein  quartz.  These 
areas  and  bands  seem  hardly  to  be  vein  quartz  in  the  sense  that  they  have  been  intro- 
duced wholly  subsequently  to  the  remainder  of  the  silica. 

Prominent  exposures. — The  various  exposures  upon  which  the  existence 
of  this  member  as  a  distinctive  horizon  is  based  are  located  carefully  on 
the  accompanying  detailed  maps  (Pis.  v  to  xiii)  so  that  it  will  not  be  desir- 
able here  to  give  a  description  of  each  of  them.  A  few  words,  however, 
with  regard  to  the  greater  exposures,  and  particularly  with  regard  to  those 
which  illustrate  best  the  general  nature  of  this  member,  may  be  of  ad- 
vantage. 

Beginning  at  the  west,  we  may  note  first  the  several  exposures  of  lime- 
stone running  along  the  south  line  of  Sees.  15  and  16,  T.  44  N.,  R.  5  W., 
Wisconsin,  in  the  vicinity  of  Marengo  river.  These  exposures  show  relatively 
little  of  the  cherty  material,  being  mainly  made  up  of  an  unusually  coarse 
crystalline  white  Or  greenish  white  limestone,  which  is  often  exceedingly 
tremolitic;  bands  occurring  in  the  rock  which  are  composed  entirely  of 
interlocking  blades  of  tremolite.  The  large  exposure  indicated  on  Plate  v 
as  occurring  not  far  north  of  the  middle  of  Sec.  24,  T.  44  N.,  R.  4  W., 
Wisconsin,  shows  a  coarsely  crystalline  limestone  very  similar  to  that  just 
described,  containing  but  little  chert,  but  seams  and  bunches  of  greenish 
white  tremolite  are  particularly  prominent.  The  extent  of  this  exposure 
appears  to  indicate  that  we  have  here  a  considerable  thickness  for  the  lime- 
stone member.  In  the  vicinity  of  Penokee  gap  the  exposures  of  this  mem- 
ber are  those  immediately  under  the  railroad  bridge  across  Bad  river  on  the 
south  side  of  the  gap,  in  the  SE.  ^  of  the  NW.  i  Sec.  14,  T.  44  N.,-R.  3  W., 
Wisconsin.^     Bad  river  here  runs  with  an  easterly  coiu'se  through  a  narrow 

1  Geol.  of  Wis.,  vol.  in,  1880,  p.  106.     Also  Atlas  to  the  Geol.  of  Wis.,  PI.  xxiii. 


U.  S    GEOLOGICAL    SURVEY 


MONOGRAPH  xrx.PL.X. 


*LUebi:j^-^Cc  n™>" 


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KEWEENAW     SERIES 


Exposures  of  eruptives 
S  Sandstona. 
-South  limit 


-  1000  feet  above  theSea    -- 
PENOKEE    SERIES. 


Upper  Slate  Member 

^^   JSSW    Si       Black  slate  exposures 
^^    'XSSSk^'"     GrayvvacKe  exposures 


Iron-bearinb  Member 

^^=     -ggjS     Exposures  of  ferruginous  schist, 
Ltcnits  of  surface  distribution 


Quartz-Slate  Member 

SI. Exposures  of  slaty  phases 
0  Exposures  ot  quarlzitic  phases 
South  limit 


SOUTHERN   COMPLEX. 


^KKH  Hb.S  Exposures  of  hornblende-schist. 
^g^^  C  S  Exposures  of  chlorire-schist 
>IHS9<    Exposures  of  granite 
,Xtf^  Exposures  of  greenstone. 


*E»posures   shoeing  str.ke  and  d.p     liW  E;<posures  showing  no  structure      Line  s  of  exploration  by  U  S  Geol  Survey Lines  of  exploration  by  Wis  Geol   Survey 

Scale  of  Map  I  inch=l  mile     Scale  of  Section  I  inch  =  1320  feet 

DETAILED    GEOLOGY  OF  THE    PENOKEE   DISTRICT,  SHEET   4. 


TIIK  GHKIITY  LIMESTONE.  139 

g-org-e,  the  nortlieru  side  of  wliicli  is  coinposed  of  a  snow-white  cliert,  while 
the  southern  si(ki  is  made  up  of  a  trenioHtic  minutely  crystalline  light  gray 
limestone.  The  limestone  and  chert  ajipear  here  then  to  be  sejjarated  into 
two  distinct  layers,  all  of  tlu*  chert  overlying  all  of  the  limestone.  The  chert 
at  this  place  has  a  thickness  something  like  40  or  50  feet.  Less  than  a 
dozen  steps  south  of  the  limestone  are  exposures  of  a  Laureutian  schistose 
gneiss. 

The  next  exposures  worthy  of  note  are  those  lying  to  the  east  of  Sun- 
day lake,  in  the  northwestern  part  of  Sec.  15,  T.  47  N.,  R.  45  W.,  Michigan. 
Here  are  extensive  exposures  of  the  siliceous  slate  member,  immediately  to 
the  south  of  which  a  number  of  test  pits  have  uncovered  chert  and  chert 
breccia  belonging  to  the  Cherty  limestone  member.  The  principal  point  of 
interest  to  be  noted  here  is  the  abundant  occurrence  of  rounded  fragments 
of  yellowish  white  chert  in  tlie  basal  layers  (8  to  10  feet  in  thickness)  of 
the  slate  series,  the  rock  being  a  chert  conglomerate.  The  fine  grained 
cementing  material,  as  well  as  the  pebbles  and  bowlders,  is  largely  chert; 
so  that  the  lower  layer  of  the  slate  has  derived  most  of  its  detritus  from  the 
Cherty  limestone  member.  In  passing  upward  the  amount  of  cherty  mate- 
rial becomes  rapidly  less  and  the  rock  soon  passes  into  the  ordinary  feld- 
spathic  quartz  slate.  In  the  northern  part  of  Section  14  is  a  long  east  and 
west  exposure  of  the  chert  rock,  immediately  to  the  north  of  which  is  seen 
recomposed  material  similar  to  that  just  described.  Here,  however,  the 
matrix  of  the  chert  conglomerate  at  the  basement  of  the  Quartz-slate  mem- 
ber is  largely  of  ordinary  detrital  material.  On  the  SE.  ^  of  Sec.  18  and 
in  the  adjoining  part  of  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan,  are  the  most 
extensive  exposures  of  limestone  to  be  met  with  anywhere  in  the  district. 
There  is  here  a  bluff  some  200  feet  in  height,  running  nearl}'  half  a  mile 
in  a  direction  somewhat  south  of  east.  The  extent  of  the  rock  here  ex- 
posed is  so  great  as  to  indicate  a  total  thickness  of  something  more  than 
300  feet.  This  limestone  is  mostly  quite  fine  grained  and  massive,  and  of 
a  gray  color.  Throughout  the  limestone  there  is  contained  much  cherty 
silica.  This  silica  is  not  only  intimately  mingled  with  the  carbonate,  as 
shown  by  the  thin  section,  but  occurs  in  nearly  pure  layers  interlaminated 
with  the  limestone.     The  peculiar  ridgy  weathering  all  over  this  exposure 


140  THE  PENOKEE  lEON-BEAEmG  SEEIES. 

is  due  to  the  presence  of  these  layers  of  chert.  The  easternmost  point  at 
which  any  rock  has  been  seen  that  can  be  with  certainty  referred  to  the 
Hmestone  member  is  near  the  northeast  corner  of  Sec.  20,  T.  47  N.,  R.  44 
W.,  Michigan,  where  chert  and  chert  breccia  are  struck  in  a  test  pit  a  few 
feet  south  of  exposures  of  the  siUceous  slate. 

Origin  of  the  limestone  and  chert. — From  the  statement  ah-eady  made,  it 
is  apparent  that  no  facts  have  been  obtained  going  to  show  that  the  lime- 
stone and  the  major  portion  of  the  cliert  of  this  belt  are  other  than  original 
water  deposited  sediments.  Whether  the  carbonates  are  of  chemical  or 
organic  origin  we  have  no  definite  proof  Early  in  the  study  of  these  rocks 
it  was  thought  that  they  were  chemical  sediments,  as  we  had  then  little  or 
no  evidence  of  life  independent  of  the  nature  of  the  rocks  themselves  and 
the  iron  carbonates  at  higher  horizons.  Later,  as  other  proofs  of  life  accu- 
mulated, the  assumed  chemical  deposition  of  these  carbonates  became  more 
and  more  doubtful.  Many  geologists  hold  that  such  carbonates  are  evidence 
of  and  could  have  been  produced  only  by  life  agencies.  Where  there  are 
no  fossils,  as  in  these  carbonates,  it  is  at  best  a  matter  of  opinion  as  to  their 
origin;  but  in  making  up  a  judgment  upon  this  point,  the  organic  beds  of 
chert  of  later  times  associated  with  limestones,  and  the  beds  of  iron  carbonate 
and  carbonaceous  material  in  higher  horizons  of  the  series,  are  facts  to  be 
taken  into  account. 

The  vein-like  character  of  a  part  of  the  chert  implies  that  the  silica  has 
been  rearranged  to  some  extent,  or  partially  introduced  subsequently  to  the 
deposition  of  the  main  body  of  the  belt.  The  deposition  of  similar  cherty 
carbonates  of  great  thickness  is  definitely  known  to  occur  in  the  Carbon- 
iferous and  Permian  periods.^  The  chert  is  here  probably  all  of  organic 
origin.  Whether  the  chert  in  the  limestones  under  discussion  is  an  organic 
or  a  chemical  substance  it  is  impossible  to  say ;  but  it  is  certain  that  in  later 
time  we  have  the  exact  analogue  of  the  deposits  described  which  are  def- 

1  On  the  Organic  Origin  ol'  the  Chert  in  the  Carboniferous  Limestone  Series  of  Ireland,  and  its 
Similarity  to  that  in  the  Corresponding  Strata  in  North  Wales  and  Yorkshire.  George  .Jennings 
Hinde.  Geol.  Mag.,  Loudon,  New  Series.,  decade  III.,  vol.  iv,  pp.  435-446.  On  the  Chert  and 
Siliceous  Schists  of  the  Permo-Carbonlferous  Strata  of  Spitzbergen,  and  on  tlie  Chai'acters  of  the 
Sponges  therefrom,  which  have  been  described  by  Ur.  K.  von  Dunikowski.  Dr.  Hinde,  ibid,  vol.  v, 
pp.  241-251. 


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THE  CHERTY  LIMESTONE.  141 

initely  known  to  ho  organic  deposits.  Tlie  origin  of  similar  cherty  deposits 
in  tlic  Iron-luiaring- formation  is  discnssed,  in  cliapter  v,  section  2.  What 
is  said  there  with  reference  to  the  "original  rock"  applies  equally  well  here. 

It  apjiears  that  tlie  chert  of  the  limestone  belt,  whether  original  or 
s(!condar}-,  had  in  the  main  reached  its  present  condition  before  the  accu- 
mulation of  the  immediately  overlying  Quartz-slate  formation,  since,  as 
already  stated,  very  numerous  fragments  of  this  chert  are  found  included 
within  the  slate  at  its  base,  and  even  in  its  middle  and  upper  parts,  and  if 
the  concentration  of  this  material  in  layers  is  due  to  secondary  causes,  there 
must  have*been  sufficient  time  between  the  deposition  of  the  Cherty  lime- 
stone and  the  Quartz-slate  to  accomplish  this.  That  the  chert  has  been 
rearranged  to  a  greater  or  less  extent  since  its  deposition,  and  that  in  the 
cracks  infiltrating  solutions  have  brought  additional  silica,  is  more  than 
probable.  At  the  time  of  this  subsequent  rearrangement  and  introduction 
of  silica,  doubtless  the  tremolite  was  formed,  although  even  this  mineral 
may  have  developed  very  early.  In  the  formation  of  the  tremolite,  the 
silica  in  solution  had  but  to  unite  with  a  portion  of  the  bases  present,  cal- 
cium and  magnesium.  The  origin  of  actinohte  from  an  analogous  rock, 
except  that  it  bore  iron,  is  discussed  in  chapter  v.  What  is  there  said 
applies  equally  well  to  the  tremolite  in  the  Cherty  limestone. 

Summary. — The  cherts  and  limestones  are  placed  together  because  as  a 
rule  the  chert  occurs  interstratified  and  thinly  interlaminated  with  the  lime- 
stone. 

This  member,  instead  of  being  continuous,  often  thins  out  and  disap- 
pears quite  suddenly,  so  that  it  is  mapped  as  occurring  only  where  actually 
found.  It  is  probable  that  •  the  Clierty  limestone  member  had  a  much 
greater  former  continuity  than  at  present.  This  is  particularly  probable 
because  fragments  of  it  are  abundantly  contained  in  the  overlying  Quartz- 
slate  member. 

Its  maximum  thickness  is  300  feet,  and  from  this  it  varies  to  nothing. 

In  petrographical  character  the  limestone  is  close  to  a  dolomite  which 
is  frequently  tremolitic. 


J  42  THE  PENOKEE  lEON-BEARING  SERIES. 

Chert  occurs  in  the  dolomite  from  minute  particles  through  thin  layers, 
which  protrude  on  the  weathered  surface  in  leaf-like  forms,  up  to  belts 
which  are  45  or  50  feet  thick. 

The  change  from  the  Cherty  limestone  to  the  overlying  Quartz-slate 
member  is  usually  abrupt,  while  in  many  places  between  the  two  there 
is  certain  evidence  of  an  erosion  interval. 

The  chert  and  limestone  are  water-deposited  sediments.  Whether 
chemical  or  organic  is  uncertain,  but  it  is  not  improbable  that  they  are  partly 
or  wholly  the  latter.  However,  if  this  is  the  case,  the  silica  has  subse- 
quently changed  to  the  mineral  form  and  has  been  extensively  rearranged, 
while  the  limestone  has  become  dolomitized. 


U.  S. GEOLOGICAL    SURVEY 


MONOGRAPH    XIX-   PL. XII. 


N. 

r 

TTB^^^sasr^ 

s. 

/            ■- 

—T^^W 

N 

^ 

ni 

-a-^jg, 

/ 

s 

-'■sKi&Jv 

II 

SI 

Gn 

Gn 

^— — s?-- 

G 

~TS5!r~— 

W'.'.y.' 

».'V.-    jRf''^ 

~~Tanmrr 

^SCi! 

7          '**'''      "T*" 

KEWEENAW     SERIES 


i  Exposures  of  erupuves 
I  S  Sandstone 
-South  limit. 


PENOKEE    SERIES. 


Upper  Slate  Member.  Iron-bearinb  Member  Quartz-Slate  Member  Cherty  Limestone  Member 


GraywacKe  exposui 


K  Exposures  of  ferruginous  schist. 
-  Limrts  of  surface  distnbirtion 


SI.Exposures  of  slaty  phases 
0  Exposures  of  quartzitic  phases 
South   limit 


Eruptives. 

(j  Exposures  of  greenstone 


SOUTHERN    COMPLEX. 


■-;■:■   Hb  S  Exposures  of  hornblende  schist. 
:.■;.-.■  MS  Exposures  of  mica-schist. 
■jrv  C   5   Eicposures  of  Chlonie-schtst. 
Jiv'^'i'^  Gn.  Exposures  of  granitoid  gneiss 


*»  E> 


posun 


i  of  oranile 


ll  Exposures  of  greenstone. 


=  Exposures   showtnd   strike  and  dip. 


J  Exposures  showing  no  structure       Lines  of  exploration  by  U  S  Geo  I  Survey 

Scale  of  Map  I  inch  =  I  mrle     Scalei  of  Sections  i  inch  =  1320  feet 

DETAILED    GEOLOGY  OF  THE    PEINOKEE   DISTRICT,  SHEET    5. 


CHAPTER  IV. 


By  R.  D.  Irving  and  C.  R.  Van  Hisb. 


THE  QUARTZ-SLATE  MEMBER. 

Applicability  of  the  namoi.  Geographical  extent.  Topographical  features.  Thickness.  General 
petrographical  character  and  stratigraphy.  Microscopical  character  of  the  feldspathic  quartz- 
slates.  Microscojjical  character  of  the  biotitie  aud  chloritie  quartz-slates.  Microscopical  char- 
acter of  the  vitreous  quartzite.  Microscopical  character  of  the  sandstone,  novaculite,  and  argil- 
laceous slates.  Tabulation  of  petrograiihical  observations.  Contacts  with  the  Cherty  limestone 
member.  Contacts  with  the  Southern  Complex.  Change  t<j  the  Iron-bearing  member.  Promi- 
nent exposures.     Mode  of  deposition  and  source  of  material.     Summary. 

Applicability  of  the  name. — Resting  directly  upon  the  limestone  or  white 
chert  of  the  formation  described  in  the  last  chapter,  or,  'in  the  absence  of  that 
formation,  directly  iipon  the  gneiss,  schist,  or  granite  of  the  Southern  Com- 
plex, follows  a  set  of  slaty  layers,  which,  though  for  the  most  part  less  than 
500  feet  in  total  thickness,  constitute  a  singularly  well  marked  horizon, 
traceable  throughout  the  entire  extent  of  the  area  occupied  by  the  Iron- 
bearing  series  of  the  district.  The  principal  ingredients  of  these  layers  are 
quartz  and  feldspar,  which  fact  has  led  to  our  selection  of  the  name  Quartz- 
slate  for  the  entu-e  member;  although,  as  shown  subsequently,  very  many 
varieties  deserving  of  distinct  lithological  names  occur  within  it.  In  the 
Wisconsin  State  Reports^  the  term  siliceous  slate  is  made  use  of  for  this 
member  of  the  series,  but  this  term  has  been  so  widely  applied,  particularly 
in  its  German  form  of  Kieselschiefer,  to  a  rock  which  is  cherty  or  nonfrag- 
mental  in  its  nature,  that  it  appears  inapplicable  in  the  present  case  where 
a  fragmental  texture  is  the  most  prominent  characteristic.     The  possession 

'Vol.  Ill,  p.  111. 

143 


144  THE  PENOKEE  lEON-BEAEING  SERIES. 

of  this  fragmental  texture  puts  the  formation  in  strong  contrast  with  the 
chert  and  limestone  of  the  limestone  member  immediately  underlying  it, 
and  with  the  various  ferruginous  and  cherty  rocks  of  the  immediately  over- 
lying Iron-bearing  member,  all  of  which  rocks  are  entirely  without  the  frag- 
mental texture,  and  whatever  their  mode  of  deposition,  are  certainly  not  in 
the  nature  of  mechanical  sediments. 

Geofjraphkal  extent. — From  the  western  end  of  the  Penokee  range,  on 
the  south  line  of  Sec.  14,  T.  44  N.,  R.  4  W.,  Wisconsin,  to  the  southeast 
corner  of  Sec.  17,  T.  47  N.,  R  43  W.,  Michigan,  a  total  distance  of  52  miles, 
the  outcropping  edge  of  the  Quartz-slate  formation  forms  a  continuous  belt. 
West  of  the  western  end  of  the  Penokee  range,  as  shown  on  the  maps  here- 
with, all  of  the  rocks  of  the  Iron-bearing  series  disappear  from  sight,  but  in 
T.  44  N.,  R.  5  W.,  Wisconsin,  they  apjDear  again,  and  here  the  same  quartz- 
slate  may  be  traced  for  a  distance  of  something  more  than  3  miles.  Whether 
the  liorizon  is  recognizable  farther  to  the  east  than  the  easternmost  point  in 
Michigan  above  mentioned  is,  however,  in  some  doubt.  This  question  is 
considered  separately  in  a  subsequent  chapter. 

From  the  western  end  of  the  Penokee  range  eastward  to  the  vicinity  of 
Sunday  lake,  the  width  of  the  outcrop  of  these  slates,  which  dip  nearly  al- 
ways at  a  high  angle  to  the  northward,  presents  only  very  slight  variations. 
To  the  east  of  Sunday  lake,  however,  there  is  a  somewhat  rapid  expansion. 
In  the  northern  part  of  Sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin,  the  width  is 
600  feet ;  at  Penokee  gap,  on  Bad  river,  it  is  450  feet ;  at  mount  Whittlesey, 
near  the  northwest  corner  of  Sec.  16,  T.  44  N.,  R.  2  W.,  Wisconsin,  it  is  450 
feet;  at  the  gorge  of  the  Potato  river.  Sec.  19,  T.  45  N.,  R.  1  E.,  Wisconsin, 
it  is  475  feet ;  at  the  passage  of  the  West  branch  of  the  Montreal  river.  Sec. 
27,  T.  46  N.,  R.  2  E.,  Wisconsin,  it  is  460  feet;  at  the  Aurora  mine,  NE.  4 
Sec.  24,  T.  47  N.,  R.  47  W.,  Michigan,  its  possible  maximum  is  580  feet; 
on  the  west  side  of  Sec.  13,  T.  47  N.,  R.  46  W.,  Michigan,  its  possible  maxi- 
mum is  600  feet;  and  in  the  southern  part  of  Sec.  10,  T.  47  N.,  R.  45  W., 
Michigan,  the  width  is  1,000  feet,  this  great  increase  in  width  being  due  in 
part  to  a  flattening  in  the  northward  dip,  and  in  part  to  an  actual  increase 
in  thickness. 


1^ 


U.  S.  GEOLOGICAL    SURVEY 


MONOGRAPH    X(X   PL.XIII 


EASTERN    SANDSTONE 


I  ft  Enposures  of  sandstone  and  conglomerate 
South  limrt. 


KEWEENAW     SERIES 


Iron-bearinb  Member  Quartz-Slate  Member  Cherty  Limestone  Member 


JWWExposuresof  erupiwes  ^  XWC<  Exposures  of  ferruginous  sc^ 

^M  5  Sandstone  Limits  of  surface  distnbutio 

South  limrt 


SI,  Exposures  of  slaty  phases 
Q  Exposi;res  of  quarlzitic  phases 
South  limit 


:  Exposuresof   limestone  and  chert=  Tf 
.  South  hmrt  


=  Exposures  showing  strike  and  dip    'fiW  Exposures  showing  no  structure 
Scale  of  Map  I  inch=I  mile     Scale  of  Section 

DETAILED    GEOLOGY  OF  THE    PENOKEE 


Ferruginous  Slates. 

'.SI.  Exposures  of  ferruginous  slate 
Limits  of  surface  distribution 


Greenstone  Conglomerates 

=  W6W  Exposures  of  greenstone  conglomer 
— ■ Lmits  of  surface  distribution 


Eruptives. 

WOOOC  Ej<posures  of  greenstone. 
Limits  of  surface  distribution 


Lines  of  exploration  by  U  5  Geo  I  Survey 

inch  =  1320  feet 

DISTRICT,  SHEET6. 


SOUTHERN  COMPLEX- 


I  W0(  HbSExposures  of  hornblende-schist 
=  »0W  C  S  Exposures  of  ctilorte-schisl 
=  WW  MS  Exposures  of  m.ca  schist 
X)OC<  Snixposures  of  gneiss 
■;    ■:  Gn  Exposures  of  gfamtoid  gno'ss 
</'/■'.'  Gr.  Exposures  of  granite 
WWGr.  Exposures  of  greenstones 


THE  QUARTZ-SLATE  MEMBER.  145 

Topofimphtral /('((titycs. — At  tlie  western  ciul  n\'  tlie  Penokee  range  the 
layers  of  the  Qunrtz-slate  inenilx'i- t'onii  the  Itnsc  ul'  tlie  southern  s]o))e  of 
the  nniov.  From  here  eastward,  however,  llie  sLite  rises  liif^'her  and 
higher  ou  tliis  slojx^,  t'oruiing  fre([uent  bohl  ;uul  uvea  precipitous  south- 
facing  exposures.  At  Bad  river  the  shite  reaches  nearly,  and  at  mount 
Whittlesey,  Sec.  I  (i,  'V.  44  N.,  R.  2  W.,  Wisconsin,  quite,  to  the  top  of  the  ridge. 
Still  farther  east  it  I'ornis  nioi-e  ;iud  more  of  the  bulk  of  the  range;  while 
to  the  east  of  Tyler's  fork  the  whole  width  of  the  outcrop  lies  on  the 
northern  slope  of  the  range,  the  summit  here  lying  within  the  Southern 
Complex.  The  same  is  A-ery  noticeably  true  in  the  Gogebic  country,  east 
of  the  state  boundary,  the  slate  as  far  as  the  vicinity  of  the  West  branch  of 
Black  river  forming  the  northern  slope  of  a  bold  ridge  whose  summit  lies 
within  the  granite.  To  the  east  of  the  West  branch  of  Black  river  another 
change  in  this  respect  takes  place,  the  ridge  itself  lying  within  the  jaspery 
iron  belt  north  of  the  quartz-slate,  which  now  lies  in  the  lower  ground  to 
the  southward;  while  beyond  Sunday  lake  and  as  far  east  as  the  East 
branch  of  Black  river,  the  quartz-slate  again  appears  in  bold  exposures, 
and  n(.)w  forms  the  principal  ridge,  with  low  ground  north  and  south  of  it. 
This  varying  position  of  the  crest  of  the  ridge  Avith  regard  to  the  different 
rock  belts  is  plainly  a  result  of  the  varying  relations  between  the  several 
belts  as  to  power-  of  resisting  erosion.  At  the  west  the  Quartz-slate 
member  contains  an  unusually  large  amount  of  soft  chloritic  slates  and 
relatiA-ely  little  of  highly  quartzose  portions,  whilst  at  the  same  time  the 
iron  belt  immediately  north  of  it  is  exceedingly  quartzose  and  resist- 
ant. Farther  east  the  quartz-slate  becomes  more  and  more  highly  quartz- 
ose, and  hence  resistant,  and  now  goes  along  with  the  iron  belt  itself  to 
make  up  the  bulk  of  the  ridge.  After  Tyler's  fork  is  passed,  however,  the 
iron  belt  begins  to  undergo  a  change  Avhereby  its  resisting  power  becomes 
less  and  less,  and,  as  the  Montreal  river  is  neared,  the  granite  to  the  south 
becomes  the  most  i-esistant  rock,  and  the  crest  of  the  ridge  is  on  it.  In  the 
same  way  a  connection  between  the  ditferent  degrees  of  resisting  power  of 
the  various  rocks,  and  the  position  of  the  crest  of  the  range,  may  be  shown 
to  hold  for  that  portion  of  the  distance  east  of  the  West  branch  of  Black 
riA^er. 

MON  XIX 10 


146  THE  PENOKEE  IRON  BEAEING  SERIES. 

Thickness. — In  several  places  tlie  full  sui'face  width  of  the  Quai'tz-slate 
member  is  coiitiimously  exposed,  while  in  other  places  exposures  of  the  rocks 
north  and  south  of  it  limit  its  possible  width  in  such  a  manner  that  we 
can  tell  very  closely  the  true  thickness.  This  surface  Avidth,  while  of 
course  primarily  dependent  upon  the  thickness  of  the  layer  itself,  is  also 
dependent  largely  upon  the  degree  of  its  inclination  to  the  north,  which 
varies  from  4U°  as  a  miniuuuu,  to  75°  as  a  maximum,  although  the 
extremes  are  rarely  reached,  the  u.sual  figures  lying  between  55°  and  65°. 
The  following  are  the  approximate  thicknesses  observed  at  various  points, 
beginning  on  the  west:  In  the  northern  part  of  Sec.  24,  T.  44.  N.,  R.  4  W., 
Wisconsin,  400  feet;  at  the  passage  of  Bad  river.  Sec.  14,  T.44N.,  R.  3  W., 
Wisconsin,  41 0  feet;  at  mount  Whittlesey,  near  the  uorthwestcornerof  Sec.  16, 
T.  44  N.,  R.  2  W.,  Wisconsin,  400  feet;  at  the  passage  of  the  Potato  river,  Sec. 
19,  T.  45  N.,  R.  1  E.,  Wisconsin,  425  feet ;  at  the  passage  of  the  West  branch  of 
the  Montreal  river,  Sec.27,  T.46  N.,  R.2  E.,  Wiscoushi,  467feet.  At  the  Aurora 
mine,  NE.  \  Sec.  24,  T.  47  N.,  R.  47  W.,  Michigan,  the  observed  thickness  is 
300  feet,  but  the  entire  thickness  is  not  exposed,  the  greatest  possible  thick- 
ness between  the  granite  o\\  the  south  and  the  Iron-bearing  member  on  the 
north  being  545  feet.  Similarl}-  on  the  west  side  of  Sec.  13,  T.  47  N.,  R. 
46  W.,  Michigan,  the  greatest  possible  thickness  is  564  feet.  Farther  east  the 
formation  thickens  somewhat  rapidly,  particularly  east  of  Sunday  lake.  In 
the  northern  part  of  Sec.  10,  T.  47  N.,  R.  45  W.,  Michigan,  there  is  a  contin- 
uous exposure  ab(-)ut  '  ,000  feet  wide,  which,  with  the  dip  of  55°,  corre- 
sponds to  a  thickness  of  at  least  800  feet.  Thus  it  appears  that  from  the 
western  end  of  the  Penokee  range  to  the  passage  of  the  Potato  river  there 
is  an  almost  constant  thickness  of  400  feet;  that  east  of  Potato  river  there 
is  a  very  gradual  increase  in  thickness,  which,  at  the  passage  of  the  West 
branch  of  the  Montreal,  is  nearly  500  feet;  and  that  this  gradual  increase 
continues  until  the  vicinit}'  of  Sunday  lake  is  reached,  when  a  more  rapid 
increase  occurs,  the  maximum  of  800  feet  being  reached  about  2  miles  east 
of  that  lake. 

General  i)etrogmphkal  character  and  stratigraphy. — The  rocks  of  this 
formation  have  in  connnon,  with  the  exceptioi  of  its  uppermost  horizon,  a 
strong  slaty  tendency  and  liglit  colored  weathering;  the  slaty  structure 


THE  QUARTZ-SLATE  MEMI5EK.  147 

lying-,  liowin'er,  parallel  to  the  beddiiij,'-,  mid  not,  l)eing  in  the  nature  of  the 
nrdinaiy  slaty  cleavage.     These  two  eharacters  serve  to  mark  this  forma- 
tion sii  strongly  that  after  onee  having  become  familiar  with  them  one  is 
never  at  a  loss  to  refer  its  exposm-es  to  their  true  horizon;   but  while  there 
is  this  general  similarity  of  appearance,  there  are  at  the  same  time  a  great 
many  snhonlinate  phases  presented.     As  macroscopically  distinguished,  the 
two   most   prominent  phases  are:    (1)  A  thinly  slaty,  rather  soft,  usually 
very  Hue  grained,   dark  colored  kind,   having  (jfteu  a  distinct  tendency 
toward  a  greenish  tint;  and  (2)  a  kind  shading -into  the  last,  but  differing 
from  it  in  being  harder,  and  in  tending  to  a  paler  or  even  light  gray  color, 
often  showing  mica  flakes  on  the  slaty  surface,  but  as  often  presenting  a 
nearly  aphanitic  appearance.      From  the  tirst  of  these  phases  there  are 
gradations  into  (3)  a  very  thinly  laminated,  highly  chloritic,  and  very  soft 
kind;   while  from  the  second  phase  there  are  gradations  in  one  direction 
into  (4)  a  genuine  vitreous  quartzite ;  in  another  into  (5)  a  slightly  indurated 
sandstone;  and  in  a  third  into  ('>)  a  light  gray  novaculite.     Besides  these 
there   are  certain   phases    of  relatively    rare   occurrence.     These  are   (7) 
red,   green,   and    purple   clay-shales    or    clay-slates;    (8)    a  conglomerate 
in  which  fragments  of  white  cliert  are  "imbedded  in  a  greenish  chloritic 
matrix;     (9)    a    magnetitic    conglomerate,   like    the    last,    but    containing 
a  large  pro})ortion  of  magnetite  (these  last  two  occur  where  the  formation 
is  in  contact  with  the  clierty  limestone);  and  (10)  a  peculiar  greenish  con- 
glomerate-slate found  only  on  the  Potato  and  West  branch  of  the  Montreal 
rivers,    immediately  at   the    contact  with    an    unconformably  underlying 
greenish  schist.     These  distinctions  are  based  entirely  upon  the  macroscopic 
appearances,  which  appearances,  however,  are  quite  well  borne  out  by  the 
study  of  the  thin  sections.     The  second  phase  mentioned  includes  kinds 
which  prove  to  have,  in  addition  to  the  jn-edominant  quartz,  a  considerable 
proportion  of  a  fragmental  feldspathic  ingredient,   to  whose  presence  the 
characteristic   whitish    to    straw-colored   weathering   is   undoubtedly  due. 
The  gradations  fi'om  this  phase  towards  vitreous  quartzite  arise  from  a 
lessening  in  the  amount  of  the  feldspathic  constituent ;  those  towards  novac- 
ulite from  an  increase  in  the  proportion  of  this  feldspathic  constituent, 
accompanied  by  a  great  decrease  in  coarseness  of  grain.     The  rocks  of  the 


148  THE  PENOKEE  IKON-BEAEIKG  SEEIES. 

first  phase  pi'ove  microscopically  to  differ  from  those  of  the  second  in  con- 
taining a  relatively  large  proportion  of  fine  micaceous  particles,  these  in- 
gredients, including  biotite,  chlorite,  and  sericite,  occurring  either  singly  or 
together.  A  still  further  increase  in  these  ingredients,  accompanied  by  an 
increasing  fineness  in  grain,  gives  i"ise  to  the  third  phase;  while  a  still 
greater  increase  in  fineness  of  grain,  along  with  an  increase  in  the  amovint 
of  clayey  material  from  the  decomposition  of  the  feldspar,  leads  to  the 
argillaceous  slates  or  shales  of  the  seventh  variety.  The  microscopical  study 
shows  further  that  the  difference  between  these  several  ])hases,  so  fai  as 
they  are  not  the  result  of  unusual  conditions,  as  in  the  case  of  the  cherty 
and  niagnetitic  breccias  above  mentioned,  are  almost  entirely  dependent 
upon  the  original  proportions  and  degrees  of  fineness  of  the  two  main  frag- 
mental  constituents ;  that  is,  the  quartz  and  feldspar.  Some  of  these  mica- 
ceous ingredients  are  taken  to  be  of  fragmental  origin.  This  is  particu- 
larly true  of  some  of  the  large  flaked  sericite  or  muscovite,  but  in  the  main 
these  materials,  including  also  kaolinite,  appear  to  have  resulted  from  a 
decomposition  of  the  feldspathic  particles.  This  decomposition  was  accom- 
panied by  the  separation  of  a  secondary  silica,  which  is  now  apparent  in 
most  of  the  thin  sections  in  the  shape  of  a  minutely  crystalline  quartz. 

Disregarding  the  special  and  rarer  phases,  the  important  kinds  may  be 
microscopically  listed  under  the  following  heads:  Chloritic  and  biotitic 
quarts-slates;  feldspathic  quartz-slates ;  vitreous  quartzite;  sandstone;  novacidite; 
argillaceous  slates.  These  are  variously  interstratified  with  one  another. 
However,  the  vitreous  quartzite  layer  is  a  persistent  element  in  the  stratig- 
raphy, composing  the  uppermost  part  of  the  foraiation  wherever  it  is 
exposed.  The  conglomerates  referred  to  (Nos.  8,  9,  and  10,)  lie  always  at 
or  near  the  base  of  the  formation,  their  peculiarities  having  been  caused  by 
proximity  to  the  underlying  rocks ;  but  exposures  of  this  kind  are  not  suffi- 
ciently numerous  to  demonstrate  that  such  rocks  are  a  persistent  element  in 
the  stratigraphy,  although  this  is  quite  probable.  At  one  place  a  vitreous 
quartzite  is  at  or  near  the  base  of  the  series.  However,  while  no  further 
definite  subordinate  arrangement  is  observable  in  the  cross-section,  a  change 
in  character  is  to  be  noted  as  the  belt  is  followed  from  west  to  east,  the 
biotitic  phase  (Nos.  1  and  3,)  predominating  in  the  western  portion  of  this 


Till':  (.ilAlvTZ  SLATK   M  KM  ISKI;.  149 

belt,  thonj^-h  steadily  hisscniuy-  in  relative  aiiuiuiit  as  one  moves  eastward. 
East  of  Sec.  ;U,  T.  45  N.,  K.  1  \V.,  AViscousin,  altlumgli  tlie  uppermost 
(piartzite  rontiimes  well  marked,  the  cross  section  l)efonies  varic^d,  t'eld- 
spatliic  (piartzites,  feMspathie  ([uartz-slates,  sandstones,  and  clay-shales  alter- 
natinj;'  with  one  another. 

Microsfopical  rhardctcr  of  the  fcldspathk  (puii-t^-dak-'i  (Phase  2,  see  PI. 
xviir,  Figs.  1,  2,  3,  4). — In  thin  sections,  from  the  typical  specimens  of  this 
phase,  Avhich  is  the  prevailing- rock  of  all  that  portion  of  the  quartz-slate  belt 
to  tlie  east  of  Sec.  34,  T.  45  N.,  R.  1  W.,  Wisconsin,  a  single  glance 
through  the  microscope  generally  suffices  to  show  that  it  is  composed  of 
two  parts,  a  coarser  plainly  fragmental  portion  and  a  finer  interstitial 
•  material.  The  relative  proportions  of  these  materials  vary  greatly,  the 
coarser  portion  at  times  sinking  to  quite  a  subordinate  position,  and  again 
nearly  excluding  the  matrix.  Between  the  finer  and  coarser  portions  thei-e 
is  often  a  material  of  an  intermediate  fineness,  and  the  whole  appearance 
suggests  that  the  two  portions  are  in  large  measure  f»uly  fined'  and  coarser 
particles  of  the  same  minerals. 

The  coarser  portion  in  these  sections  always  comprises  fragments  of 
both  quartz  and  feldsjjar.  With  these  is  very  often  more  or  less  rather 
coarse  gi'ained  mica,  which  appears  also  in  a  finer  condition  in  the  inter- 
stitial material.  This  coarser  grained  mica  is  taken  to  be  in  the  main  frag- 
mental also,  although,  as  will  be  seen  subsequently,  secondary  micaceous 
minerals  are  plentifully  developed  in  the  rocks  of  this 'member.  These 
coarser  grained  micas  are  the  ones  which  appear  as  biilliant  flakes  to  the 
naked  eye  on  the  surfaces  of  the  laminae.  There  is  considerable  variation 
in  size  among  the  coarser  pieces,  which,  as  already  said,  grade  down- 
ward into  the  matrix  material  itself.  This  mingling  of  coarser  and  finer 
material,  since  it  is  made  up  of  fragments  of  difi'erent  minerals,  is  taken 
to  indicate  that  the  detritus  of  which  this  rock  is  composed  had  received 
relatively  little  sorting  before  deposition. 

The  fragments  of  quartz  are  for  the  most  pai-t  portions  of  single  indi- 
viduals, but  not  unfrequently  they  are  minutely  complex,  having  been 
derived  from  some  chert}-  or  fli'nty  rock.  These  particles  vary  greatly  as 
to   the  degree    of  rounding  which  they  have   received.     In  general  the 


150  THK  PENOKEB  lEOK-BEARING  SERIES. 

amount  of  rounding-  appears  to  be  in  a  direct  relation  to  the  size  of  the 
particles,  the  more  minute  pieces  having-  remained  quite  angular.  In 
speaking  of  tliese  pieces  as  rounded,  however,  -vve  refer  ahvays  to  the 
original  frasrments,  whose  outlines  for  the  most  part  still  remain  distinct; 
but  as  they  now  stand,  a  large  proportion  of  them  are  built  out  by  second- 
ary enlargement;  the  added  portions  varjdng  greatly  in  width,  but  often 
extending  beyond  the  original  fragments  a  distance  equal  to  a  fifth  or  sixth  of 
their  diameters.  These  enlargements  are,  as  usual,  optically  continuous  with 
the  original  fragments,  and  have  frequently  interlocked  with  one  anotlier 
in  such  a  fashion  as  to  produce  very  irregular  outlines.  It  is  noticed  that 
these  secondary  enlargements  are  narrowest  in  those  sections  which  have  a 
considerable  quantity  of  brown  iron  oxide  among  the  interstitial  materials.  • 
The  outlines  of  the  original  .quartz  fragments  where  they  have  received 
enlargements  are  emphasized,  as  is  usual  in  such  cases,  by  particles  of 
brown  iron  oxide,  and  by  the  presence  of  minute  cavities.  In  a  few  cases, 
mingled  with  this  brown  iron  oxide,  and  at  times  almost  excluding  it,  are 
films  of  a  greenish  chlorite. 

The  feldspar  fragments  include  three  distinct  kinds.  The  most 
abundant  is  unstriated,  and  its  appearance  is,  in  every  respect  that  of  the 
ordinary  orthoclase  of  the  granitic  rocks,  and  there  can  be  no  reasonable 
doubt  that  it  is  of  this  nature.  The  difficulty  of  separating  tliese  supposed 
orthoclase  particles  from  the  other  ingredients  of  the  rock,  particularl}'  from 
the  microcline,  would  be  so  great  that  it  is  not  thought  worth  while  to  make 
the  determination  any  more  certain  than  this.  A  second  variety  is  the 
ordinary  cross  twinned  microcline,  and  the  third  is  a  striated  plagio- 
clase.  On  making  many  measurements  of  the  extinction  angles  of  the 
last  named  variety  by  Pumpelly's  method,  we  fail  to  find  any  angles 
which  would  suggest  the  presence  of  a  plagioclase  more  basic  than  one 
belonging  in  the  oligoclase  series.  We  may  therefore,  with  some  confi- 
dence, say  that  the  feldspar  particles  of  these  rocks  include  pieces  of  ortho- 
clase, microcline,  and  albite  or  oligoclase,  or  both,  an  association  which  is 
that  of  the  ordinary  granites.  Often  these  feldspar  fragments  are  very 
fresh,  but  in  other  cases  many  of  them  are  much  altered  or  decomposed,  a 
gradation  in  this  respect  being  found  to  obtain  between  those  kinds  in 


TIIH  QUAKTZ-SliATl';  iM  KIM  liKi;.  151 

wliicli  the  |)arti(4os  ure  still  very  fresli  and  tlic  cliloritic  slates  or  the  argilla- 
ceous shales;  the  most  plentiful  ])ro(hie,ts  of  the  (leronn)osition  of  the 
feldspars  hein{:f  ehlorite  and  kaolinite.  In  the  decomposition  of  the  feld- 
spars to  chlorite,  the  particles  of  the  latter  mineral  "are  found  to  form  in  the 
iirst  place  in  the  nei<>'hl)orhood  of  the  edges  of  the  feldspar  fragments,  the 
alteratictn  having  progressed  regularly  from  the  outsides  of  the  grains ;  in 
the  case  of  the  kaolinitic  alteration,  the  kaolinite  particles  appear  as  usual 
in  nuuierous  minute  flakes  throughout  the  entire  feldspar  grains. 

The  I'arge  particles  oif  white  mica  which  are  supposed  to  be  of  a  frag- 
mental  nature  are  taken  to  he  an  altered  muscovite.' 

The  fine  interstitial  material  in  these  rocks,  aside  from  the  minute 
grains  of  quartz  and  feldspar,  is  a  mixture  of  a  minutely  di^nded 
silica,  flakes  of  kaolinite,  chlorite,  and  a  fine  mica,  Avhich  is  taken  to 
be  sericite  and  muscovite,  in  proportions  which  vary  between  very  wide 
limits,  although  in  nearly  every  case  all  of  these  minerals  appear  to  be 
present  in  the  matrix,  except  perliaps  the  sericite.  The  siliceous  portion  of 
this  matrix  is  at  times  completely  though  very  minutely  crystalline,  ])ut  in 
other  cases  it  takes  on  a  chalcedonic  or  even  an  amorphous  form.  While 
some  of  the  larger  particles  of  the  quartz  in  the  matrix  are  doubtless  of  a 
fragmental  nature,  much  of  this  interstitial  silica  is  evidei^tly  an  original 
crystallization.  The  kaolinite  and  chlorite  are  in  minute  flakes  intimately 
mingled  with  the  fine  silica ;  the  brown  iron  oxide  occurs  in  irregular  In-own 
patches,  being  usually  in  a  rather  minute  quantity,  though  at  times  deeply 
staining  the  entire  matrix.  The  sericite  particles  of  the  matrix  are  usually 
of  somewhat  larger  size  than  the  particles  of  the  other  minerals,  on  which 
account  it  is  perhaps  a  question  whether  this  sericite  should  not  rather  be 
regarded  as  belonging  with  the  larger  mica  flakes  which  we  have  already 
mentioned  as  of  a  fragmental  nature.  All  of  these  interstitial  minerals  are 
ones  which,  as  is  known,  result  from  the  alteration  of  feldspars  and  micas, 
and  it  is  supposed  that  they  are  the  result  of  metasomatic  changes  cari-ied 
out  particularly  in  the  finer  detrital  material  subsequently  to  the  original 
deposition  of  the  rock. 

MicroscopicaJ  character  of  the  hiotitic  and  cMoritic  quarts-slates. — (Phases 
1  and  3,  see  PL  xix,  Fig.  1.)     The  thin  sections  of  these  slates,  which  are,  as  a 


152  THE  PENOKEE  IRON-BEAEING  SERIES. 

whole,  mucli  finer  grained  tlian  the  slates  just  described  and  which  do  not 
show  the  same  separation  into  a  coarser  fragmental  portion  and  a  finer 
interstitial  portion,  show  a  background  which  is  composed  mainly  of 
quartz,  but  also  contaiifs  usually  more  or  less  feldspar.  In  some  sections 
the  feldspar  particles  are  quite  abundant,  while  in  otliers  they  are 
almost,  or  qtiite,  wanting.  Scattered  through  this  background  in  vary- 
ing quantities  in  the  different  sections  are  green  flakes  of  chlorite  and 
brown  ones  of  biotite,  the  chlorite  predominating  in  some  sections,  the 
biotite  in  others. 

The  quartz  particles  as  now  constituted  are  minutely  angulai'.  How- 
eA'er,  manj-  of  the  larger  have  very  plainly  marked  fragmental  cores  whose 
outlines  are  emphasized  by  films  made  up  of  minute. flakes  of  chlorite  and 
biotite.  It  is  exceedingly  difficult  to  say  how  much  of  the  finer  grained 
quartz  is  of  a  fragmental  origin.  Judging  from  the  sections  of  the  coarser 
grained  varieties  of  phase  2,  described  immediately  above,  we  conclude 
that  part  of  this  finer  silica  is  fragmental,  but  that  part  of  it  is  also  an 
original  crystallization. 

Tlie  flakes  of  chlorite  and  biotite  are  usually  quite  small,  but  very 
Avell  defined,  though  occasionall}'  large  sized  scales  of  uniaxial  chlorite  are 
seen.  It  is  taken  as  probable  that  all  of  these  two  minerals,  along  with  the 
nonfragmental  silica,  have  resulted  from  the  decomposition  of  a  detrital 
feldspathic  material.  At  all  events,  in  some  sections  all  three  of  the 
minerals  occur  within  the  outlines  of  a  single  original  feldspar  fragment  in 
such  a  manner  as  to  show  their  derivation  from  the  feldspar.  This  is  a 
process  of  alteration,  particularly  as  regards  the  biotite,  which,  as  shown 
on  a  subsequent  page,  has  been  carried  out  in  a  very  striking  manner 
among  the  slates  of  certain  portions  of  the  Upper  slate  member  of  this 
district,  fragmental  feldspathic  rocks  having  been  thus  altered  to  rocks 
that  would  ordinarily  be  taken  as  crystalline  mica-schists.  If  this  is  also 
the  origin  of  the  micaceous  ingredients  of  the  rocks  now  especiallj^ 
under  consideration,  we  should  expect  feldspar  fragments,  recognizable 
as  such,  to  be  ])resent  in  an  abundance  standing  in  an  inverse  ratio  to  the 
amount  of  chlorite  and  biotite  present.  The  thin  sections  show  that  this  is 
actually  the  case.  In  the  most  highly  chloritic  and  biotitic  varieties  there 
is  recognizable  little  or  no  feldspar. 


THE  qi;ai{T/-slatk  imkmi'.kk'.  I'yp, 

ll()\vc\cr  tliis  may  bo,  that  all  of  these  roeks  are  oC  a  f'ragmental 
ori-^iii  is  rendered  siitticiently  evident  by  tlie  fi;radation  of  those  kinds,  in 
wliieli  tile  fraj>-niental  textui'e  is  lost  into  those  ])hases  in  which  it  still 
remains  distinct.  It  should  be  said  that  these  gradations  occur  constantly 
within  short  distances,  and  that  those  j)ortions  in  which  no  distinct  trace  of 
frag-niental  origin  is  now  perceptible  are  all  relatively  of  very  small  extent 
and  rare  occurrence.  It  is  a  noticeable  cohicidence  that,  the  most  thor- 
oughly changed  varieties  met  with  in  this  slate  belt  occur  in  its  more 
western  portions,  this  being  at  the  same  time  true  of  the  uppermost  member 
of  the  series.     A  reason  for  *this  coincidence  is  later  suersrested. 

Microscopical  character  of  the  vitreous  quartzite. — (Phase  4,  PI.  xx,  Figs. 
1,  2,  3,  4.)  The  vitreous  quartzites^  prove,  as  one  would  expect  from  their 
macroscopic  appearance,  to  be  composed  almost  entirely  of  relatively  large 
sized  quartz  fragments,  each  one  of  which  has  received  an  enlargement, 
the  several  secondary  enlargements  interlocking  with  one  another  in  a 
more  or  less  intricate  manner.  P'Sldspathic  quartzites  are  interstratified 
in  thin  seams  at  various  horizons  in  the  quartz-slate  formations,  particu- 
larly so  in  the  more  eastern  portion  of  the  belt;  but  the  only  occurrence 
of  pure  quartzite  in  this  formation,  as  already  noted,  is  that  persistent 
band  which  forms  throughout  the  entire  extent  of  the  formation  its 
uppermost  horizon.  The  specimens  brought  from  this  particular  laj^er 
furnisli  some  of  the  handsomest  illustrations  which  we  have  ever  met 
with  of  a  transformation  of  a  sandstone  to  a  vitreous  quartzite  by  the 
enlargement  process.^  The  outlines  of  the  original  fragments  of  these 
rocks  are  in  many  cases  emphasized  by  a  brown  iron  oxide,  in  which 
case  the  rock  has  usuiilly  a  more  or  less  distinctly  brownish  tinge ;  but  in 
many  cases  the  emphasizing  mineral  is  chlorite  in  minute  flakes.  In  the 
latter  case  the  rock  is  either  of  a  light  gray  color,  or,  if  the  chlorite  is 

'  As  used  in  this  memoir,  the  word  "quartzite"  i.s  restricted  to  rocks  which  have  been  derived  from 
Iragmeutals.  The  timdameutal  dift'erence  wliit^h  exists  between  roclcs  of  this  kind  and  tliose  iji 
which  the  qnartz  is  an  original  crystallization,  we  believe,  is  in  this  volume  for  the  first  time  fully  recog- 
nized. However  quartzite-like  the  nonfragmeutal  (juartzose  rocks  of  the  Iron-bearing  and  Cherty 
limestone   members  are,  they  are  always  designated  by  some  other  name  than  quartzite. 

-For  a  more  complete  explanation  of  this  enlargement  process,  with  illustrations  drawn  from 
very  many  localities  and  from  different  geological  horizons,  see  Bull.  U.  S.  Geol.  Survey  No.  8,  by 
E.  D.  Irviug  and  C.  R.  Van  Hise. 


154  THE  PEiSTOKEP]  lEON-BEAEING  SERIES. 

somewhat  plentiful,  of  a  distinct  greenisn  tinge.  The  entire,  or  nearly 
entire,  absence  V)f  particles  of  any  other  mineral  than  quartz  in  these 
rocks  is  taken  to  indicate  a  greater  amount  of  sorting  than  has  been 
received  by  materials  which  have  composed  the  other  phases  of  this  forma- 
tion. This  conclusion  is  borne  out  by  the  generally  uniform  size  of  the 
quartz  fragments  of  the  vitreous  quartzites,  the  sorting  having  been 
carried  so  tar  as  to  remo^'e  not  only  the  particles  of  other  minerals,  but  the 
smaller  particles  of  quartz. 

Microscopical  character  of  the  sandstone,  novactilite,  and  argillaceous 
slates. — (Pliases  5,  6,  and  7.)  These  phases  of  the  Quartz-slate  member, 
because  of  their  relatively  small  importance,  may  be  more  rapidly 
dismissed.  The  sandstone  phase  (PI.  xix.  Fig.  3)  is  an  unusual  one,  having 
been  so  far  observed  only  in  three  localities,  viz:  In  the  vicinity  of  the 
Ashland  mine.  Sec.  27,  T.  47  N.,  R.  47  W.,  Michigan;  in  the  Aacinity  of  the 
Aurora  mine.  Sec.  23,  T.  47  N.,  R.  47  W.,  Michigan,  and  at  a  point  about 
three-fourths  of  a  mile  east  of  Sunday  lake.  Sec.  10,  T.  47  N.,  R.  46  W.,  Mich- 
igan. In  the  first  two  localities  it  occurs  at  a  high  horizon  immediately 
beneath  the  overlying  vitreous  quartzite;  but  in  the  last  place  the  sandstone 
occurs  on  the  contrary  at  a  low  horizon.  Macroscopically  these  sandstones  are 
of  a  rather  fine  grained  arenaceous  texture  and  from  red  to  white  in  color, 
these  colors  being  often  very  irregularly  blotched.  An  examination  of  the 
hand  specimens,  without  reference  to  their  source,  would  certainly  suggest 
that  they  came  from  some  modem  sandstone  formation  rather  than  from 
so  ancient  a  terrane  as  that  we  are  now  concerned  with.  Microscopical 
examinations  show,  however,  that  thej'^  are  merely  less  consolidated  phases 
of  the  quartzites  or  feldspathic  quartz-slates  above  described,  these  rocks 
being  in  fact  no  whit  less  frag-mental  in  texture  than  the  sandstones  them- 
selves. 

The  novaculite  or  whetstone-like  phase  (PI.  xviii,  Fig.  4)  of  the  Quartz- 
slate  member  occurs  here  and  there  in  thin  seams  in  the  eastern  half  of  the 
belt.  One  of  the  most  noteworthy  localities  for  this  phase  is  the  gorge  of 
Tyler's  fork  where  the  novaculite  shows  on  the  west  bank  of  the  river  at 
about  200  paces  north  of  the  quarter  post  on  the  east  line  of  Sec.  33,  T.  45 
N.,  R.  1  W.,  Wisconsin.  This  novaculite  is  nothing  more  than  a  very  fine 
and  even  grained  variety  of  the  feldspathic  quartz-slates,  above  described. 


Tino  (H'AKTZ-WLATE  MEMHKH.  155 

Tlio  aro-illiU'cous  sLitcs  (I'l.  \i\,  F"'io\  4)  occiir  in  ,i  iiiiiuIxt  of  |(lii('(^s,  l)at 
iicai-ly  always  in  i-clatixcly  tliiii  scams,  iiiter.strati(ic(|  witli  the  coarser  vari- 
eties iiitii  wliicli  tliey  li'rade.  'I'liese  slates  a|)])eai'  tii  diH'er  IVoiii  the  teld- 
spathic  (|uart/,-slates  merely  in  ha\in<i'  the  coarser  fragmental  jjortion  almost, 
althon-ih  luit  (|uito,  excluded;  that  is  to  say,  they  are  of  the  same  natxire 
as  the  matrix  portion  of  the  feldspathic  quartz-slates,  with  perhaps  an  un- 
usual amount  of  kaolinitic  material  from  the  alteration  of  feldspar  detritus. 

Tabulation  of  jicfrrn/raphicnl  ohservailons. — The  following  tabulation  ex- 
hibits a  series  of  observations  made  on  specimens  from  this  formation.  As 
in  the  case  of  the  limestone  formation,  these  observations  are  taljulated  in  the 
first  place  in  g-eog-rapliical  order,  proceeding  from  the  west  to  the  east.  In 
the  second  place  in  stratigraphical  order  for  each  cross-section  examined, 
beginning  with  the  lower  horizons  in  each  case  and  proceeding  to  the  higher. 

From  the  section  in  Sec.  34,  T.  44  N.,  R.  4  W.,  Wisconsin. 

1.  Oliloritic  and  biotitic  slate,  from  a  middle  horizon.  Specimeus  9645  (slide 
3155),  9C4G  (slide  3150). '     From  l.->00  N.,  0  W.,  Sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin. 

A  tiiielj'  grannlar,  dark  gray)  uniformly  textured  material,  of  almost  qnartzitic 
eoinpaetuess,  is  interbanded  with  a  finely  laminated  material  of  a  greenish  gray  color, 
whicli  upon  the  snrface  of  the  lamina^  shows  the  sheen  of  mica. 

The  sections  consist  chiefly  of  quartz,  feldspar,  kaolin,  chlorite,  and  biotite,  the 
first  t\yo  minerals  being  fragmental.  In  the  more  qnartzitic  phases  the  quartz  in 
rather  small  nniform  sized  fragments,  some  of  which  have  received  a  secondary  en- 
largement, constitutes  two-thirds  of  the  mass  of  the  rock.  The  feldspar  fragments 
include  both  orthoclase  and  plagioclase.  Scattered  through  the  mixture  of  quartz  and 
feldspar,  and  composing  the  greater  part  of  the  remaining  third,  are  small  flakes  of 
chlorite  and  biotite,  the  former  the  more  abundant  of  the  two.  The  chlorite  is  in  well 
defined  flakes  which  extinguish  rectangularly.  The  schistose  part  of  the  rock  con- 
tains a  greater  proportion  of  feldspar,  kaolin,  and  biotite  than  the  more  qnartzitic  por- 
tion. The  biotite  and  chlorite  both  appear  to  be  secondary  developments  from  the 
feldspar  fragments.    Magnetite  is  an  unimportant  accessory. 

2.  Hornblendic  and  chloritic  quartzites,  from  the  uppermost  layers,  above  9645  and 
9646,  and  immediately  underneath  the  Iron-bearing  member.     Specimens  9647  (slide 

'The  numbers  of  specimeus  and  slides  are  usually  those  of  the  collection  of  the  lake  Sujierior 
division.  Specimens  with  Wr.  after  the  numbers  are  from  the  collection  of  the  late  Mr.  Charles  E. 
Wright.  Specimeus  with  Wis.  after  the  numbers  are  from  the  collection  of  the  Wisconsin  Geo- 
logical Survey.  Locations  are  given  from  the  southeast  corner  of  the  sections,  in  steps  of  2,000 
per  mile. 


156  THE  PENOKEE  IRON-BEAEING  SERIES. 

3157),  from  1575  K,  0  W.,  9648;  (slide  3158)  from  1635  K,  0  W.,  Sec.  24,  T.  44  K, 
R.  4  W.,  Wisconsin. 

A  medium  grained,  massive,  semi-vitreotis  quartzite,  mottled  light  gray  and  dark 
green. 

Fragments  of  a  very  limpid  qnartz  compose  three-fourtlis  of  the  thin  sections. 
These  fragments  are  often  partly  or  wholly  separated  from  each  other  by  an  interstitial 
material,  composed  mainly  of  chlorite  in  aggregates  of  pale  green  flakes,  and  horn- 
blende in  small  greenish  needles  and  blades.  However,  over  the  greater  part  of  the 
sections  tlie  quartz  grains  lit  closely,  or  even  interlock,  and  at  times  the  original  out- 
lines of  the  fragments  are  quite  obliterated.  Here  and  there  a  little  interstitial  car- 
bonate 'is  seen. 

From  the  section  in  Sec,  17,  T.  44  N.,  R.  3  W.,  Wisconsin. 

3.  Chloritic  quartzite.  Specimen  4521  (slide  1402).  From  SW.  i,  Sec.  17,  T.  44 
N.,  R.  3  W.,  Wisconsin. 

Rather  large  interlocking  quartz  grains  compose  the  greater  part  of  the  section. 
These  are  believed  to  be  enlarged  fragments  of  quartz,  mainly  because  in  juost  of  the 
similar  rocks  of  this  belt  they  are  manifestly  so;  but  taking  this  section  alone,  the 
proof  of  the  fragmental  origin  is  not  conclusive.  In  masses  and  Alms  between  the 
grains  are  aggregates  of  chlorite,  the  section  as  a  whole  being  closely  allied  to  those  of  2. 

4.  Biotitic  slate.  Specimen  9644  (slide  3154).  From  near  the  center  of  Sec.  17, 
T.  44  N.,  R.  3  W.,  Wisconsin. 

This  is  for  the  most  part  a  finely  laminated  slate,  but  occasionally  bands  of  greater 
width  and  coarser  grain  occur.  It  is  of  a  greenish  gray  color  and  shows  strongly  the 
sheen  of  mica. 

The  thin  section  is  composed  mainly  ofvery  fine  grained  quartz  and  mica,  the  two 
minerals  being  about  equally  abundant  and  evenly  mingled.  The  fragmental  char- 
acter of  this  fine  matrix  is  not  entirely  plain,  but  a  coarser  band  running  across  the 
section  shows  the  usual  large  fragments  of  qnartz,  each  with  a  wide  secondary  enlarge- 
ment, while  occasional  quartz  grains  of  the  same  character  are  scattered  through  the 
fine  grained  groundmass.  The  line  of  demarkation  between  these  quartz  fragments 
and  the  enlargements  is  commonly  marked  by  minute  flakes  of  mica.  There  are  also 
occasionally  present  large  flakes  of  a  rectangularly  extinguishing  chlorite,  and  some 
fragments  of  feldspar.  It  is  not  improbable  that  much  of  the  fine  grained  quartz  has 
developed  from  the  alteration  of  the  feldspar.     (PI.  xix.  Figs.  1  and  2.) 

5.  Chloritic  quartzite.    Specimen  164  Wr.    From  1000  N.,  1000  W.,  Sec.  17,  T. 
44  N.,  R.  3  W.,  Wisconsin. 

This  is  a  medium  grained,  gray,  vitreous  quartzite,  which  resembles  closely  2, 
but  differs  in  lacking  the  peculiar  mottling  shown  by  those  rocks. 

Intricately  interlocking  clear  quartz  grains  compose  nearly  the  whole  section, 
the  only  other  minerals  present  being  chlorite  and  ferrite,  unless  some  of  the  greenish 


THE  QUARTZ  SLATE  MEMBER.  157 

interstitial  inimn-al  he  aiiipliiholc.  Most  of  tlie  quartz  individuals  have,  plainly  been 
enlai-jjcd,  aiul  tlic  lock  is  certainly  an  onliiiary  fia},nncnlal  (|uaitzit('.  In  tlie  case  of 
a  few  ol'  till'  iari^c  (|uartz  individuals  the outliiifs  of  the  i)rif;inal  tra^iincntal  cores 
arc  iHit  visihlc;  hiil  llicse  areas  arc  so  coniplctcly  like  the  otiicrs,  in  wliich  the  cores 
arc  pcrccplihic,  dial  all  nuisthc  taken  as  of  the  same  nature.  IJoth  (Mdarf^enicnts  and 
ori;;inal  cores  are  composed  of  a.  sin};ularly  pure  limpid  (inartz ;  whence  the  occasional 
invisibility  of  the  outlines  of  the  cores.  This  section,  then,  furinshcs  us  abundant 
proof  of  the  possibility,  under  favorable  cirtHimstancies,  of  the  development,  from  an 
ordinary  sandstone  by  enlaij;-cnicnr,  of  the,  (piartz  fragments,  of  a  quartzite  whose 
fragmenta!  character  would  never  be  sus])ccted  from  the  thin  section.  In  this  same 
connection  reference  should  be  made  to  U  and  li,  above  described,  whose  fragmental 
oi'igin  is  less  clear  than  in  the  section  now  under  consideration. 

From  the  section,  in  Sec.  16,  T.  41  N.,  R.  3  W.,  Wisconsin. 

G.  Hornblendic  and  chloritic  quartzite,  from  the  ui)permost  layers  and  imme 
diately  beneath  the  Iron-bearing  member.  Specimen  9tJJ:o  (slide  31.33).  From  1850 
N.,  1050  W.,  Sec.  10,  T.  14  :N.,  E.  3  W.,  Wisconsin. 

A  rock  similar  to  2,  both  macroscopically  and  microscopically. 

From  the  Fenokce  (/up  section. 

7.  Magnetitic  chert-breccias,  from  the  base  of  the  Quartz  slate  niendjer.  Speci- 
mens 9534  (slide  3140),  9535  (slide  3141),  1424,  Wis.  (slide  252).  From  1470  N.,  1200 
W. ;  1455  Wis.  (slide  265).    From  15D0  X.,  1635  W.,  Sec.  14,  T.  44  K,  R.  3  VV.,  Wisconsin. 

In  these  peculiar  rocks  a  greenish  to  black  schistose  uuitrix  contains  numerous 
angular  pieces  of  white  chert,  and  fewer  smaller  ones  of  clear  (piartz. 

The  groundmass  of  the  thhi  sections  consists  mainly  of  a  minutely  crystalline 
silica  of  nonfragmental  appearance.  This  is  explained  by  the  fact  that  this  rock 
rests  directly  upon  the  friable  (juartz  i-ock  (Nos.  7  and  8,  ]>.  136)  of  the  cherty  lime- 
stone and  has  derived  most  (>{  its  material  from  this  underlying  formation.  It 
approaches  closely  to  a  recomposed  quartz  rock,  the  particles  of  which  are  separate 
grains.  With  the  quartz  are  mingled  more  or  less  chlorite  and  actiuolite,  some 
brown  and  red  iron  oxide,  and  a  large  ([uantity  of  magnetite.  In  3140  and  3141  this 
magnetite  plays  a  very  ^subordinate  part.  In  252,  how.wer,  it  is  very  abundant, 
while  in  265  it  predomhiates  over  the  silica  of  the  cement,  occui-ring  in  most  beauti- 
fully outlined  crystals,  often  of  a  very  considerable  size.  There  is  little  iit  first  sight  in 
the  matrix  of  any  of  these  rocks  to  suggest  a  fragmental  origin.  However,  there  are 
contained  in  it,  in  quantity  sufficient  to  compose  from  one- third  to  three-fourths  of  the 
area  of  the  thiu  sections,  fragments  of  quartz  and  of  a  minutely  crystalline  silica  or 
chert.  The  quartz  fragments  are  well  rounded,  and  the  pic:'cs  of  chert  are  only  partly 
SO,  being  often  quite  angular,     The  chert  pieces  reach  as  much  as  a  quarter  of  an  jnc^ 


]^58  THE  PENOKBE  IKON  BEAEING  SERIES. 


in  diameter.  The  quartz  fragmeuts  are  both  simple  and  complex,  and  have  often 
received  secondary  enlargement's.  The  most  notable  thing  about  these  rocks  is  the 
fact  that  the  crystals  of  magnetite,  and  also  numerous  minute  needles  of  actinolite, 
besides  being  contained  in  the  matrix,  are  included  abundantly  in  the  enlargements 
of  the  fragmental  grains  of  quartz.  These  minerals  have  developed  in  situ.  These 
very  singular  rocks  come  from  a  belt  lying  above  the  chert,  which  at  Penokee  gap 
forms  all  of  the  upper  part  of  the  limestone  member,  and  the  fragmental  slates  of  the 
member  now  uader  consideration.     (PI.  xvii,  Pigs.  1  and  2.) 

8.  Biotitic  and  chloritic  slates,  from  low  horizons.  Specimens  9562  (slide  3143), 
9563  (slide  3097),  from  14S0  N.,  1700  W.;  9564  (slides  3144,  3145),  9565  (slide  3098), 
from  1530  N.,  1160  W.,  Sec.  14,  T.  44  X.,  R.  3  W.,  Wisconsin. 

These  are  fine  grained,  thinly  laminated  rocks,  breaking  at  times  with  a  sub- 
conchoidal  fracture  across' the  planes  of  lamination.  In  color  they  range  from  dark 
brown  or  black,  through  gray,  to  various  shades  of  green. 

The  thin  sections  present  a  ground  mass  which  is  always  chieily  quartzose,  the 
grains  being  of  small  but  uniform  size  and  generally  of  roundish  or  oval  shapes.  In 
detail,  however,  the  outlines  of  the  grains  are  angular,  and  the  projections  of  the 
different  particles  commonly  interlock.  In  many  of  these  minute  particles  the  out- 
lines of  the  original  cores  are  perceptible,  the  enlargements  being  narrow  and  pro- 
ducing the  angular  projections  referred  to.  The  line  of  division  between  the  enlarge- 
ment and  the  original  core  is  generally  emphasized  by  secondary  developments  of 
particles  of  chlorite  and  biotite.  Many  more  quartz  particles,  however,  do  not  show 
any  traces  of  the  original  cores,  and  it  is  not  certain  that  a  portion  of  them  may  not 
have  crystallized  in  situ.  Small  fragments  of  feldspar  make  up  a  part  of  the  ground- 
mass,  scattered  through  which  are  particles  and  clusters  of  particles  of  chlorite  and 
biotite.  The  single  flakes  of  these  two  minerals  are  larger  than  the  particles  of  the 
groundmass  itself,  but  are  still  very  minute.  In  some  slides  tlie  biotite  almost 
excludes  the  chlorite,  wliile  in  others  the  reverse  of  this  is  true.  A  considerable 
portion  of  these  two  minerals  seems  plainly  to  be  due  to  the  decomposition  of  the 
feldspar  fragments.     Accessory  constituents  are  sericite,  kaolin,  tourmaline,    and 


magnetite. 


9.  Quartzite,  from  the  uppermost  part  of  the  slate,  and  immediately  beneath  the 
Iron-bearing  member.  Specimens  9560  (slide  3192),  9561  (slide  3193).  From  1675  N., 
1280  W.,  Sec.  14,  T.  44  N.,  11.  3  W.,  Wisconsin. 

A  flue  grained,  quite  grayish  quartzite,  having  a  conchoidal  fracture. 

Quartz  in  rather  small  but  uniform  sized  particles  composes  three- fourths  of  the 
sections.  The  i»articles  of  this  mineral,  although  they  very  often  closely  fit  and  inter- 
lock, are  plainly  in  the  main  of  a  fragmental  nature.  Biotite  in  3192  and  chlorite  in 
3193  occur  sparingly  between  the  (luartz  particles.  Numerous  fragments  of  feldspar, 
flakes  of  sericite,  and  a  few  particles  of  limonite  are  seen.  The  sections  are  closely 
allied  to  those  of  8.  . 


THE  CHJAKTZ-SLATH  MEMBER.  159 

10.  Chloiitic-  Mild  hiotilic  sl:itc.  S|.i'(im«'n  l.V^i  (slide  UOr,).  From  NE.  J  Sec. 
17,  T.  44  N.,  li.  li  \V.,  Wisconsin. 

Till'  tliiii  section  is  cDiiipiiscd  ol'  c\<-cc(lMi.;;ly  line  |>;nliclcs  of  iiuartz,  feldspar, 
biotite,  chlorite,  and  scricite.  Tlic  general  ai)|icarauce  is  a  fnij-iiieutal  oue,  but  the 
grain  is  so  minute  that  it  would  he  ditliciilt  lo  |Mdvr  a  fragiueiital  nature  from  this 
section  aloue. 

From  the  Tykys  fork  nccfion. 

11.  (Jhloritic  and  biotitic  shitc.  Specimen  Olil.j  (slide  .ilMtfJ).  Prom  1135  N.,  1980 
W.,  Sec.  34,  T.  45  N.,  R.  I  W.,  Wi.scoQsiii. 

A  lino  grained,  slaty  rock,  breaking  with  eoucihoidal  fracture  across  the  lamina- 
tion. Upon  cleaved  surfaces  are  seen  numerous  flakes  of  white  mica  of  a  light  gray 
color. 

The  chief  constituents  of  the  thin  section  are  (piartz  and  feldspar  in  plainly  frag- 
mental  particles,  the  former  being  very  plentiful,  and  having  the  fragments  ordinarily 
enlarged.  Between  the  grains  of  (piartz  and  feldspar,  and  also  included  in  the  feld- 
spar fragments  as  secondary  products,  are  numerous  small  folia  of  chlorite  biotite 
and  sericite. 

'^  From  tlic  I'utato  river  section. 

13.  Magnetitic  quartz-slate  and  conglonierate,  from  the  base  of  the  Quartz-slate 
member.  Specimens  0094  (slide  3297),  9()9G  (slide  1!S<»5),  9175  (slides  2994,  2995).  All 
from  888  N.,  35  VV.,  See.  19,  T.  45  K,  R.  1  E.,  Wisconsin. 

The  lowest  portion  of  the  Quartz-slate  member  ou  Potato  river,  at  and  near  the 
junction  with  the  underlying  green  schists,  diffei's  from  the  body  of  the  member  above 
described  only  in  the  presence  of  mucjh  magnetite,  along  with  numerous  pebbles  and 
bowlders  derivedfrom  the  lower  schists,  also  fewer  small  tiaginents  of  jasper  and  chert. 

The  background  of  the  sections  differs  from  the  slates  before  described  (mly  in  that 
they  contain  a  very  considerable  (luantity  of  magnetite  in  well  defined  crystals,  and 
aggregates  of  crystals,  these  being  at  times  so  large  and  numerous  as  to  make  up  juost 
of  the  sections.  As  indicated  in  the  macroscopic  description,  these  rocks  contain  peb- 
bles and  bowlders  derived  from  the  green  schist  immediately  below,  and  also  fewer  of 
quartz,  chert,  and  jasper.  In  sections  2994  and  2995  is  seen  a  large  clierty  area  in 
which  are  numerous  magnetite  particles.  The  occurrence  in  these  rocks  of  numerous 
fragments  ofcherty  and  jaspery  material,  associated  with  an  abundance  of  magnetite, 
caUs  to  mind  the  peculiar  magnetitic  breccia  whicli  lies  at  this  same  horizon  on  Bad 
river,  where,  however,  there  lies  between  it  and  the  lower  schists  a  thickness  of  som(> 
90  feet  of  limestone  and  white  chert.  The  Cherty  limestone  member  is  entirely  lack- 
ing at  Potato  river,  but  the  occurrence  just  described  suggests  that  it  and  a  jasper 
formation  may  have  formerly  been  here.     (PI.  xvti,  Fiy.  i.) 

13.  Feldspathic  quartzites,  or  gray  wackes,  ehloritic  (piartz-slates,  and  graywacke- 
slates,  from  middle  and  lower  horizons.    Specimens  9083  (slide  2781),  from  950  N.,  180 


IQQ  THE  PENOKEE  lEON-BEAEING  SERIES. 

W.;  9084  (slide  li782),  from  970  N.,  200  W.;  9086  (slide  2783),  from  9(30  N.,  185  W.; 
9088  (slide  2784),  from  960  K,  185  W.;  9089  (slide  2785),  from  910  N.,  105  W.;  9099 
(slide  2788),  from  888  I^.,  36  W.;  9100  (slide  2896),  from  889  N.,  37  W.;  9101  (slide 
2897),  from  890  N.,  38  W.,  Sec.  19,  T.  45  N.,  R.  1  E.,  Wisconsin. 

Except  at  the  extreme  north,  where  the  quartzite  14  is  found,  the  exposures  of  • 
the  Quartz-slate    member  on   Potato   river  consist  of    interstratiflcations  of  thinly- 
bedded  pale  pinkish  quartzitic  layers,  and  greenish  (chloritic)  gray  and  brown  argil- 
laceous slates.    These  grade  into  one  another  and  are  all  described  here  together. 

The  thin  sections  from  the  more  quartzitic  portions  of  the  Potato  river  exposures 
show  fragments  of  quartz  as  the  predominant  ingredient,  but  mingled  with  the  quartz 
fragments  are  plentiful  ones  of  feldspar,  including  orthoclase,  microcline,  and  plagio- 
clase.  The  quartz  fragments  have  nearly  always  received  enlargements  which 
have  produced  an  interlocking  of  the  (quartz  areas.  The  feldspar  fragments  on  the 
whole  are  singnlarly  fresh.  Chlorite,  ferrite,  and  calcite  are  present  as  accessories  in 
some  of  the  sections,  occurring  as  narrow  films  and  areas  between  the  fragmental 
particles.  From  the  quartzitic  phases  there  is  found  a  gradation  to  those  vai'ieties 
which  are  entirely  aphanitic  in  the  hand  specimens,  and  are  so  exceedingly  fine 
grained  and  clayey  that  in  the  thin  section  little  is  to  be  made  out  save  that  minute 
particles  of  (piartz  and  feldspar  are  scattered  through  the  clayey  background,  which 
is  taken  to  be  composed  of  pulverized  and  kaolinized  felds])ar,  mingled  with  chlorite, 
sericite,  etc.  Slide  2788  is  at  the  extreme  as  to  fineness  of  grain  and  small  number  of 
recognizable  quartz  and  feklspar  particles.  In  the  intermediate  varieties  half  or  more 
than  half  of  each  section  is  composed  of  minute  but  still  distinctly  recognizable  frag- 
ments of  quartz  and  feldspar,  the  quartz  grains  being  almost  always  provided  with 
plainly  visible  enlargements,  even  when  widely  separated  from  one  another  by  tlie 
intermediate  clayey  matrix.  This  matrix  in  many  sections,  in  additi(m  to  the  minerals 
mentioned  above  (namely,  chlorite,  sericite,  kaolin,  and  ferrite),  contains  calcite  and 
occasionally  crystals  of  magnetite,  which  last  mineral  is  particularly  met  with  in  sec- 
tions from  specimens  which  come  from  not  very  far  from  the  underlying  green  schists. 

14.  Quartzite,  from  the  uppermovst  horizon  of  the  Quartz-slate  member,  imme- 
diately beneath  the  Iron-bearing  member.  Specimen  9082  (slide  2780).  From  1000 
N.,  210  W.,  Sec.  19,  T.  45  N.,  R.  1  E.,  Wisconsin. 

A  coarse  grained,  vitreous  (quartzite,  of  a  pale  pink  color. 

The  section  is  almost  wholly  composed  of  clear  quartz  in  large  intricately  inter- 
locking areas.  Each  one  of  these  areas,  however,  is  furnished  with  a  very  distinctly 
outlined  fragmental  core,  and  occasionally  the  secondary  enlargement  lias  received 
■crystal  outlines.  The  cores  and  enlargements  are  separated  from  one  another  by  films 
of  ferritic  material.  This  section  furnishes  one  of  the  very  finest  illustrations  tliat  we 
have  met  with  of  the  production  of  a  vitreous  quartzite  from  a  completely  fragmental 
.sandstone  by  the  eulargenieut  process.     (PI.  XX,  Figs.  1  and  2.) 


THE  QUAKTZ-aLATlO  MEMBER.  161 

From  the  section  in- Sec.  li,  T.  IS  N.,  li.  2  E.,  Wiscomm. 

15.  Chloritio  slato,  from  a  low  horizon  underlying  9145  (Hi).  Specimen  9133 
(slide  3300).     From  1000  N.,  KJOO  W.,  Sec.  6,  T.  45  N.,  K.  2  E.,  Wiscousiu. 

A  line  grained,  thinly  ch^avalile,  grayish  green  slate. 

Minute  fragments  of  (juartz  and  feldspar,  the  former  enlarged  in  the  usual 
manner,  arc  mingled  with  a  liner  material  composed  of  kaolin,  sericite,  and  ferrite. 

10.  (Jhloritic  quartzite,  from  the  uppermost  lioi'izon.  Specimen  9145  (slide  2800). 
From  lOOO  N.,  1535  W.,  Sec.  0,  T.  45  N.,  R.  2  E.,  Wisconsin. 

A  line  grained,  vitreous,  greenish  gray  quartzite. 

The  thin  section  is  composed  mainly  of  rather  small  quartz  fragments,  which  are 
provided,  however,  with  wide  and  deeply  interlocking  enlargements.  Chloritic  flakes 
with  a  little  ferrite  occur  in  the  interstices  of  the  grains. 

From  the  West  branch  of  the  Montreal  section. 

17.  Chloritic  slate  and  conglomerate,  from  the  basal  portion  of  the  Quartz-slate 
member,  and  iu  contact  with  the  underlying  greeu  schist.  Specimens  9149  (slide 
2912),  9171  (slide  2914).    From  175  N.,  1035  W.,  Sec.  27,  T.  46  K,  R.  2  E.,  Wisconsin. 

Specimen  9171  contains  large  pebbles  of  green  schist,  cemented  by  a  dark  green, 
uniform  grained  matrix,  in  which  are  recognizable  numerous  grains  of  quartz,  feld- 
spar, and  calcite.  Specimen  9140  is  like  the  matrix  of  9171.  They  are  both  from  the 
junction  with  the  underlying  greenish  schists. 

The  slides  are  like  those  described  in  12,  except  that  they  carry  numerous  frag- 
nients  of  the  greeuish  schist  beneath  them,  and  much  secondary  calcite  and  chlorite, 
presumably  derived  from  the  secondary  alteration  of  the  schist  fragments. 

IS.  Sericitic  and  chloritic  graywackes  and  graywacke-slates,  from  the  middle 
horizons.  Specimens  9035  (slide  2919),  from  190  K,  1130  W.;  9038  (slide  2771),  from 
210  N.,  1080  W.;  9040  (slide  2772),  from  210  N.,  1020  W.;  9043  (slide  2773),  from  270 
if.,  1000  W. ;  9044  (slide  2774),  ti-om  320  N.,  1010  W.,  Sec.  27,  T.  46  K,  R.  2  E.,  Wis- 
consin. 

The  rocks  are  flue  grained,  compact,  of  a  uniform  texture  and  subconchoidal 
fracture  when  broken  across  the  lamination;  are  often  readily  cleavable  parallel  to 
the  lamination,  into  large  thin  plates,  the  cleavage  surfaces  showing  the  sheen  of 
mica.    The  colors  vary  through  shades  of  green,  pink,  gray,  and  purple. 

The  slides  differ  somewhat  widely  iu  the  quautityof  fragniental  material  which  is 
recognizable  as  such.  The  finer  interstitial  material,  varying  from  a  quite  subordinate 
quantity  to  a  predomiiuiting  amount,  is  composed  of  flakes  of  sericite  and  chlorite, 
along  with  some  minutely  crystalline  quartz  and  also  kaolin  and  brown  iron  oxide. 
The  quartz  and  feldspar  fragments  occur,  as  in  thin  sections  of  similar  rock  previously 
described.  The  feldspar  particles,  while  fresh  in  the  inner  portions,  are  commonly 
altered  upon  the  exterior  to  chlorite  and  kaolin,  which  minerals,  along  with  the  seri- 
cite and  minutely  crystalline  quartz,  are  taken  to  be  due  to  metasomatic  changes. 
MON  XIX 11 


162  THE  PENOKEE  lEON-BEARmG  SERIES. 

19.  Ferruginous  and  feldspathic  quartzites,  from  the  upper  horizons.  Specimens 
9151  (slide  2802),  from  310  N.,  1120  W.;  9152  (slide  3104),  from  310  N.,  1130  W.;  9153 
(slide  2803),  from  310  N.,  1140  W.;  9154  (slide  2804),  from  350  N.,  1132  W.;  9045  (slide 
2885),  from  395  N.,  1055  W.,  Sec.  27,  T.  46  N.,  E.  2  E.,  Wisconsin. 

The  specimens  vary  from  those  of  a  medium  grain,  vitreous  luster,  and  con- 
choidal  fracture,  to  those  which  are  very  line  grained  and  lusterless.  They  are 
stained  from  a  light  to  a  dark  brown  by  hydrated  iron  oxide. 

Kather  small  and  well  rounded  fragments  of  quartz,  now  provided  with  inter- 
locking enlargements,  predominate  in  the  thin  sections.  Fragments  of  orthoclase 
and  plagioclase,  commonly  quite  fresh,  occur  in  some  abundance.  In  some  sections 
large  films  and  areas  of  hydrated  oxides  of  iron  are  plentiful  between  the  fragments; 
in  other  cases  (2802)  chlorite  flakes  occur  mingled  with  the  iron  oxide.  (PI.  xx, 
Figs.  3  and  4.) 

20.  Quartzite,  from  the  uppermost  layers  of  the  Quartz-slate  member,  being  the 
foot  wall  at  the  Bessemer  mine.  Specimen  9054  (slide  2920),  from  1000  N.,  1838  W., 
Sec.  26,  T.  46  K,  E.  2  E.,  Wisconsin. 

Both  macroscopically  and  microscopically  this  quartzite  is  exactly  like  14. 

From  the  Germania  mine  section. 

21.  Slaty  feldspathic  quartzite,  from  a  high  horizon  and  immediately  beneath 
9013.  Specimen  9011  (slide  2766).  From  133  N.,  1500  W.,  Sec.  24,  T.  46  N.,  R.  2  E., 
Wisconsin. 

The  rock  is  composed  of  interstratified  seams  of  a  coarse  pinkish  quartzite  and 
a  flue  grained  green  material. 

Fragments  of  quartz  and  feldspar  make  up  most  of  the  section,  the  particles 
being  quite  angular.  In  the  case  of  the  quartz  this  angularity  is  pla,inly  due  to  a 
secondary  enlargement.  Between  these  fragments  is  an  interstitial  material,  com- 
posed ill  part  of  a  minutely  crystalline  quartz,  with  limonite,  kaolin,  and  sericite  in 
minute  flakes.    The  section  resembles  closely  2802  in  19,  above  described. 

22.  Quartzite,  from  the  uppermost  horizon,  immediately  in  contact  with  the  Iron- 
bearing  member,  being  the  foot  wall  of  the  Germania  mine.  Specimen  9013  (slide 
2767).    From  200  N.,  1560  W.,  Sec.  24,  T.  46  N.,  R.  2  E.,  Wisconsin. 

Both  macroscopically  and  microscopically  this  rock  is  like  14. 

From  the  Ashland  mine  seetioti. 

23.  Slaty  and  cherty  quartzite,  from  a  low  horizon.  Specimen  7621  (slide  2017). 
From  1805  N.,  1910  W.,  Sec.  27,  T.  47  N.,  R.  47  W.,  Michigan. 

This  rock  is  composed  of  thinly  interstratified  seams  of  a  greenish  or  yellowish 
gray  vitreous  quartzite,  and  of  a  dark  green  aphanitic  material. 

The  thin  section. appears  to  be  cut  wholly  from  one  of  the  quartzitic  seams.    It 


TMK  QUAKTZ  SLATE  MEMHIOK'.  ]  63 

shows  a  mixture  of  tViiginciits  oC  (iiuutz  and  of  a  niiimlcly  crystalline  cherty  silica, 
iiiiboddcd  in  a  cement  which  composes  about  one-third  of  the  section,  and  consists  of 
a  mixture  of  very  minutely  crystalline  silica,  with  particles  of  chlorite  and  ferrite. 
The  (|uart/,  fratinu^nts  at  times  show  very  distinct  secondary  enlargements.  A  few 
feldspar  fragments  are  seen. 

'2i.  Quartzites,  from  a  high  horizon.  Specimens  9005  (slide  3i01i),  7620  (slide 
201(i).     From  18S0  N.,  1910  W.,  Sec.  27,  T.  47  N.,  li.  17  W.,  Michigan. 

These  are  compact,  coarse  grained,  vitreous  (luartzites,  of  ai  gray  color. 

Quartz  fragments,  with  the  usual  interlocking  enlargements,  and  mingled  with 
some  pieces  of  feldspar,  make  up  most  of  the  sections.  The  enlargements  of  the  quartz 
fragments  are  unusually  wide.  The  outlines  of  the  cores  within  these  enlarge- 
ments are  marked  in  a  distinct  manner  by  films  of  chlorite  and  brown  iron  ore,  which 
minerals  also  occur  along  the  line  of  junction  between  the  enlargements  of  difl'erent 
grains.  A  number  of  instances  are  noted  of  the  enlargement  of  complex  quartz  frag- 
ments, each  individual  area  withiu  the  fragment  having  received  its  own  enlargement. 

25.  Sericitic  ferruginous  sandstone,  from  a  high  horizon.  Specimens  9004  (slide 
2880),  7(>18  (slide  2015).     From  1880  N.,  1910  W.,  Sec.  27,  T.  47  N.,  E.  47  W.,  Michigan. 

Specimen  7018  is  massive,  medium  grained,  dark  reddish  brown,  and  of  a  sub- 
vitreous  luster;  9004  is  nearer  a  slate,  the  quartzitic  parts  being  interstratified  with 
narrow  green  belts. 

These  slides  are  intermediate  between  the  fragmental  quartzites  and  the  argil- 
laceous slates  of  this  member.  The  fragmental  quartz  particles  still  predominate, 
along  with  some  fragmental  feldspar,  the  quartz  grains  being  either  without  enlarge- 
ments or  with  only  very  narrow  ones.  Contained  between  the  fragments  and  serving 
as  a  matrix  to  them  is  a  very  fine  nmterial  composed  of  browm  iron  oxide,  kaolin, 
minutely  crystalline  quartz,  and  chlorite.  Numerous  flakes  of  sericite  occur  also,  ap- 
pearing rather  as  if  fragmental,  or  altered  from  the  fragmental  mica,  than  as  if  sec- 
ondary to  some  of  the  other  fragmental  constituents  of  the  rock  (PI.  xix,  Fig.  3). 

From  the  Aurora  mine  section. 

26.  Slaty  and  cherty  quartzite,  from  the  uppermost  layers,  being  the  foot  wall  of 
the  Aurora  nnne.  Specimen  7614  (slide  2014).  From  624  N.,  1000  W.,  Sec.  23,  T.  47 
N.,  E.  47  W.,  Michigan. 

The  thin  section  shows  an  interlamination  of  coarser  and  finer  materials.  The 
coarser  bands  are  composed  in  about  equal  quantity  of  fragments  of  fine  quartz  indi- 
viduals and  of  a  chloritic  chert.  The  finer  grained  bands  are  made  up  of  fragments 
of  quartz  and  feldspar,  along  with  particles  of  probably  secondary  silica,  chlorite, 
and  iron  oxides. 


164  THE  PENOKEE  lEON-BEAEING  SERIES. 

From  the  section  at  the  Mount  Hope  mine. 

37.  Ferruginous  quartzite,  from  the  uppermost  layers,  being  the  foot-wall  at 
the  Mount  Hope  mine.  Specimen  7611  (slide  2307).  From  1140  N.,  0  W.,  Sec.  23,  T. 
47  N.,  E.  47  W.,  Michigan. 

The  thin  section  is  composed  of  large  interlocking  quartz  areas,  at  the  junctions 
of  which  are  often  films  of  brown  iron  oxide.  Each  quartz  area  is  furnished  with  a 
very  distinct,  well  rounded,  fragmental  core. 

From  the  section  between  the  Blue  Jachet  and  First  National  mines. 

28.  Cherty  quartzite,  from  a  low  horizon.  Specimen  7602  («lide  2010).  From 
1900  N.,  1150  W.,  Sec.  19,  T.  47  N.,  E.  46  W.,  Michigan. 

Both  in  the  hand  si)ecimen  and  in  the  thin  section  this  rock  is  like  23,  which 
occurs  at  a  similar  horizon. 

29.  Ferruginous  quartzite,  from  a  middle  horizon.  Specimen  7603  (slide  2303). 
From  1980  N.,  900  W.,  Sec.  19,  T.  47  N'.,  E.  46  W.,  Michigan. 

The  specimen  and  thin  section  are  as  in  27. 

From  the  Colby  mine  section. 

30.  Sericitic  and  chloritic  slate  (iiovaculite),  from  a  low  horizon.  Specimen 
9523  (slide  3091).     From  775  N.,  770  W.,  Sec.  16,  T.  47  K,  E.  46  W.,  Michigan. 

This  rock  in  specimen  and  thin  section  resembles  closely  9038  aud  9044  in  18, 
from  the  West  branch  of  the  Montreal  river.     (PI.  xviir,  Fig.  4.) 

31.  Quartzite,  from  a  high  horizon.  Specimen  7591  (slide  2006).  From  1000 
N  ,  548  W.,  Sec.  16,  T.  47  N.,  E.  46  AV.,  Michigan. 

This  rock  in  specimen  and  thin  section  is  lik*  7620  in  24  from  the  Ashland 
mine  section. 

From  the  Tilden  mine  section. 

32.  Quartzite,  from  an  upper  horizon.  Specimen  12524  (slide  5333).  From  1228 
N.  to  1153  N.,  1166  W.,  Sec.  15,  T.  47  N.,  E.  46  W.,  Michigan. 

The  rock  is  a  fine  grained,  massive,  greenish  gray  vitreous  quartzite,  having  a 
conchoidal  fracture. 

The  thin  section  consists  chiefly  of  rather  small  interlocking  quartz  areas,  each 
with  its  fragmental  core.  A  considerable  quantity  of  fragmental  feldspar  is  also 
present.  Outlining  the  original  fragments  of  quartz  are  Alms  of  chlorite  and  ferrite, 
the  greenish  color  of  the  rock  being  produced  by  the  former.  The  chlorite  and  fer- 
rite particles  also  occur  where  the  eidargements  of  the  different  grains  meet  each 

other. 

From  the  Palms  mine  section. 

33.  Conglomerate.  Specimens  12522  (slide  5388),  12893  (slide  5511).  From 
SE.  4  of  XE.  4,  Sec.  15,  T.  47  N.,  E.  46  W.,  Michigan. 


TniO  QKAltTZ-SLATE  MEMUKH.  105 

A  breccia  composed  chietly  of  wliilc,  ^ray,  black,  and  red  chert,  Imt  contaniiiif;- 
also  rrat-iiiiMits  of  other  niiiierals  aii<l  of  j^raidte,  and  rc^stiiif^-  directly  upon  the 
firaiiite. 

The  thin  sections  Jire  made  up  mainly  of  lounded  fragments  of  chert  included 
ill  a  nuitrix  of  the  same  material.  This  chert,  in  both  fragments  and  matrix,  consists 
of  an  exceedingly  ndniitely  crystalline  ipiartz,  with  possibly  some  intermingled 
amori)hous  silica.  There  arc  also  included  in  the  matrix  rounded  i)articles  of  quartz 
and  feldspar,  areas  of  chlorite  ami  of  some  carbonate,  and  complex  fragments  of 

granite. 

From  the  Black  river  section. 

34.  (Jhloritic  and  cherty  slates  and  quartzites,  from  a  nuddle  liorizon.  Speci- 
mens 9400  (slide  ;5078),  from  1445  N.,  1185  W.;  0401  (slide  3;51(»),  from  1460  N.,  1185 
W.;940l»  (slide. '5079),  from  1475  N.,  1185  W.;  9403  (slide  3080),  from  1490  N.,  1185 
W.,  Sec.  13,  T.  47  N.,  R.  40  W.,  Michigan. 

These  specimens  represent  interstratiftcations  of  slaty  and  quartzitic  phases. 
The  slaty  phases  are  tine  grained,  in  places  green,  in  places  brown,  with  variations 
from  one  color  to  the  other.    The  quartzitic  layers  are  gray  and  vitreous. 

In  thin  sections  the  slaty  and  quartzitic  phases  differ  from  each  other  only  in 
coarseness  of  grain  and  proportions  of  the  different  mineral  constituents,  the  chief 
ones  of  which  are  quartz,  feldspar,  and  chlorite,  the  two  former  being  plainly  in  a 
fragmental  condition.  In  3079  the  quartz  fragments  are  greatly  preponderant  and 
widely  enlarged.  In  the  slaty  portions,  mixed  with  the  small  quartz  grains,  are  many 
fragments  of  a  minutely  complex  silica,  which  at  times  is  in  part  probably  even  amor- 
phous or  opaline.  The  chlorite  is  particularly  plentiful  in  the  slaty  phases.  (PI. 
XVIII,  Pig.  3.) 

35.  Sandstones,  fi'om  a  high  horizon.  Specimens  9512  (slide  3088),  9513  (slide 
3089),  from  1510  N.,  1950  W.;  7557rt.  (slide  1980),  from  1570  N.,  1950  W.,  Sec.  13,  T.  47 
N.,  E.  40  W.,  Michigan. 

These  are  fine  grained,  arenaceous  rocks,  having  strangely  marked  sedimentation 
laminae.  Some  specimens  are  light  greenish  gray,  others  are  dark  brown;  while  in 
still  others  are  found  both  colors  arranged  in  the  most  irregular  manner. 

The  thm  sections  are  composed  mainly  of  medium  sized  fragments  of  quartz  and 
feldspar.  The  quartz  fi^agments  are  mostly  from  single  individuals,  and  have  gener- 
ally received  secondaiy  enlargements,  but  some  of  them  are  composed  of  finely 
crystalline  cherty  silica.  The  rather  plentiful  cementing  material  appears  to  be  a  mix- 
ture of  minutely  crystalline  quartz  with  flakes  of  kaolin,  chlorite,  and  brown  iron  oxide, 
the  browner  portions,  as  seen  macroscopically,  differing  from  those  that  are  of  paler 
color  only  in  containing  a  larger  quantity  of  the  iron  oxide. 

From  the  Sunday  lake  section. 

36.  Graywacke-slates  (feldspathic  quartz-slate),  from  a  middle  horizon.  Speci- 
mens 9442  (slide  3071),  from  360  N.,  1600  W.;  9443  (slide  3072),  from  325  N.,  1600  W., 
Sec.  10,  T.  47  N.,  E.  45  W.,  Michigan. 


166  THE  PENOKEE  IRO:i!f-BEARI]SrG  SEEIES. 

These  rocks  resemble  closely  9038  to  90M  of  18.  The  sheen  of  hydro-mica  is  less 
marked  than  in  those  rocks. 

The  fragmental  particles  of  quartz  and  feldspar,  of  which  these  sections  are 
mainly  composed,  are  more  rounded  and  larger  than  in  2771  and  2774  of  18,  to  which 
in  all  other  respects  these  rocks  are  closely  similar.  The  enlargements  of  the  quartz 
fragments  are  broad  and  well  marked.  In  3071  chlorite  appears  to  be  the  chief 
interstitial  material ;  in  3072  the  chlorite  is  mingled  with  much  kaolin  and  minutely 
crystalline  silica.     (PI.  xviii,  Figs.  1  and  2.) 

37.  Vitreous  quartzites,  from  near  the  top  of  the  Quartz-slate  member.  Speci 
mens  9440  (slide  3069),  from  450  N.,  1625  W.;  9441  (slide  3070),  ti^om  400  N.,  1625  W., 
Sec.  10,  T.  47  N.,  E.  45  W.,  Michigan. 

The  rock  9440  is  compact,  gray,  and  coarse  grained,  while  9441  is  fine  grained 
and  pink.    Both  break  with  a  conchoidal  fracture. 

The  thin  section  3069  furnishes  a  beautiful  illustration  of  the  true  nature  of  the 
ordinary  vitreous  quartzite;  the  large  and  deeply  interlocking  areas  of  quartz  which 
mostly  compose  it  having  each  its  plainly  outlined  fragmental  core.  These  cores  are 
outlined  particularly  by  the  broad  films  of  the  chlorite  and  limonite  with  which  they 
are  furnished.  Slide  3070  differs  from  3069  only  in  being  much  finer  grained  and  in 
having  mingled  with  tlie  quartz  a  good  deal  of  fragmental  feldspar. 

From  the  section  in  the  adjoining  portions  of  Sees.  10,  15,  and  14,  T.  47  N.,  E.  45 

W.,  Michigan. 

38.  Sideritic  and  chloritic  schists,  from  the  bottom  of  the  Quartz-slate  member, 
immediately  beneath  the  succeeding  number.  Specimens  12545  (slide  5337),  12546 
(slide  5338).     From  1975  N.,  800  W.,  Sec.  15,  T.  47  N.,  E.  45  W.,  Michigan. 

The  rocks  are  greenish  gray,  fine  grained,  ajid  have  a  well  developed  fibrous 
structure.  They  show  lamination  only  by  somewhat  difiicult  and  irregular  parallel 
fracture.     Contained  in  the  fine  material  are  numerous  larger  grains  of  feldspar. 

While  the  thin  sections  have  a  strongly  crystalline  appearance,  they  show  that 
the  rocks  are  unmistakably  fragmental.  Fragments  of  quartz  and  feldspar  of  medium 
size  are  contained  in  a  fine  grained  matrix  of  a  very  complex  character.  The  clastic 
quartz  and  feldspar  are  in  part  well  rounded,  but  most  of  the  grains  are  quite  angular, 
indicating  the  probability  that  they  are  near  their  original  source.  Some  of  the  quartz- 
grains  are  distinctly  enlarged.  Many  of  the  feldspar  fragments  are  much  altered, 
the  resultant  products  being  chlorite,  kaolin,  sericite,  and  probably  quartz.  A  few 
rounded  complex  fragments  from  a  basic  eruptive  are  present.  Tlie  matrix  of  the 
sections  is  a  finely  comijlex  mass  of  quartz,  feldspar,  chlorite,  sericite,  kaolin,  and  a 
little  ferrite. 


THE  (,)rAi;T/-SLATK  MKMBKH.  167 

39.  Quartzitcs,  Iroiii  the  bottom  of  tin'  (iuiirtz-slatc  inombcr,  iimncdiatdy  bcncatli 
40.  SpwiiiuMis  !>444  (sli.lc  .'U;!;?),  7.507  (slido  1!>52).  From  1985  N.,  750  W.,  Sec.  15,  T. 
47  N.,  K.  45  W.,  Micliifian. 

Tlic  siK^ciiiit'iis  and  tliiii  sections  rcscnihlc,  (-loscly  those  oC  24. 

40.  (Jliertcoiif>loiiu'i'ate,  IVoni  near  the  base  of  the  (Jiiartz-shite  member,  aucl 
immediately  beneath  7505  (41).  Specimens  0440  (slide  31;M),  7.500  (sli(h^  10.51 ).  From 
1908  N.,  750  VV.,  Sec.  15,  T.  47  N.,  K.  45  \V.,  Michigan. 

The  matrix  of  this  rock  is  fine  grained,  compact,  and  of  a  gray  color.  In  this 
matrix  ai'c  very  numerous  rounded  pebbles  of  chert,  which  are  sometimes  several 
inches  in  diameter. 

lu  the  thin  sections  the  matrix  of  this  conglomerate  is  seen  to  be  composed  of 
materials  of  two  degrees  of  fineness.  The  finer  appears,  as  is  general  with  the  slates 
of  this  member,  to  be  composed  of  minutely  crystalline,  and,  at  times,  even  an  amor- 
phous silica,  with  which  are  mingled  some  kaolin  and  ferrite.  The  coarser  i)ortion  of 
the  matrix  is  distinctly  fragmental  and  is  made  up  wholly  of  quartz  pieces,  nearly  all 
t)f  which  show  the  usual  secondary  enlargements.  The  pebbles  of  the  conglomerate 
are  wholly  of  a  chert  identical  with  that  characteristic  of  the  limestone  member,  as 
previously  described.  They  are  seen  to  be  made  up  of  an  intimate  mixture  of  a 
minutely  crystalline  and  an  amorphous  or  opaline  silica. 

41.  Sandstone,  from  a  very  low  horizon.  Specimen  7.505  (slide  1948)  From  0 
N.,  750  W.,  Sec.  10,  T.  47  N.,  R.  45  W.,  Michigan. 

This  rock  in  specimen  and  thin  section  resembles  35  from  the  Black  river  section. 

42.  Argillaceous  slates  or  shales,  from  a  low  horizon.  Specimens  7502  (slide 
1944),  from  2  N.,  177  W.;  7503  (.slide  1945),  from  10  N.,  177  W.,  Sec.  10,  T.  47  N.,  R. 
45. W.,  Michigan. 

Aphanitic,  highly  laminated  slates  or  shales  having  a  strong  clayey  odor,  and 
colors  varying  from  reddish  brown  to  greenish  gray. 

The  thin  sections  differ  from  those  of  43  in  that  the  fragmental  particles  of  quartz 
and  feldspar  are  now  insignificant  in  quantity,  the  clayey  matrix  assiiming  a  relatively 
important  role. 

43.  Argillaceous  slates  or  shales.  Specimens  7504  (slide  194C),  from  1985  N.,  450 
W.;  7505a  (slide  1949),  from  0  N.,  750  W.,  Sees.  10  and  15,  T.  47  N.,  E.  45  W.,  Michigan. 

Compact  clayey  slates  or  shales,  not  always  showing  the  hues  of  sedimentation  ; 
in  color  varying  from  greenish  gray  to  reddish  brown,  some  specimens  presenting  both 

colors. 

The  thin  sections  show  small  fragments  of  quartz  a^d  feldspar,  the  former  the 
more  abundant  and  commonly  provided  with  secondary  enlargements,  imbedded  in  a 
groundmass  composed  of  ferrite,  chlorite,  kaolin,  and  minutely  crystalline  silica.     (PI. 

XIX,  Fig.  4.) 

44.  Sericitic  graywackes,  from  a  low  horizon.  Specimens  9432  (slide  3130),  from 
1925  N.,  1915  W.;  9435  (shde  3132),  from  1935  N.,  1940  W.;  9436  (slide  3068),  from 


168  THE  PENOKEE  IRON-BEARINa  SERIES. 

1935  N.,  1940  W.;  7510  (slide  1966),  from  1925  K,  1890  W.,  Sec.  14,  T.  47  N.,  R.  45  W., 
Michigan. 

Ihese  are  fine  grained  slaty  rocks,  of  a  uniform  texture,  and  greenisli  to  pinkish 
gray  colors  upon  the  cleavage  surfaces.     Except  in  9432  the  sheen  of  mica  is  seen. 

The  thin  sections  of  these  rocks,  with  the  excei)tion  of  3130,  which  is  peculiar  in 
containing  a  relatively  small  quantity  of  sericite,  are  very  close  to  2771  and  2774  in  18. 

45.  Quartz-slate  and  cUoi'itic  slate.  Specimens  9453  (slide  3074),  9452  (slide 
3073).     From  200  N.,  800  W.,  Sec.  10,  T.  47  N.,  R.  45  W.,  Michigan. 

These  slates  consist  of  alternating  bands  of  coarse  quartzitic  and  aphanitic 
greenish  material. 

•  The  aphanitic  portions  of  these  rocks,  as  seen  niacroscopically,  show  in  the  thin 
section  an  intimate  mixture  of  minute  particles  of  quartz,  feldspar,  chlorite,  kaolin, 
and  biotite.  The  coarser  seams  show  a  groundmass  of  the  same  natui'e,  in  which  are 
carried  an  abundance  of  rounded  fragments  of  quartz,  with  which  are  associated 
numerous  fragments  composed  of  an  exceedingly  minutely  crystalline  silica,  mingled 
with  flakes  of  kaolin,  or  of  a  micaceous  material.  It  seems  not  improbable  that  each 
of  these  fragments  may  be  due  to  the  decomposition  of  a  single  feldspar. 

46.  Quartzite  and  slate,  from  an  upper  middle  horizon.  Specimens  7495  (slide 
1940),  from  .300  N.,  0  W.;  7496  (slide  1941),  from  140  N.,  0  W.,  Sec.  10,  T.  47  N.,  R.  45 
W.,  Mic^higan. 

These  specimens  represent  an  interstratiflcation  of  greenish  gray,  semivitreous, 
medium  grained  quartzite,  and  argillaceous  slate. 

The  thin  sections  of  these  rocks  are  from  the  more  quartzitic  portions  of  the 
specimens.    They  closely  resemble  those  of  37. 

47.  Chloritic  and  sericitic  quartzite,  from  a  high  horizon.  Specimen  94.54  (slide 
3075),  from  370  N.,  800  W.,  Sec.  10,  T.  47  N.,  R.  45  W.,  Michigan. 

A  fine  grained,  greenish  gray,  massive  quartzite,  with  a  conchoidal  fracture. 

The  thin  section  shows  large  fragments  of  quartz  and  feldspar  buried  in  a 
groundmass  of  minutely  crystalline  silica,  mingled  with  cldorite  and  sericite  flakes. 
The  feldspar  fragments  are  nmch  altered,  the  resulting  products  being  chlorite,  seri- 
cite, and  quartz.  The  manifest  production  of  these  thi'ce  secondary  minerals  from 
the  lai'ger  fragments  of  feldspar  makes  exceedingly  probable  a^  similar  origin  for  the 
same  minerals  in  the  matrix. 

48.  Quartzite,  from  immediately  beneath  the  Iron-bearing  member  at  the  top  of 
the  Quartz-slate  member.  Specimen  7513  (slide  1954),  from  420  N.,  873  W.,  Sec.  10, 
T.  47  "N.,  R.  45  W.,  Michigan. 

A  medium  grained,  massive,  semivitreous  quartzite. 
The  thin  section  of  this  rock  closely  resembles  3069  in  37. 


TlIK  CiUAUTZ-HLATE  MKMIJER.  109 

From  tht;  sectlou  iti  norlltrm  part  of  Sec.  If,  T.  47  N.,  R.  4.'>  W.,  Michigan. 

W.  Obert-conglouio.riite,  IVoiii  the  bottom  of  the  Quartz-slato  iiicinbei-.  Speci- 
iiit'iis  !tns  (slide  ;5(r)i)),  ittlO  (slides  ;ill>(J,  ;iiul  3127),  Irom  1775  N.,  1075  VV.,  Sec.  14,  T. 
47  N.,  R.  45  \V.,  Mi<^iiiKiiii. 

The  matrix  of  this  eouf'loinerate  is  tine  gi'ained  aiid  of  a  dark  green  eolov.  In 
the  matrix  are  buried  very  plentiful  rouuded.  IVagments  of  translucent  oijalinti  cht^rt, 
which  vary  in  size  from  three-fourtlis  of  an  inch  to  two  inches  in  diameter. 

The  matrix  of  this  conglomerate  is  seen  in  the  thin  section  to  be  composed  of 
rather  small  fragments  of  (puirtz  and  feldspar,  the  former  pi'edominating,  mingled, 
with  a  considerable  proportion  of  interstitial  cherty  and  amorpiious  silica,  along  with 
some  cldorite,  brown  iron  ore,  and  kaolin.  Large  particles  of  mica,  apparently  frag- 
meutal  in  nature,  are  also  seen.  The  pebbles  are  wholly  composed  of  a  ehcu't  identical 
with  that  of  the  limestone  member  below,  being  in  part  a  minutely  crystalline  and  in 
part  an  amorphous  silica.  The  slides  closely  resemble  3134  and  1951  in  40.  (PL 
XVII,  Fig.  3.) 

50.  Chert-breccia  or  conglomerate,  from  the  bottom  of  the  Quartz-slate  member. 
Specimen  94L'0  (slide  3000),  from  1775  N.,  1225  W.,  Sec.  14,  T-  47  N.,  R.  45  W.,  Mich- 
igan. 

This  rock  differs  from  49  in  that  its  matrix  aijpears  to  be  a  chloritic  and  ferru- 
ginous chert,  while  in  the  latter  the  matrix  is  entirely  fragmental. 

The  pebbles  and  matrix  in  thin  section  differ  from  each  other  only  in  pvirity; 
the  pebbles  being  composed  of  a  semicrystalline  cherty  silica,  while  the  matrix  is 
composed  of  the  same  silica  mingled  with  much  ferrite  and  chlorite.  A  few  large 
fragments  of  quartz  are  seen. 

51.  Quartzite,  from  near  the  bottom  of  the  Quartz-slate  member.  Specimen  9421 
(slide  3061),  from  1775  N.,  1200  W.,  Sec.  14,  T.  47  N.,  R.  45  W.,  Michigan. 

A  dark  reddish,  semi  vitreous  quartzite. 
,     In  the  thin  section  well  rounded  quartz  fragments  with  small  enlargements  are 
buried  in  a  fine  interstitial  material  composed  of  minutely  crystalline  and  amorphous 
silica,  kaolin,  chlorite,  and  toown  iron  oxide. 

From  the  exposures  on  the  Peninsular  Mining  Gompany^s  property. 

52.  Ferruginous  quartzite.  Specimen  12789  (slide  547G),  from  1910  N.,  1040  W., 
Sec.  13,  T.  47  N.,  E.  45  W.,  Michigan. 

A  medium  grained,  vitreous  quartzite,  stained  a  dark  brown  color  by  iron  oxide. 

About  .one-half  of  the  thin  section  is  comj)osed  of  widely  enlarged  fragments  of 
quartz;  the  main  part  of  the  remainder  of  the  section  consists  of  minutely  crystalline 
silica,  mingled  with  chlorite  and  ferrite  particles,  with  a  few  fragments  of  orthoclas^e, 
microcUne,  aud  plagioclase. 


170  THE  PENOKEB  IRON-BEARING  SERIES. 

From  the  exposures  in  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 

53.  Feldspathic  quartzites  and  quartz- slates,  from  near  the  bottom  of  the  Quartz- 
slate  member.  Specimens  9410  (slides  3125  and  3271),  9411  (slide  3052),  from  525  N., 
1925  W.;  9389  (slide  3043),  from  350  N.,  1550  W.;  9390  (slide  3044),  from  350  N.,  1550 
W.,  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 

Some  of  these  specimens  are  slaty,  while  others  are  more  quartzitic  and  vitreous. 
The  color  varies  from  gray  to  black,  being  in  places  mottled  with  red  spots. 

The  thin  sections  are  made  upof  fragments  of  quartz,  feldspar,  chert,  and  jasper. 
The  quartz  fragments  are  usually  widely  enlarged ;  the  cherty  and  jaspery  pieces 
seem  in  the  main  to  represent  the  chert  of  the  limestone  belt  immediately  below. 
There  are,  however,  a  good  many  fragments  which  appear  to  be  intimate  mixtures  of 
chlorite,  minutely  crystalline  quartz,  ferrite,  and  a  koalin-like  mineral.  Tliese  may 
possibly  be  altered  feldspars.  The  sparse  matrix  is  very  heavily  stained  with  brown 
iron  oxide. 

54. .  Chloritic  slate,  from  near  the  base  of  the  Quartz-slate  member.  SiDCcimen 
9409  (slide  3051),  from  525  N.,  1925  W.,  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 

An  aphanitic  light  green  slate  or  shale,  cleavable  into  tliin  plates  parallel  to  the 
lamination. 

The  thin  section  shows  an  intimate  mixtui'e  of  an  exceedingly  minutely  crystal- 
line quartz,  with  flakes  of  pale  green  chlorite. 

55.  Clay-shales  or  clay-slates,  from  near  the  base  of  the  Quartz-slate  member. 
Specimens  9391  (slide  3045),  from  350  N.,  1550  W.;  9412  (sbde  3053),  from  525  N., 
1925  W.,  Sec.  17,  T.  47  N.,  Rj  44  W.,  Michigan. 

These  are  flue  grained  and  finely  laminated  slates  or  shales  ;  9412  is  gray,  and 
9391  reddish  purple.     The  latter  shows  plentiful  flakes  of  sericite. 
The  thin  sections  of  these  rocks  are  closely  like  those  of  42. 

56.  Graywackes  and  chloritic  slates,  from  a  low  horizon.  Specimens  12067  (slide 
5391),  12608  (slide  5392),  from  15  N.,  05  W.;  12669  (slide  5393),  from  78  N.,  42  W.; 
12670  (slide  5394),  from  100  N.,  105  W.,  Sec.  17,  T.  47  N.,  R.  44  W.,  Micliigan. 

These  specimens  range  ft'om  slaty  to  more  massive  kinds,  varying  in  color  through 
dark  green,  reddish,  dark  brown,  gray,  and  greenish  shades.    All  are  aphanitic. 

The  thin  sections  show  that  they  are  of  the  typical  slate  of  the  Quartz-slate 
member,  being  composed  of  fragments  of  quartz  and  feldspar,  buried  in  the  usual 
argillaceous  matrix,  in  which  chlorite,  kaolin,  sericite,  and  a  minutely  crystalline 
quartz  are  recognizable. 

57.  Feldspathic  quartzite  or  graywacke,  from  a  middle  horizon.  Specimen  9394 
(slide  3122).    From  .470  N.,  975  W.,  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 

A  coarse  grained,  greenish  gray,  vitreous  quartzite. 

The  thin  section  of  this  rock  is  closely  similar  to  2016  in  24. 

58.  Chloritic  slate,  from  a  high  horizon.  Specimen  12682  (slide  5404).  From  477 
N.,  790  W.,  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 


THE  (^UAUTZ-SLATE  INFEMBEK.  171 

An  olive  }i:reeu  rock  of  nearly  aphanitit!  toxturo,  and  slaty  structure. 

TIk^  tiiin  section  shows  a  ijredoniiuatiiif;'  line  i^raiiuxl  background  conii)oscd  of 
flakes  of  clilorit(^,  sericite,  and  kaolin,  witli  sonic  ininutc^ly  crystalline  (juurtz,  tliron<'ii 
wliicli  aie  scattei'ed  rather  plentifully  small  fragments  of  quartz  and  rcldsjtar.  The 
section  is  cut  across  the  lamination  of  the  slate,  and  shows  flakes  ofcldoritc,  sericite, 
etc.,  arranged  with  their  longer  axes  in  a comnion  direction. 

.">!>.  (inart/.iles,  from  a  high  horizon.  Spc^cimens  lli(>SO  (slide  5402),  ll'tilSl  (slide 
54();$).     From  477  N.,  795  to  825  W.,  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 

These  si)ecimens,  taken  from  the  same  pit,  ditt'er  in  character;  120S()  is  a  massive, 
medium  grained,  ordinary  quart/Jte;  12(i.Sl,  a  massive,  strongly  magiietitic  (juartzite. 

The  thin  section  5402  is  that  of  a  typical  (juartzite,  wliose  induration  and 
vitreims  appearance  are  due  to  the  wide  enlargement  of  each  grain.  Broad  films  of 
chlorite  and  ferrite  separate  cores  and  enlargements  and  occur  again  along  the  con- 
tacts of  the  eidargements  themselves.  There  are  some  irregular  areas  com])osed  of 
hematite,  magnetite,  and  a  minutely  crystalline  quartz.  The  quartz  areas  are  all 
arranged  with  their  longer  axes  in  a  common  direction  parallel  to  the  structure,  which 
perhaps  indicates  subjection  to  pressure.  The  thin  section  5403  differs  from  5402  iu 
tliat  the  fragmental  ([uartz  grains  compose  only  about  half  of  the  mass  of  the  rock, 
there  being  a  very  plentiful  interstitial  material  consisting  mainly  of  magnetite  and 
containing  also  a  considerable  quantity  of  hematite,  chlorite,  and  minutely  crystalline 
quartz.  The  magnetite  in  this  matrix  is  for  the  most  part  in  very  regular  outlined 
crystals.  These  crystals  occur  also  within  tlie  enlargements  of  the  (piartz  fragments, 
and  occasionally  appear  to  be  even  within  the  cores  themselves,  although  this  appear- 
ance may  be  due  to  the  positions  at  which  these  grains  are  cut.  They  also  occur 
somewhat  jdentifully  along  the  bounding  line  between  fragmental  cores  and  enlarge- 
ments. That  portion  of  the  magnetite  which  occurs  within  the  cores  themselves 
appears  to  be  very  distinctly  more  plentiful  in  the  neighborhood  of  their  outlines. 
As  to  this  singular  occurrence,  see  the  description  of  7. 

Contacts  ivith  the  Cherty  limestone  member. — As  a  rule  the  contact  of 
the  Quartz-slate  member  with  the  more  southerly  rocks  is  concealed.  At 
several  points,  liowever,  it  may  be  seen,  either  in  contact  with  or  very  close 
indeed  to  exposures  of  the  white  chert  or  of  the  limestone  itself  of  the 
underlying  formation,  and  in  other  places  again  with  the  granite,  gneiss,  or 
schist  of  the  Southern  Complex.  The  contacts  with  white  chert  are  at 
Penokee  gap,  Sec.  14,  T.  44  N.,  R.  3  W.,  Wisconsin ;  in  the  NW.  |  Sec.  1 6, 
T.  44  N.,  R.  2  W.,  Wisconsin ;  in  the  northern  part  of  Sec.  14,  T.  47  N.,  R. 
45  W.,  Michigan;  and  in  the  SW.  \  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 
It  has  already  been  intimated  that  in  these  places  there  are  found  in  the 


172 


THE  PENOKEE  IRON-BEAEING  SERIES. 


lower  layers  of  the  quartz-slate  numerous  fragments  of  the  chert  which 
immediately  underlies  it,  and  sometimes  the  rock  becom.es  a  recomposed 
quartz  rock,  disting-uishable  from  the  underlying  formation  onl}^  by  the 
presence  of  a  small  portion  of  plainly  recognizable  fragmental  material. 
These  peculiar  occurrences  are  taken  to  indicate  that  between  the  two 
formations  was  a  more  or  less  extended  erosion  interval.  The  chert 
fragments  in  these  places  occur  both  in  basal  conglomerates  and  in  thin 
bands  interstratified  with  a  nonconglomeratic  slate.  As  the  contact  is 
receded  from  the  chert  fragments  become  very  soon  of  smaller  size,  although 
pieces  of  the  same  chert  are  occasionally  microscopically  recognizable  even 
in  the  higher  liorizons  of  tlie  fomtiation.  . 

Contacts  with  the  Southern  Complex. — At  Potato  river  and  West  branch 
of  Montreal  river  the  Quartz-slate  m'^ember  is  seen  in  contact  with 
greenish  schists  on  the  south.    The  Potato  river  junction  is  illustrated  by  Fig. 


^^     %FerruffmoiLsC?iert.  Xr"^ 


Fig.  5. — M.ip  <if  exposures  at  Potato  i-iver.  'Scale;  1  iTich=2C4  feet. 

5,  which  shows  the  position  of  the  various  exposures.     The  river  liere  has 
a  bold  bank  some  75  feet  in  height,  along  the  face  of  which  the  contact 


THi:  (JUAUTZ-HLATK   MEMBER.  ]  73 

between  tlio  slate  and  the  underlyin<r  sehists  is  linely  (sxposed.  Tlic  l)t\st 
view  of  the  contact  is  that  (il)t.iiiie(l  at  the  foot  of  the  l)aiik,  where  there  is 
a  perpendicuh^r  clitr  nf  l)ar('  rock.  The  (h-tails  of  tlic  junction  sliown  on 
this  cUff  are  represented  in  Fiys.  (i  and  7.     The  more  southerly  rock  is  a 


/•,; 


^. 


,^7Tr*-^  I  i-ina 


lomeratB. 


) 


! 


I 


Fia.  0 — Junction  of  quartz-slate  and  green  schists  at  Potato  river. 
Scaie :  1  iucli=20  feot. 

greenish  chloritic  schist,  with  a  fibrous  or  parallel  structure  in  a  direction 
almost  exactly  at  right  angles  to  the  junction  line.  The  thin  sections  show 
it  to  be  probable  that  this  rock  was  once  some  sort  of  a  porphyritic  eruptive. 
Whatever  its  original  nature,  however,  it  has  certainly  .been  most  intensely 
altered,  the  minerals  having  been  rearranged  into  new  combinations  and  a 
parallel  structure  superinduced.  Moreover,  this  alteration  was  all  carried 
out  previous  to  the  dei)osition  against  it  of  the  quartz-slate,  the  lowest  layers 
of  which  are  crowded  with  fragments  from  the  schist  of  all  sizes,  from  a  fine 
detritus  to  blocks  several  feet  hi  diameter.  This  exposure,  then,  shows  one  of 
the  handsomest  instances  of  unconformity  that  we  have  ever  seen,  the  worn 
upper  surface  of  the  scliist  being  traceable  and  having  fitted  into  it  a  finer 
detrital  material  belonging  to  the  overlying  fragmental  slate.  The  accom- 
panying sketches  of  this  contact  were  drawn  on  the  ground  and  represent 
actual  occurrences,  the  sizes  of  all  the  larger  fragments  of  the  conglomerate 
band  being  drawn  in  Fig.  7  to  scale,  while  the  structures  of  the  under- 
lying schists  and  overlying  slates  are  exactly  as  represented.  The  junction 
at  the  West  branch  of  the  Montreal  is  in  all  respects  similar  to  that  on  the 
Potato,  except  that  the  exposure  of  the  contact  is  much  smaller,  and  there- 
fore less  satisfactory.  The  similarity  of  the  rocks  at  the  two  places  is  such 
as  to  render  it  extremely  probable  that  the  same  contact  extends  for  all  the 
distance  between  the  two  streams. 


174 


THE  PENOKEE  IROISr-BEARING  SERIES. 


Directly  south  of  the  Pahns  imue  (Sec.  14,  T.  47  N,  R  l)  W.,  Mich- 
igan), upon  a  raih-oad  spur,  and  about  one-fourth  of  a  mile  to  the  west  along 
the  same  track,  are  seen  contacts  of  the  quartz-slate  and  the, granite.  The 
basal  conglomerate  is  a  mere  facing.  The  fragmental  material  also  pene- 
trates the  granite  in  cracks  which  existed  at  the  time  of  tlie  deposition  of 
the  quartz-slate.  Some  of  these  seams  of  conglomerate  extend  from  a  foot 
to  several  feet  into  the  granite.  The  pebbles  of  the  conglomerate  include 
jasper,  chert,  and  quartz,  as  well  as  granite.  The  chert  was  probably 
derived  from  the  Cherty  limestone  formation,  while  the  jasper,  perhaps, 
came  from  an  iron-bearing  formation  belonging  with  the  Cherty  hmestone 
series,  but  now  wholly  removed  by  erosion. 


Congljom^rttt&. 


Green,  J'WroiMi 


Fig.  7.— Large-scale  drawing  of  junction  of  quartz-slate  and  green  schists  at  Potato  river. 

Scale:  1  inch^ofuct. 

A  contact  between  the  Quartz-slate  and  the  granite,  very  similar  to 
that  at  the  Palms,  occurs  just  south  of  the  Aurora  mine. 


Till-:  (,>i:ai.tz-slath  mi:mi!i:u.  175 

Clian/fr  Id  the  fniii-lirdrinf/  inniihcr. — 'I'lie  c-oiitiu^t  l)etweeu  tlio  Quartz- 
slate  ami  file  Inni-luiariiif^-  iiieiuber  wliicli  imiiicdiately  overlies  it  is  exposed 
in  scores  of  places,  as  a  result  of  mining-  operations.  Nowhere  where 
it  lias  been  seen,  however,  does  tliis  contact  sugf^est  anything  but  the  most 
abru})t  cliange  from  one  formation  to  Ihe  other.  In  places  the  upper  part 
of  the  (piartzite  appears  to  be  no  more  tlian  a  coarse  sand,  and  occasionally 
blocks  of  it  are  contained  in  the  basement  layers  of  the  iron  formation. 
This  would  seem  to  indicate  that  here  and  there  the  quartzite  was  some- 
what broken  before  the  beginning  of  the  deposition  of  the  overlying  mem- 
ber or  else  by  dynamic  movements.  Also,  the  upper  part  of  the  quartzite 
is  often  heavily  stained  with  iron  oxide  which  has  been  carried  down  along 
the  cracks  by  leaching  action.  Nevertheless,  the  change  from  one  forma- 
tion to  the  other  is  astonishingly  abrupt,  it  often  being  possible  to  locate  to 
a  fraction  of  an  inch  the  plane  between  the  two  formations.  Upon  one  side 
of  this  plane  is  the  coarse  fragmental  quartzite;  upon  the  other  the  nonfrag- 
meutal  varied  rocks  of  the  Iron-bearing  member. 

Prominent  exposures. — As  stated  at  the  beginuing  of  this  chapter,  the 
Quartz-slate  is  one  of  the  best  and  most  continuously  exposed  members  of 
the  entire  Penokee  sjeries.  This  will  be  realized  from  an  inspection  of  the 
detailed  maps  herewith  (Pis.  v  to  xiii),  upon  which,  however,  are  placed 
only  the  more  accurately  located  exposures.  Further  detailed  work  and 
measurements  would  undoubtedly  enable  us  to  locate  about  as  many  more. 
In  the  following  notes  we  refer  only  to  those  exposures  which  are  particu- 
larly prominent,  eitlier  on  account  of  their  size  or  because  they  show  some 
noteworthy  peculiarity.  Beginning  at  the  west,  the  first  exposures  worthy 
of  mention  are. the  large  ones  running  along  the  north  side  of  the  Marengo 
river  in  the  extreme  southwestern  portion  of  Sec.  14,  and  northwestern 
portion  of  Sec.  23,  T.  44  N.,  R.  5  W.,  Wisconsin.  The  river  runs  nearly 
along  the  strike  of  the  slate,  and  also  apparently  along  its  contact  with  the 
more  southern  granite,  which  shows  in  bold  exposures  all  along  the  south 
side  of  the  river.  The  slate  is  one  of  the  biotitic  and  chloritic  varieties.  In 
the  extreme  northeastern  part  of  Sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin, 
several  exposures  of  the  quartzite,  which  forms  the  uppermost  horizon  of 
the  quartz-slate,  are  to  be  seen.     One  of  these  lies  on  the  eastern  line  of 


176  THE  PENOKEE  IRON-BEARING  SERIES. 

the  section  at  about  400  steps  south  of  the  northeast  corner.  Immediately 
-  to  the  north  of  this  quartzite,  but  not  directly  in  contact  with  it,  are  expos- 
ures of  the  Iron-bearing  member,  and  immediately  to  the  south  larger  ones 
of  a  biotitic  and  cldoritic  slate  of  the  Quartz-slate.  Still  larger  exposures  of 
quartzite  and  biotitic  quartz-slate  are  shown  along  the  crest  of  the  Penokee 
range  in  its  course  across  Sec.  17,  T.  44  N.,  R.  3  W.,  Wisconsin.  The  east- 
ernmost one  of  these  exposures  shows  a  contact  on  the  north  with  a  mag- 
netitic  schist.  This  contact  lies  550  steps  south  and  400  steps  west  of  the 
northeast  corner  of  the  section.  Similar  large  exposures  show  in  the 
northern  part  of  Sees.  16  and  16,  but  much  more  striking  and  complete 
exposures  are  those  met  with  in  the  vicinity  of  Penokee  gap,  where  Bad 
river  passes  the  Penokee  range  from  south  to  north.  The  position  and 
extent  of  these  exposures  will  be  best  understood  by  an  inspection  of  PI. 
xxxvi.  The  m6st  striking  of  them  is  that  which  forms  a  bold  hill  at  the 
southern  end  of  Penokee  gap  itself  Here  the  entire  width  of  the  Quartz- 
slate  is  seen,  from  its  contact  with  the  chert  of  the  underlying  limestone 
formation  on  the  south,  northward  to  its  contact  with  the  iron  formation. 
The  latter  contact  is  between  the  usual  uppermost  vitreous  quartzite  of  the 
Quartz-slate  and  an  actinolite-magnetite-slate  of  the  Iron-bearing  member. 
Contacts  are  also  seen  at  two  points  respectively  300  and  400  paces  farther 
west.  Except  in  the  uppermost  horizons  where  we  have  the  usual  viti'eous 
and  relatively  massive  quartzite,  the  greater  portion  of  the  rock  hei'e  exposed 
is  quite  thinly  slaty,  seams  of  light  and  dark  gi'ay  and  light  and  dark 
greenish  slate  alternating  rapidly  with  one  another.  On  the  whole  these 
slates  are  more  generally  of  the  chloritic  and  biotitic  phases  above  described. 
Proceeding  farther  eastward  the  next  very  noteworthy  exposures  are  those 
at  mount  Whittlesey  on  the  south  side  of  the  Penokee  range.  This  bold 
south-facing  cliff  lies  at  the  corner  of  Sees.  8,  9,  16,  and  17,  T.  44  N.,  R.  2 
W.,  Wisconsin.  The  northward  angle  of  inclination  of  the  slate  here  varies 
from  56^  to  65°,  the  edges  of  the  inclined  layers  projecting  on  the  face  of 
the  cliff  in  such  a  manner  as  to  make  an  exceedingly  jagged  and  irregular 
precipice.  The  greater  part  of  the  slate  here  belongs  to  the  chloritic  and 
biotitic  quartz-slate  phase.  Quite  a  large  portion  of  the  thickness  of  the 
formation  is  exposed  at  the  gorges  of  Carrie's  creek,  NE.  |  Sec.  11,  T.  44  N., 


THE  (iUAKTZ-SLATK  MEMBER. 


177 


K.  2  W.,  Wisf-oiisiu:  iiiid  ajriuu  in  the  X  Iv  '  oftlic  SK.  ',  Sec.  1,  T.  44  X., 
R.  2  W.,  ^^'isl•()Ilsi^,  \\liure  are  to  Ix'  iiidcil  aiiiono-  the  iiidic  usual  phast^s  of 
the  slate  a  iimiil)ci-  of  laxcrs  of  a  pink  to  ln'ick  i'c(|  and  luatl  j;'i';iy  shale,  a 
[)liase  wliirli  lias  tluis  far  not  been  noted  to  the  west  of  this  point,  though 
oerurring  somewhat  fretiueutly  further  to  tlu^  i^ast.  'I'hi^  greater  part  of  the 
thickness  of  the  Quart/,-slate  formation  is  exposed  again  at  tlie  gorge  of 
Tylers  fork,  SK.  \  of  tiie  XE.  \  Sec.  33,  T.  45  N.,  R.  1  W.,  Wisconsin,  the 
contact  with  the  overh'ing  iron  formation  Ijeing  finely  exposed  on  the  Avest 
bank  of  the  river,  as  indicated  on  the  foUowhig  sketch  map  (i^'ig.  8).     The 


"■f' 


Itonbeannrfinen  '   i  Quat Iz  date  Tnember 

Fio.  8 Map  and  sectiou  ahowui!;  iiusituin  of  rock  exiiosiircs  at    lylers   loik.     (Frmii  Plate  MV,  toI.  hi,  Wiaconsin 

Geologi<-al  Survey.)     Scale:  1  in.  =^  310  feet. 

uppermost  vitreous  quartzite  shows  here  very  plainly,  while  the  body  of 
the  formation  is  distinctly  nearer  in  character  to  the  feldspathic  quartz- 
slate  phase  above  described,  although  there  are  interleaved  with  the  pre- 


MON  xis- 


-12 


178 


THE  PENOKEE  IEO]S"-BEARmG  SERIES. 


dominating  variety  some  of  the  biotitic  and  ^cldoritic  slates,  novaculite, 
and  a  little  red  clay  shale.  At  the  gorge  of  the  Potato  river,  in  the  NE.  \  of 
the  SE.  \  Sec.  19,  T.  45  N.,  R.  1  E.,  Wisconsin,  the  exposures  are  again  large, 
showing  the  entire  width  of  tlie  quartz-slate.  The  positions  of  these  ex- 
posures afe  shown  in  Fig.  5.  Tlie  contact  on  the  south  between  this  slate 
and  the  underlying  greenish  schist  hasf  already  been  described.  The  usual 
vitreous  quartzite  is  found  as  the  uppermost  horizon  and  the  body  of  the 
formation  is  made  up  of  thinly  laminated  feldspathic  (juartz-slates.  The 
exposures  at  the   gorge  of  the  West  branch  of  the  Montreal  (Fig.  9)  are 


Quarti 


Southern.  CompUx.. 


Fig.  9.— Map  of  exposures  at  West  branch  of  Montreal  river. 

quite  similar  to  those  on  the  Potato,  showing  the  lower  junction  but  not  the 
upper.  From  here  eastward  to  the  vicinity  of  Sunday  lake  the  natural 
exposures  of  the  Quartz-slate  member  are  much  rarer -and  smaller  than 
those  already  described.  However,  in  a  very  large  number  of  places  the 
mining  or  exploring  operations  of  the  district  have  uncovered  this  forma- 
tion for  small  distances.  Naturally  the  most  commonly  uncovered  portion 
of  the  formation  is  its  uppermost  quartzite  member,  which  is  ordinarily 


THE  QUARTZ  SLATE  MEMBER.  179 

the  foot-wall  of  the  mines  of  tlu^  (Jo^^ciljic  district;  while  tlie  lower  portions 
of  the  foninitioii  h.-nc  often  been  exposed  in  test-pitting  operations,  so  that 
the  course  and  width  of  the  belt  can  l)e  (juite  accurately  laid  down.  It  may 
be  noted  that  tliere  ju-e  larg-e  exjwsures  on  the  west  side  of  Sec.  14,  T.  47 
N.,  1\.  4(!  W,,  Michio-an,  where  the  several  railroad  spurs  running  to  the 
I'ahus  and  Anvil  mines  liave  cut  deeply  into  the  slate.  On  the  Black  river, 
in  the  northern  part  of  Sec.  13,  T.  47  N.,  R.  47  W.,  ]\Iichigan,  are  natural 
exposures  of  the  formation.  To  the  east  of' Sunday  lake,  in  Sec.  10,  T.  47 
'N.,  R.  45  W.,  Michigan,  where  the  (:iuartz-slate  formation  has  an  unusual 
thickness  and  its  outcrop  belt  an  unusual  width,  there  are  again  very  large 
and  continuous  exposures,  displa}'iug  the  entire  thickness  of  the  formation, 
which  here  reaches  something  like  800  feet.  The  Quartz-slate  here  makes 
up  the  bulk  of  two  bold  bluifs,  one  in  the  soathwestern  portion  and  the 
other  in  the  southeastern  portion  of  Sec.  10,  and  extending  thence  into 
Sees.  11  and  14.  About  all  the  phases  that  are  characteristic  of  the  Quartz- 
slate  formation,  with  the  exception  of  the  chloritic  and  biotitic  slates,  are 
displayed  in  these  exposures.  The  last  exposures  of  this  formation  met 
with  before  reaching  the  Presque  Isle  river  are  those  in  the  southern  part 
of  Sec.  17,  T.  47  N.,  R.  44  W.,  Michigan. 

Mode  of  deposition  and  source  of  material. — It  has  already  been  made  evi- 
dent that  the  various  rock  phases  of  which  the  Quartz-slate  member  is  com- 
posed are  of  a  detrital  nature.  Even  the  finest  grained  phases  and  the^  finer 
interstitial  material  of  the  coarser  phases,  although  at  times  not  now  plainly 
showing  their  fragmental  character,  are  taken,  notwithstanding  this,  to  be 
wholly  of  detrital  origin,  being  composed  of  the  same  materials  as  the 
coarser  phases,  either  in  the  original  unaltered  condition,  or,  as  is  often  the 
case,  somcAvhat  changed  by  metasomatosis,  the  chlorite,  kaoliuite,  micas, 
and  finely  crystalline  quartz  being  in  the  main  secondary  derivatives  from 
feldspathic  detrital  material. 

The  nature  of  the  most  of  the  detritus  of  which  this  formation  is  mainly 
composed  is  such  as  to  suggest  very  strongly  the  derivation  of  its  material 
from  some  sort  of  granitic  or  gneissic  rock.  The  pieces  of  quartz,  orthoclase, 
microcline,  plagioclase  (probably  albite  and  oligoclase),  and  mica,  are  in 
their  association  and  in  their  peculiarities  just  what  we  would  expect  in  a 


130  THE  PENOKEE  IRON  BEARING  SERIES. 

granitic  detritus.  And  more  than  this,  they  are  entirely  similar  to  the  par- 
ticles of  these  same  minerals  as  they  appear  in  granite  and  gneiss  which  is 
so  largely  exposed  immediately  to  the  south.  One  of  the  most  noteworthy 
things  about  the  formation  (excepting  the  upper  horizon)  is  the  relatively 
small  degree  of  assortment  that  the  detrital  material  of  which  it  is  composed 
has  received.  The  detritus,  which,  while  quartz  predominates,  is  still  largely 
composed  of  feldspar  and  mica  particles,  can  not  have  been  carried  to  any 
great  distance  from  its  source.  The  quartzite,  however,  which  forms  tlie 
uppermost  horizon  of  the  formations,  represents,  of  course,' a  more  thoroughly 
assorted  detritus,  and  it  is  very  interesting  in  this  connection  to  note  the 
extraordinary  persistency  of  this  horizon. 

At  those  places  in  which  the  Quartz-slate  is  in  contact  with  the  Cherty- 
limestone  formation,  or  outcrops  of  it  are  not  distant,  a  considerable  por- 
tion of  the  detritus  is  often  derived  from  this  formation,  and  close  to  the 
contact  the  cherty  detritus  sometimes  becomes  predominant,  when  we  have 
true  basal  conglomerates  or  recomposed  rocks.  In  the  higher  horizons  of 
the  Quartz-slate  the  cherty  detritus  is  usually  sparse  or  absent.  The  pres- 
ence of  red  jasper  fragments  in  certain  of  the  basal  conglomerates  suggests 
the  former  presence  of  an  earlier  Iron-bearing  formation. 

Suinman/. — The  Quartz-slate  member  is  given  this  name  because  of  its 
slaty  character,  and  of  the  fact  that  quartz  is  its  prominent  constituent. 

In  geographical  extent  it  is  the  most  continuous  belt  of  the  Penokee 
range,  extending  from  the  westernmost  exposures  of  the  series  nearly  to 
Gogebic  lake. 

The  thickness  of  the  greater  portion  of  the  belt  varies  from  300  to  4'  0 
feet,  but  its  maximum  thickness  is  as  great  as  800  feet. 

Petrographically  the  Quartz-slate  has  many  varieties;  the  most  persist- 
ent of  these  is  a  vitreous  quartzite,  which  is  found  at  the  uppermost  horizon 
of  the  member. 

The  quartz-slates  are  always  fragmental.  Their  induration  is  due  (1) 
to  the  enlargement  of  quartz  fragments,  rarely  to  enlargement  of  the 
feldspars ;  and  (2)  to  the  alteration  of  the  feldspars  to  other  minerals,  the 
most  abundant  of  which  are  biotite,  chlorite,  and  quartz. 


THK  (,)|tai;tx-slatk  memuei:.  •        i8l 

At  sevorul  plucc^s  tlio  Quiirtz-slati^  is  in  cniitact  with  tiic  (Micrty  lime- 
stone, and  here  a  lui-<>-e  part  of  its  dc'hris  is  from  this  Idwcr  mcmlx-r.  It  is 
more  tVeciueiitly  in  contact  witli  tlu*  Sonthern  (Jomph-x.  At  these  ((nitacts 
trne  basal  conglomerates  are  found.  In  one  or  two  jdaces  at  those  contacts 
unconformities  are  nicely  shown. 

Tiie  change  t(^  the  overlying  Iron-bearing  nieni])er  is  always  alirujit, 
its  nonfragmental  rocks  being  found  upon  the  vitreous  (piartzite. 

The  greater  part  of  the  material  for  the  Quartz-slate  member  is  derived 
from  the  underlying  rocks  of  the  Southern  Complex,  although  locally  it 
contains  material  froui  the  Cherty  limestone  member. 


CHAPTER  V. 


By  E.  D.  Irving  and  0.  E.  Van  Hise. 


THE  IRON-BEARING  MEMBER. 

Section  I.     Details. 

Applicability  of  tbe  name.  Abruptness  of  transition  from  the  underlying  Quartz-slate  raember. 
Geographical  extent.  Topographical  features.  Thickness.  General  petrograpbical  character. 
Distribution  of  the  three  types  of  rock.  Microscopical  character  of  the  oherty  iron  carbonates. 
Microscopical  character  of  the  ferruginous  slates  and  ferrngiuons  cherts.  Microscopical  char- 
acter of  the  actinolitic  slate.s.  Tabulation  of  petrograpbical  observations. 
Section  II.     Origin  of  tbe  Rocks  of  the  Iron-bearing  Member. 

The  original  rock.     The  ferruginous  slates.     Tbe  ferruginous  cherts.     The  actinolitic  slates. 
Section  III.     The  Animikie  Iron-bearing  Series. 

The  oherty  iron  carbonates.     The  ferruginous  slates.     Tbe  ferruginous  cherts.     The  actinolitic 
slates.     General. 
Section  IV.     The  Iron  Ores. 

Position  of  the  ores  in  the  Iron-bearing  member.  Dikes  in  Iron-bearing  member.  Position  of  ore 
in  reference  to  the  dikes.  Kock  above  tbe  ore.  Practical  deductions  to  be  applied  iu  lu-ospeetiug 
and  miuiug.  Nature  of  the  rocks  of  the  Iroii-be.iring  member  adjacent  to  the  ore  bodies.  Tbe 
character  of  the  ore.  A  particular  occurrence  of  iron  ore.  Chemistry  of  the  process  of  ccmcen- 
tratiou.  Time  at  wbicb  concentration  of  the  main  ore  bodies  occurred.  Process  of  concentra- 
tion. Exceptional  localities.  Probable  extent  in  depth  of  ore  bodies.  Emmons  on  ore  deposits. 
Iron  ores  in  other  parts  of  lake  Superior  country.     Summary  of  more  important  conclusions. 

SECTION  I.     DETAILS. 

ApplicahUity  of  the  name. — The  name  given  to  this  member  is  justified 
by  its  large  content  of  iron.  Certain  phases  of  the  belt  contain  little,  or  no 
iron,  being  wholly  made  up  of  silica  in  one  form  or  another,  but  such 
phases  have  no  very  great  extent,  there  being  nearly  always  present  at 
least  a  little  iron  oxide,  while  throughout  the  greater  portion  of  the  belt 
the  content  of  metallic  iron  certainly  exceeds  10  per  cent.  Very  consider- 
able thicknesses  are  met  with  in  which  the  amount  of  iron  reaches  20,  30, 

182 


Till'.  li;(»N-HKAl{IN(i  MICMHKH.  lB3 

40,  ami  even  ;"»(»  |)i'i-  cent.  Carct'iil  iiiciisiirciucnt  ami  saiiipliiif^'  oi'tlie  west 
dirt"  of  Feiiokee  g-ap  at  tlic  passajic  oi"  Had  river  tliroiioli  tlic  I'euokee 
range  showed  that  tliis  clitV  is  made  u|)  oi'  the  inllowiiif^'  dixisioiis — these 
divisions  beiiij^' measured  at  riglit  angles  to  tlie  strike,  he<^-inuing- with  the 
h)\vest  parr  of  the  clirt':  1!)  feet,  containing  44-94  i)er  rent  of  nietallic 
iron;  7  feet  G  indies,  containing  171(1  ))er  cent;  18  feet,  containing  49-4 
per  cent;  fi  inches,  nearly  free  from  iron;  (i  feet,  containing  3(j-64  per  cent; 
36  feet,  containing  45-87  per  cent.  Above  and  below  the  layers  shown  in 
this  cliff  are  others  nearl}^  or  equally  rich  in  metallic  iron,  alternating  with 
layers  in  which  the  iron  is  present,  but  in  smaller  quantity. 

The  following  figures  indicate  the  richness  of  various  other  thicknesses 
at  Penokee  gap:  7  feet,  containing  43-29  per  cent;  2  inches,  included 
within  the  preceding  measurement,  62-21  per  cent;  10-incli  rich  seam  from 
a  different  place,  67-52  per  cent.  The  following  are  other  figures,  indicat- 
ing the  content  of  iron  in  considerable  thicknesses  at  various  points  on  the 
Penokee  range  from  the  Potato  river  westward:  (1)  5  feet,  containino- 
11-23  per  cent;  (2)  15  feet,  containing  12-99  per  cent;  (3)  15  feet,  contain- 
ing 25-81  percent;  (4)  75  feet,  contaimng  26-64  per  cent;  (5)  25  feet,  con- 
taining 36-14  percent;  (6)  10  feet,  containing  37  percent;  (7)  40  feet,  con- 
taining 37-75  pel-  cent;  (8)  20  feet,  containing  37-88  per  cent;  (9)  20  feet, 
containing  37-93  per  cent;  (10)  3  feet,  containing  38-75  per  cent;  (11)  58 
feet,  containing  41-93  per  cent;  (12)  5  feet,  containing  44-43  per  cent;  (13) 
150  feet,  containing  45  per  cent;  (14)  3  feet,  containing  45-07  per  cent; 
(15)  75  feet,  containing  48-12  per  cent;  (16)  15  feet,  containing  49-73  per 
cent;  (17)  50  feet,  containing  53*46  per  cent.^     These  figures,  of  course, 

'These  figures  are  taken  from  taWes  of  analyses  given  iu  the  third  volume  of  the  GeoloK-y  of  Wis- 
,  cousin,  pp.  156-160.  The  samples  were  mainly  selecti'd  hy  R.  D.  Irving ;  in  a  few  cases  hy  E.  T.  Sweet. 
They  were  all  made  by  taking  a  large  numher  of  small  pieces  across  the  entire  thickness  indicated  in 
each  case.  The  places  from  which  the  samples  were  selected  are  as  follows:  (1)  Exploring  trench, 
NE.  i  Sec.  15,  T.  44  N.,  R.  3  W.,  Wisconsin;  (2)  exposure  SW.  i  Sec.  1,  T.  44  N.,  R.  2  W.,  Wisconsin; 
(3)  exposure  SE.  i  Sec.  35,  T.  45  N.,  R.  1  W.,  Wisconsin;  (4;  exposure  NW.  i  Sec.  21,  T.  44  N.,  R. 
5  VV.,  Wisconsin;  (5)  is  from  the  west  side  of  the  passage  of  the  Potato  river  through  the  Penokee 
range;  (6)  exposure  NE.  i  Sec.  18,  T.  44  N.,  R.  2  W.,  Wisconsin;  (7)  exposure  SW.  i  Sec.  17,  T.  44 
N.,  R.  3  W,,  Wisconsin;  (8)  exposure  SW.  i  Sec.  1,  T.  44  N.,  R.  2  W.,  Wisconsin;  (9)  exposure  on  the 
fourth  principal  meridian,  ,Sec.  19,  T.  45 N.,  R.  1  E.,  Wisconsin;  (10)  exposurenearcenter  of  Sec.  18,  T. 
44  N.,  R.  3  W.,  Wisconsin;  (11)  trench  SW.  j-  Sec.  10,  T.  44  N.,  R.  2  W.,  Wisconsin;  (12)  exposure 
NE.  i  Sec.  14,  T.  44  N.,  R.  3  W.,  Wisconsin;  (13)  exposure  NW.  i  Sec.  23,  T.   44  N.,  R*  5  W.,  Wiscon- 


184  THE  PENOKBE  lEON-BEARING  SERIES. 

represent  the  richer  portions  of  the  belt,  the  layers  from  which  they  come 
alternating  with  others  which  run  down  to  only  a  small  percentage  of  iron. 

In  the  more  eastern  portion  of  the  belt  the  total  amount  of  metallic  iron 
may  perhaps  be  nearly  as  great  as  at  Penokee  gap,  but  it  tends  to  greater 
concentration,  the  result  of  which  is  the  production  of  genuine  ore  bodies 
and  of  considerable  thicknesses  of  rock,  in  which  relatively  little  iron  is 
present,  though  in  these  cases  iron  oxide  is  usually  contained  in  bands, 
seams,  and  finely  disseminated  particles. 

Abruptness  of  transition  from  the  vmderlying  Quarts-slate  member. — As  in- 
dicated alread}"  in  Chapter  II,  the  fundamental  distinction  between  the 
Quartz-slate  and  Iron-bearing  members  lies  in  the  fact  that  the  former  is 
wholl}"  of  clastic  origin,  being  still  made  up  mainly  of  fragmental  material 
readily  recognizable  as  such,  while  the  latter  not  only  contains  no  frag- 
mental material  whatever,  but  presents  us  with  no  evidence  at  all  of  ever 
having  accumulated  in  a  detrital  condition.  Upon  the  other  hand,  the  slaty 
rocks  which  overlie  the  Iron-beai'ing  member,  though  occasionally  so  nmch 
changed  by  metasomatic  processes  as  to  have  lost  their  fragmental  character, 
have  in  the  main  preserved  it  thoroughly,  and  are  plainly  of  a  detrital 
origin.  The  transition,  then,  from  the  Quartz-slate  member  to  the  Iron- 
bearing  member  is  one  from  a  detrital  to  a  noridetrital  formation.  The 
contact  between-these  two  formations  is  to  be  seen  at  numerous  points,  and 
at  all  of  them,  the  change  is  abrupt.  This  'conclusion  has  been  reached, 
not  merely  by  an  examination  in  the  field,  but  as  a  result  of  a  careful  study 
of  thin  sections  of  specimens  collected  on  each  side  of  the  contact  with 
the  verjr  object  of  obtaining  light  upon  this  point.  In  the  more  eastern 
portion,  of  the  district,  and  so  far  west  as  the  vicinity  of  the  passage  of 
Tylers  fork,  in  T.  45  N.,  R.  1  W.,  Wisconsin,  this  contact  has  been 
brought  to  \\eyv  by  mining  operations  at  many  points,  the  rule  being,  as 
further  explained  hereafter,  that  the  principal  deposits  of  ore  lie  at  the  base 

siu;  (14)  trench  NE.  i  Sec.  15,  T.  14  N.,  E.  3  W.,  Wisconsin;  (15)  exposure  SW.  i  Sec.  17,  T.  44  N.,  E. 
3  W.',  Wisconsin;  (16)  exposure  NW.  i  Sec.  16,  T.  44  N.,  R.  3  W.,  AVisconsin ;  (17)  exposure  NE.  J  Sec. 
14,  T.  44  N.,  K.  3  W.,  Wisconsin.  The  samijles  from  which  these  analyses  were  made  were  all  selected 
with  reference  to  determining  the  value  of  these  schists  as  iron  ores,  and  therefoi-e,  while  fairly 
lepresentino;  the  percentages  of  iron  in  the  thicknesses  named,  were  of  course  selected  from  the 
richer  portions  of  the  exposures. 


TIIK   llJON-ltEAIMNC    .M  i:.M  liKli.  l8il 

(it' the  lnm-l)('ariiiji-  inciuhfi-  iiud  iiuinediatcly  iipnii  llic  \  itrciuis  (|iiarty.ito 
wliicli  t'oi'iiis  the  iipiicnnost  la\cr  nt'  llic  (Quartz-slate  mcnil»cr.  In  tlicsc 
iiiiiics  till-  ciintact  is  lictwceii  tliis  ([luutzitc  on  the  unc  side  and- citlicr  th« 
ore  (ir  a  liciiiatitic  clicrtx-  niattM-ial  of  an  eutirel}'  iiDidVajiincntal  cliaractci- on 
the  other.  I)nt  tlie  coiitaet  between  the  two  foriiiatious  is  also  visible  at  a 
uuinl»(M-  of  natural  exposures,  as,  for  Instance,  at  the  passage  of 'r}lers  fork, 
in  T.  4;')  N.,  H.  1  W.,  Wisconsin  (Fig-.  H),  and  the  passage  of  Had  river  at 
I'enokee  gap,  in  T.  44  N.,  H.  o  W.,  Wisconsin.  (See  PI.  xxxvi.)  In  these 
cases  on  one  side  is  the  niagnetitic  cherty  and  slaty  rock,  and  on  tlu'  other 
the  ^•itreons  quartzite;  and,  as  usual,  the  two  classes  of  materials  come 
directly  against  each  other  without  transition  jjhases.  West  of  Penokee 
gaj)  but  few  contacts  were  seen.  In  a  number  of  places,  however,  the 
exposures  of  the  two  formations  were  found  in  very  close  proximity  to 
one  another.  (Pis.  v,  vi.)  As  already  stated,  the  uppermost  jjortiou  of 
the  Quartz-slate  is  everywhere  a  vitreous  quartzite,  i.  e.,  a  sandstone  in 
which  the  interstices  of  the  quartz  fragments  have  been  filled  with  a 
secondary  silica,  which  has  in  the  main  coordinated  itself  with  the  original 
fragments.  However,  as  is  usually  the  case  in  such  quartzites,  some  of 
the  silica  has  deposited  in  a  more  or  less  minute  mosaic  in  the  interstices. 
This  mosaic,  itself  of  direct  chemical  origin,  in  a  few  sections  is  not  far 
different  from  the  coarser  grained  phase  of  the  nonfragmental  silica,  which 
forms  the  groundwork  of  most  of  the  rock  of  the  Iron-bearing  member. 
But  in  any  case  the  chemically  deposited  silica  is  of  a  secondar}'  nature, 
and  can  not  be  taken  as  indicating  a  transition  between  the  modes  of 
deposition  of  the  two  formations. 

Geographical  extent. — Longitudinally  the  Iron-bearing  member  is  co- 
extensive in  distribution  with  the  underlying  Quartz-slate;  that  is  to  say,  is 
continuous  from  the  western  end  of  the  Penokee  range  in  the  north  half  of 
Sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin,  to  the  NE.  \  of  Sec.  21,  T.  47  N., 
R.  44  W.,  Michigan,  a  distance  of  more  than  50  miles.  To  the  west  of 
the  western  end  of  the  Penokee  range,  as  indicated  heretofore,  the  entire 
succession  of  the  Penokee  series  is  lost  for  a  distance  of  6  miles,  in  which 
distance  exposures  of  the  itnderlying  gTanitic  and  gneissic  rocks  on  the  one 
hand  and  of  gabbros  on  the  other,  come  in  close  proximity  to  each  other 


186  THiS  PENOKEE  mON-BEARmG  SEEIES. 

and  make  it  a  question  whether  the  entire  Iron-hearing  series  is  here  cut 
out.  This  statement  is  at  variance  with  the  maps  of  the  Geology  of  Wis- 
consin, wliich  indicate  a  continuity  across  R.  4  W.,  Wisconsin,  not  only  of 
the  Iron-bearing  member,  but  of  the  underlying  and  overlying  members  of 
the  Penokee  series/  It  was  known  at  the  time  these  maps  were  made  that 
the  Penokee  range,  with  all  its  characteristic  rock  exposures,  ends  abruptly 
in  the  eastei-n  part  of  T.  45  N.,  R.  4  W.,  Wisconsin,  being  succeeded  to  the 
westward  by  a  low,  marshy  country,  with  but  rare  exposures,  the  Penokee 
range  rocks  not  reappearing  until  in  the  vicinity  of  the  bold  ridge  in  the 
NE.  4  of  Sec.  24,  T.  44  N.,  R.  5  W.,  Wisconsin.  The  late  Mr.  C.  E. 
Wright,  however,  traced  through  the  intervening  low  country  a  line  of 
rather  feeble  magnetic  attractions  which  he  considered  to  establish  the  con- 
tinuity of  the  Penokee  range  rocks  beneath  the  drift  covering.  Accepting 
Mr.  Wright's  conclusion,  the  surface  maps  were  drawn  accordingly.  As  to  the 
magnetic  attractions,  it  is  to  be  said  that  they  were  too  feeble  to  liase  an)" 
certain  conclusion  upon,  being  pei'haps  explicable  by  the  considerable 
quantity  of  magnetite  occurring  in  the  gabbro,  Avhich  appears  here  to  usurp 
the  place  of  the  Penokee  rocks.  Still,  it  is  not  impossible  that  this  line  of 
attractions  may  be  due  to  the  Penokee  iron  lielt  itself,  buried  here  under- 
neath a  considerable  drift  covering.  In  the  north  half  of  Sec.  23,  T.  44 
N.,  R.  5  W.,  Wisconsin,  as  already  stated,  outcrops  of  the  Penokee  series 
reappear,  and  from  here  for  a  distance  westward  of  4  miles  exposures  of 
the  Iron-bearing  and  Quartz-slate  members  are  sufficiently  frequent  to  indi- 
cate their  continuity  through  this  distance.  The  last  exposure  on  this  line 
is  met  with  in  the  S.  .]  of  Sec.  20,  T.  44  N.,  R.  5  W.,  Wisconsin,  beyond 
which  point  no  exposures  have  yet  been  found  until  those  met  with  in  the 
SE.  4  of  Sec.  26,  T.  44  N.,  R.  6  W.,  Wisconsin,  where  the  characteristic 
rocks  of  the  iron  belt  are  seen  again.  Still  farther  west  Mr.  Wright  traced 
a  line  of  feeble  magnetic  attractions  as  far  as  the  north  side  of  lake  Numa- 
kagon.  In  the  northern  part  of  T.  43  N.,  R.  6  W.,  Wisconsin,  beyond  lake 
Numakagon,  no  further  exposures  of  the  Penokee  series  have  been  met  with, 
while  in  Sec.  20,  T.  43  N.,  R.  7  W.,  Wisconsin,  the  occurrence  in  close 

'  See  Atlas  of  the  Geol.  of  Wis.,  Pis.  xxil  and  xxvii.     See  particularly  the  latter  plate  for  the 
position  of  all  exposures  known  at  that  time. 


TIIK  lliON  IJKAIMNC   AlKMIiKlt.  187 

pi'oxiniit  \  (p|  cxiiosiircs  ol  rocUs  clKir.ictcristlc  dl  tlic  Keweenaw  series 
ami  dl'  tli<'  iifaiiitfs  heloiiyiii}'-  beneath  the  I'enokee  series  ajjpear  to 
iiiilieatt^  tiie  teniiination  in  tliat  dii-ection,  lor  sonic  distance  at  h'ast, 
of  tlic  Tcnokee  rocks.  To  the  east  of  the  easterinnost  point  indicated  as 
reaciicd  l)\-  the  continnons  iron  l)elt  of  tlie  I'enokee  raii"e,  tli(^  NE.  I  of 
Sec.  21,  '\\  47  X.,  U.  44  \\  .,  Micliigan,  exposures  of  ferrnjiinous  rocks  allied 
to  those  of  tlie  Iron-beai'ini>-  niend)er  are  met  witli  for  a  distance  of  G  or 
7  miles,  and  to  witiiin  3  or  4  miles  of  Gogebic  lake,  but  these  occair  in  a 
peculiarly  disturbed  and  difficult  area,  whose  geology,  whatever  be  the 
true  interpretation  of  it,  is  unlike  that  of  the  Penokee  belt  proper.  The 
iron-])earing  rocks  of  this  area  receive  special  attention  in  C'hapter  VIII. 

The  width  of  the  belt  of  country  occupied  by  the  Iron-bearing  mem- 
ber is  surprisingl)^  uniform  from  the  westernmost  exposure  as  far  east  as  the 
central  porti (in  of  T.  47  N.,  R.  46  W.,  Michigan.  Throughout  this  very 
considerable  distance  this  widtli  rarely  falls  below  800  feet  and  as  rarely 
exceeds  1,000  feet;  its  variations  being  frequently  explicable  by  changes 
in  the  dip,  though  apparently  some  part  of  the  variation  may  be  due  to 
actual  difference  in  thickness.  Still,  through  most  of  this  distance  the 
thickness  luust  be  taken  as  more  nearly  constant  than  the  width  of  the 
belt  occupied.  When  R.  46  W.,  Michigan,  is  reached,  however,  a  distinct 
widening  of  the  belt  becomes  perceptible,  and  in  the  eastern  part  of  that 
township  this  becomes  so  rapid  that  when  Black  river  is  reached  it  has 
become  as  much  as  2,400  feet.  From  Black  river  eastward  there  is  a  still 
more  rapid  increase,  the  width  on  the  east  side  of  Sees.  7  and  1 8,  T.  47  N., 
R.  45  W.,  Michigan,  being  fully  4,700  feet.  A  part  of  this  great  increase  : 
in  width  is  plainly  due  to  a  very  iinusual  flattening  iu  the  degree  of  north- 
ward dip,  but  this  will  not  serve  to  ex^ilain  all  of  tlie  Avidening,  a.  part  of 
which  may  be  due  to  an  actual  increase  in  thickness,  but  is  owing  in  part  at 
least  to  the  presence  of  interbedded  greenstones.  Immediately  east  of  the 
last  named  point  the  overlying  Keweenaw  rocks,  whose  southern  boundary 
has  been  for  some  time  rapidly  ajjproached  by  the  northern  edge  of  the 
iron-bearing  belt,  are  reached  by  it.  Continuing  to  the  east,  the  iron  belt 
is  in  part  cut  off  by  the  Keweenaw  series,  so  that  at  one  place  its  surface 
width  is  not  much  more  than  500  feet.     Not  far  east  of  Sunday  lake  a 


188  THE  PENOKEE  IKON-BE AEING  SERIES. 

widening  comes  in  again.  This  is  evidentl)'  due  to'  a  cliange  in  course  of 
the  iron  belt,  which  now  trends  southward,  and  thus  diverges  from  the 
Keweenawau  beds,  the  divergence,  however,  not  being  so  great  as  to 
allow  the  reappearance  of  the  slates  belonging  above  the  Iron-bearing 
member  as  far  east  as  the  middle  of  T.  47  N.,  R.  44  W.,  Michigan. 

Topograpliical  features. — The  Penokee  iron  range,  save  for  several  trans- 
verse cuts  made  through  it  by  the  northward  flowing  streams,  is  a  contin- 
uous ridge  from  the  northern  half  of  Sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin, 
eastward  to  beyond  Sunday  lake  in  Michigan.  To  the  west  of  the  western 
termination  of  this  range,  as  indicated  in  previous  chapters,  are  again 
other  detached  ranges,  which,  being  made  up  of  like  strata  dipping  in  the 
same  northerly  direction,  may  be  looked  upon  as  forming  portions  of  the 
same  P-dneral  line  of  elevation.  The  same  is  true  east  of  the  eastern 
termination  indicated  as  far  as  the  vicinity  of  the  Little  Presque  Isle 
river.  In  places  the  ridge  rises  from  100  to  300  feet  above  the  elevated 
swampy  area  south  of  it,  and  from  100  to  600  feet  above  the  lower  area 
north.  In  its  more  western  portions  this  range  is  wide  and  has  a  rather 
narrow  serrated  crest,  while  eastward  from  Tylers  fork  it  becomes  more 
and  more  of  a  gentle  swell  until  a  point  west  of  Sunday  lake  is  reached, 
where  there  is  again  a  broader  range.  In  much  of  this  distance  the  ridge 
forms  the  most  prominent  feature  of  the  topography  of  the  country,  being 
visible  from  the  waters  of  lake  Superior  in  the  vicinity  of  the  Apostle  islands 
as  a  blue  line  against  the  horizon.  On  a  preceding  page  the  relations  of  the 
Quartz-slate  member  to  this  ridge  have  been  indicated,  and  incidentally  those 
of  the  Iron-bearing  member  also.  The  points  of  principal  interest,  however, 
may  conveniently  be  •  repeated  here.  Along  that  portion  of  the  Penokee 
range  which  is  west  of  Bad  river,  and  also  in  the  detached  ranges  above 
referred  to  as  occurring  still  farther  west,  in  T.  44  N.,  R.  5  W.,  Wisconsin, 
the  Quartz-slate  member  forms  the  foot  of  the  bold  south  face  of  the  ridge, 
the  upper  portion  of  the  southern  face,  along  with  the  crest  and  the  upper 
portion  of  the  northern  slope,  being  all  made  up  of  the  steeply  inclined 
layers  of  the  Iron-bearing  member.  The  passage  of  Bad  river  through  the 
range  has  been  determined  by  the  existence  of  a  fault,  which  has  caused 
a  discordance  in  the  layers  of  some  800  feet.     Eastward  from  Bad  river  the 


THE  IKON-HEAKING  MEMBEK.  189 

Qiiartz-slalc  mcmluT  cliinljs  hin-licr  mid  lii^licr  ini  tlic  snutlicni  fare  of  tlie 
ridrre,  all  of  which  it  makes  up  \)y  the  time  iikkiiiI  Whittlesey  is  reached. 
Oorrespoudiuu^ly,  the  Irou-beariii<'-  ineinber  creeps  i'arther  and  farther  down 
the  northern  slope.  At  Tylers  fork  this  change  has  gone  on  so  far  that 
the  crest  of  the  ridsj'e  is  now  entirely  within  the  (piartz-slate,  while  east  of 
that  stream  and  all  the  way  to  the  West  branch  of  Black  river  the  crest 
of  the  ridfje  is  in  the  granitic  rocks  belonging-  to  the  Southern  Complex. 
In  this  area  the  Iron-bearing  member  continues  to  creep  down  the 
northern  face  of  the  ridge  until  it  lias  the  greater  portion  of  its  width  in  the 
low  ground  t(»  the  northward.  East  of  the  West  branch  of  Black  river 
for  a  short  distance  the  ridge  again  lies  within  the  iron  Ijelt,  which  here 
contains  an  unusually  large  amount  of  resistant  jaspery  material.  The 
changing  relation  thus  indicated  as  obtaining  between  the  positions  of  the 
ridge  and  that  of  the  outcrop  belt  of  the  Iron  member  has  been  explained 
on  a  previous- page  as  a  result  of  a  variation  in  the  mineralogical  character 
of  the  member,  which,  where  resistant,  forms  the  upi)er  jiortions  of  the 
ridges,  while  in  other  places  the  rocks  to  the  south  Ijeing  more  resistant 
the  com-se  of  the  iron  belt  lies  in  the  lower  ground  to  the  north. 

Thkkmss.— It  has  already  been  said  that  the  outcrop  belt  of  the  Iron- 
bearing  member  has  a  singularly  constant  width  from  800  to  1,000  feet  for 
all  of  the  distance  between  the  western  extremity  of  the  Penokee  ranffe  to 
the  middle  of  T.  47  N.,  R.  46  W.,  Michigan.  The  thickness  must  be  yet 
more  constant  than  is  indicated  by  these  figures,  the  variations  between  the 
limits  indicated  being  generally  explicable  by  a  somewhat  changing  degree 
of  northward  inclination.  Throughout  this  distance  the  actual  thickness  of 
the  formation  can  not  vary  much  on  either  side  of  850  feet.  How  far  the 
great  increase  in  surface  width,  which  has  already  been  indicated  as 
obtaining  in  the  eastern  portion  of  T.  47  N.,  R.  46  AV.,  Michigan,  and 
farther  to  the  east,  is  due  to  an  increase  in  actual  thickness  is  .exceedingly 
difficult  to  tell.  Some  of  the  increased  width  is  plainly  the  result  of  an 
unusually  low  angle  of  northward  inclination.  More  of  it  is  evidently  due 
to  the  intercalation  of  eruptive  greenstone  sheets;  but  after  these  two 
causes  of  widening  have  been  considered,  how  much  remains  to  be 
accounted  for  by  an  increased  thickness  of  the  Iron-bearing  member  we 


190  THE  PElSrOKEE  IEOIn-BEAEING  SEEIES. 

have  no  means  of  determining  accurately.  Throughout  much  of  this  area, 
moreover,  the  uppermost  portion  of  the  iron  belt  is  missing,  but  how  much 
is  gone  there  is  no  means  of  teUing.  It  is  only  possible  to  say  in  a  general 
way  that  it  would  seem  that  in  the  eastern  part  of  the  formation  its  thick- 
ness may  be  very  materially  increased. 

General  petrograpkical  character. — Three  main  types  of  rock  make  up 
the  Iron-bearing  member.  Between  these  three  types  there  are  various 
gradation  phases,  while  each  main  type  presents  itself  in  a  number  of 
forms  between  which  there  are  minor  differences.  In  certain  rare  instances 
a  little  detrital  material  has  been  introduced  during  the  original  deposition 
of  these  rocks,  but  ordinarily  this  is  completely  lacking.  The  three  types 
referred  to  may  be  briefly  characterized  as  (1)  slaty  and  often  clierty  iron 
carbonate,  (2)  ferruginous  slates  and  ferruginous  cherts,  and  (3)  actinolitic  and 
magnetitic  slates. 

The  first  type  is  a  very  well  marked  one,  and  is  present  in  very  con- 
siderable thickness.  It  is  characterized  by  the  invariable  presence  of  iron 
carbonate  as  a  chief  constituent.  In  some  cases  the  iron  carbonate  con- 
stitutes the  only  important  mineral  in  the  rock,  but  usually  it  is  mingled 
with  more  or  less  of  calcium  carbonate,  magnesium  carbonate  and  cherty 
silica,  the  latter  ranging  in  character  from  minutely  crystalline  to  amorphous. 
Other  minor  ingredients,  one  or  more  of  which  may  be  present,  are  hema- 
tite, limonite  or  other  brown  iron  oxide,  magnetite,  carbonaceous  or  graph- 
itic matter,  iron  pyrites,  a  chloritic  or  viriditic  ingredient,  an  excessively 
fine  clayey  substance,  and,  very  rarely,  pieces  of  a  fragmental  quartz.  In 
texture  these  rocks  are  commonly  earthy  and  aphanitic,  but  in  some  cases 
the  iron  carbonate  is  sufficiently  coarsely  crystalline  for  thfe  cleavage  sur- 
face of  the  minute  crystals  of  siderite  to  be  perceptible  to  the  naked  eye. 
There  is  very  commonly  a  regular  and  thin  lamination  produced  by  an 
alternation  of  lighter  and  darker  gray  shades  of  the  carbonate.  Frequently, 
however,  the  gray  carbonate  is  interlaminated  with  seams  of  black  graphitic 
matter,  red  jasper,  red  hematite,  black  flint,  greenish  black  or  viriditic  car- 
bonate, or  with  seams  of  carbonate  in  which  magnetite  particles  are  partic- 
idarly  abundant.  While  the  platy  or  thinly  stratiforuA  habit  is  very  char- 
acteristic of  these  rocks,  these  laminee  often  become  irregular,  presenting 


THE  IHON-HEARING  MEMISEH.  191 

tlie  Ji])pearanco  of  liiiviiii^-  licrii  hrokeii  aparl  and  rccpniciifi'd,  in  which  case 
there  is  usually  a  (•(>iisi(lerat)U^  quantitx'  nf  the  clicrtN'  silica  present.  Tlie 
observations  thus  t'jir  made  have  aj)[)H(Ml  particuhirly  to  fresh  surfaces.  On 
exposed  surfaces  there  is  very  apt  to  he  a  prevalent  brownish  or  reddish  iron 
stain  from  peroxichition  of  the  iron  of  the  carbonate.  In  specific  g'ravity 
these  rocks  range  generall}-  between  tlie  two  precedino-  tyj)es.  The  specific 
gravity  of  jnxre  iron  carbonate  is  given  in  the  mineralogies  as  between  3  1 
and  ;V!l.  The  rocks  now  considered  fall  below  this  figure  because  of  the 
presence  of  other  lighter  substances,  particularly  the  calcareous,  siliceous 
and  clayey  ingredients.  The  following  figures  are  the  results  of  specific 
gravity  determinations:  2-97,  3-04,  3-20,  3-22,  3-24,  3-29,  3-40,  and  3-50. 

Of  the  following  analyses  the  first  five  represent  the  composition  of 
iron  carbonates  from  the  Penokee-Gogebic  district.  The  remainder  are  of 
similar  carbonates  from  other  districts  about  lake  Superior,  and  are  inserted 
here  for  the  sake  of  comparison.  No.  I  (specimen  9191),  analyzed  by  Mr 
R  B.  Riggs,  of  the  U.  S.  Geological  Survey,  is  of  a  rock  exposed  in  a 
test-pit  in  the  NE.  |  of  the  NE.  ^  of  Sec.  6,  T.  45  N.,  R.  2  E.,  Wisconsin. 
No.  II  (specimen  9472),  made  by  Mr.  W  F.  Hillebrand,  of  the  U.  S.  Geo- 
logical Survey,  is  of  a  specimen  from  the  large  precipitous  exposure  on 
the  south  side  of  the  outlet  of  Sunday  lake,  NE.  J  of  Sec.  13,  T.  47  N„  R. 
46  W.,  Michigan.  No.  Ill  (specimen  12885),  made  by  Mr.  Thomas  M. 
Chatard,  of  the  U.  S.  Geological  Survey,  is  of  a  carbonate  occurring  near 
the  base  of  the  Iron-bearing  member  on  the  Miner  &  Wells  option,  Sec.  13, 
T.  47  N.,  R.  46  W.,  Michigan.  No.  IV  (specimen  12887),  made  by  Mr.  Hil- 
lebrand, is  of  a  specimen  representing  a  large  natiu-al  exposure  on  the 
Palms  property,  Sec.  14,  T.  47  N.,  R.  46  W.,  Michigan.  No.  V  (speci- 
men 12543),  made  by  Mr.  Chatard,  represents  a  carbonate  occurring  at  a 
low  horizon  in  the  member,  in  the  NW.^  of  Sec.  18,  T.  47  N.,  R.  45  W., 
Michigan.  Of  the  remaining  analyses.  No.  VI  (specimen  9264),  bv  Mr.  Hil- 
lebrapd,  represents  a  peculiar  carbonate  occurring  in  the  SE.  4  of  Sec.  20, 
T.  47  N.,  R.  43  W.,  Michigan,  in  that  confused  area  to  the  east  of  the  Presque 
Isle  river,  which  is  considered  in  the  present  volume  separateh'  from  the 
regular  Penokee  succession.  No.  VII  (specimen  10575),  by  Mr.  Chatard, 
is  an  iron  carbonate  from  the  so-called  gunfiint  beds  exposed  on  the  eastern 


192 


THE  PEJfOKEE  lEON-BEAEmG  SBEIES. 


side  of  the  outlet  of  Gunflint  lake,  situated  on  the  national  boundary 
between  Minnesota  and  Canada.  No.  VIII  (specimen  10598),  by  Mr, 
Chatard,  is  from  the  same  beds,  but  from  an  exposure  on  the  northern  side  of 
Gunflint  lake.  No.  IX  (specimen  10588),  also  by  Mr.  Chatard,  is  a  ferrif- 
erous carbonate  from  another  part  of  the  north  side  of  Gunflint  lake.  No. 
X  (specimen  10157),  by  Mr.  Riggs,  is  a  black  slaty  and  carbonaceous  iron 
carbonate,  exposed  at  Kakabikka  falls  on  the  Kaministiquia  river,  Canada. 
And  No.  XI  (specimen  10160),  also  by  Mr.  Riggs,  is  from  a  less  carbona- 
ceous phase,  exposed  at  the  same  place  as  No.  X.  The  rocks  represented  by 
Nos.  VII  and  XI,  inclusive,  of  these  analyses  are  all  from  the  so-called 
Animikie  series  of  northeastern  Minnesota  and  the  adjoining  portions  of 
Canada. 

Analyses  of  iron-bearmy  carhoiiates. 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

vni. 

IX. 

X. 

XI. 

Silii-a            

15-62 

28-86 
0-20 
1-29 
101 

4601 
0-12 
0-83 
1-35 

46-47 

0-10 

■     0-70 

0-86 

36-73 
0-19 
0-38 
0-98 

3-16 

0-08 
0-93 

68-23 

trace 

0-06 

5-01 

46-46 

trace 

0-24 

0-64 

23-90 

none 

0-07 

0-44 

37-73 

54-26 

4-27 
8-14 

3-41 
6-42 

2-57 
3-62 

Iron  protoxide 

32-85 

37-37  ■ 

26-00 

28-57 

34-74 

15-18 

18-41 

26-28 

10-66 

22-93 

19-6.1 

Maiiiianoiis  oxide 

5-06 

0.97 

2-09 

0-40 

0-52 

1-15 

0.25 

0-21 

0-28 

0-40 

0-19 

0-81 
2-66 

0-74 
;)-64 

0-63 
2-86 

0-49 
2-,30 

0-48 
2-74 

26-66 
11-01 

0-38 
9-59 

1-87 
3-10 

22-25 
8-52 

1-26 
3-98 

1-07 
2-93 

ilajiUfsium  oxide 

Cai'ljon  dioxido. 

30-32 

■^5-21 

17-72 

19-24 

22-44 

41-10 

5-22 

19-96 

32-42 

18-01 

14-93 

Plioaphoric  arid 

0-07 

0-06 

013 

0-11 

0-12 

0-34 

0-14 

0-11 

0-13 

"Water  at,  10S° 

0-12 

none. 



0-07 

0-07 

■ 

AVatiT  at  red  heat 

0-68 

0-68 

1-71 

0-60 

1-40 

0-54 

2-01 

1-15 

0-99 

1-20 

Water  at  red    heat,   partly 

2-74 

(*) 

(*) 

(*) 

(*) 

(*) 

n 

(*) 

:.-54 

0-45 

1 

100-41 

99-97 

99-50 

99-73 

100-84 

100-20 

99-40 

100-32 

99-66 

100-41 

lOU-85 

'  Undetermhied. 


The  rocks  of  the  second  type,  which  we  have  collected  under  the  gen- 
eral designation  of  ferruginous  slates  and  ferruginous  cherts,  have  in  common 
a  prominent  siliceous  constituent,  which  is  always  of  a  nonfragmental  nature, 
and  ranges  in  crystallization  from  a  wholly  though  minuteh'  crystalline  con- 
dition, through  partly  crystalline  and  chalcedonic  phases  to  an  entirely  amor- 
phous phase,  the  several  phases  being  frequently  associated  in  the  same  thin 


TJllO  1IH)N  liEAKING   aiEMBEE.  193 

section.     Compared  witli  the  siliceoius  ingredient  of  the  rocks  of  the  third 
type,  })etvveen  which  and  those  now  nnder  consideration  there  are  distinct 
<jra(hition  varieties,  the  silica  of  the  rocks  of  the  second  type  is  in  f>eneral 
very  nuich  liner  in  <>rain.     Indeed,  those  phases  in  whicli  the  silica  is  either 
in  so  minute  individuals  that  they  can  only  with  extreme  difficulty  be  sepa- 
rated laider  the  microscope,  or  in  which  it  is  mingled  with  more  or  less 
chalcedonic  or  even  amorphous  fonns,  g-reatly  predominate.     In  addition  to 
this  siliceous  g-rouuchnass,  to  all  of  the  phases  of  which  we  apply  the  general 
term  of  chert,  the  principal  ingredients  of  these  rocks  are  the  several  iron, 
oxides:  that  is,  magnetite,  hematite,  and  a  brown  hydrated  oxide.     These 
are  generally  greatly  subordinate  in  quantity  to  the  silica,  at  times  sinking 
almost  completely  out  of  sight.     In  other  cases  they  are  present  in  consid- 
erable quantity,  and  frequently  occur  so  plentifully  as  to  furnish  interme- 
diate phases  between  the  cherts  and  the  iron  ores  of  the  district.     The 
principal  one  of  these  oxides  is  hematite.     When  magnetite  is  present  in 
any  considerable  quantity,  it  is  generally  accompanied  by  more  or  less 
actinolite  in  minute  needles,   and  phases  of  gradation  between  the  cherts 
and  the  actiuolitic  rocks  of  the  third  type,  subsequently  described,  are  thus 
produced.     On  the  other  hand,  more  or  less  iron  carbonate  is  found  in 
remnants    in   the    many  sections  of  these   cherts,   and  by  its    increasing 
quantity  leads  us  through  phases  of  gradation  into  the  rocks  of  the  first 
type  above  described.     In  color  these  rocks  vary  greatly,  presenting  red, 
brown,  gray,  and  white  colors,  depending  upon  the  amount  of  iron  oxide 
present.     Perfectly  white  nonferruginous  phases  are  occasionally  met  with; 
also. light  to  dark  gray  kinds,  and  grayish  kinds  mottled  irregularly  with 
black,  in  which  phase  the  iron  oxide  is  mainly  magnetite.     When  hematite 
is  present  in  large  quantity  the  rock  may  have  a  uniform  red  color.     Occa- 
sionally the  red  iron  oxide  is  present  in  just  such  quantity  and  condition  as 
to  make  of  the  chert  a  genuine  jasper,  but  such  phases  are  not  common. 
A  rather  unusual  kind  is  quite  black,   apparently  from  the  presence  of 
carbonaceous  matter. 

In  structure  these  rocks  vary  from  the  regular  lamination  of  the  first 
type  to  those  that  are  much  less  regular,  being  either  without  any  uniform 
banding,  or,  if  the  banding  is  present,  the  laminae  present  the  appearance 
MON  xix 13 


194  THE  PENOKEE  lEON-BEARmG  SERIES. 

of  having  been  disrupted  and  the  broken  fragments  recemented.  Both  the 
regular  and  irregularly  laminated  phases  are  found  in  large  areas. 

The  uniformly  laminated  phase  to  which  the  name  ferruginous  slate 
is  given  has  a  uniform  texture  and  even  lamination,  and  varies  in  color 
from  yellow  to  deep  red,  the  change  in  color  being  due  to  different 
oxides  of  iron.  This  phase  might  be  taken  for  a  slaty  iron  ore,  but  its 
low  specific  gravity  and  its  large  content  of  silica,  as  shown  by  analysis, 
exclude  it  from  the  iron  ores.  The  silica  and  iron  oxide  are  so  uniformly 
.mingled  and  the  silica  is  in  such  small  particles  that  it  is  not  macroscop- 
ically  visible. 

The  second  phase,  the  ferruginous  cherts,  is  distinguished  from  the  first 
phase  by  the  greater  .concentration  of  the  iron  oxides.  They  occur  in 
irregular  bands  aiid  areas,  interlaminated  with  chert  layers  of  greater  or 
less  purity.  There  are  all  gradations  in  exposure,  from  the  regularly 
laminated  slates  of  the  first  phase  through  the  ferruginous  cherts  to  areas 
in  which  the  iron  oxide  is  concentrated  in  such  large  masses  as  to  be 
workable  deposits  of  iron  ore.  In  general  the  texture  of  the  chert  is 
aphanitic ;  occasionally  it  presents  a  chalcedony-like  appearance.  In  many 
instances  there  is  present  a  more  or  less  porous  texture,  minute  cavities 
occurring  tlu'oughout  the  specimens.  Also  a  minute  brecciation  of  the 
rock  is  not  unfrequently  perceptible  to  the  naked  eye,  though  this  peculiar 
characteristic  comes  out  more  prominently  in  the  thin  section,  as  subse- 
quently explained.  When  perceptible  to  the  naked  eye,  it  shows  itself  as 
an  irregular  mottling  of  dark  and  light  gray,  or  of  black  and  gray;  the 
appearance  being  that  of  more  or  less  angular  fragments  imbedded  in  a 
lighter  colored  matrix.  The  following  are  the  results  of  specific  gravity 
determinations  of  the  ferruginous  cherts:  2-65,  2-67,  2-69,  2-76,  2-90,  2-92, 
2'95,  3"21,  3"25,  3"26,  3"39.  Of  these  amounts  those  above  three  represent 
the  more  ferruginous  kinds. 

The  rocks  belonging  to  the  third  type,  the  actinolitic  and  magnetitic 
slates,  are  in  thq  main  very  dark  colored,  being  often  black.  Occasion- 
ally, however,  in  bands  the  color  is  lighter,  in  which  case  the  light  and  dark 
colored  bands  present  a  striking  striped  appearance  in  natm-al  exposure. 
The  grain  is  usually  a  very  fine  one,  being  at  times  quite  aphanitic.  Occa- 
sionally each  of  the  main  individual  constituents  may  be  detected  with  the 


THE  1KON-J5EAKING  MEMBER.  195 

naked  eye.  More  commonly,  however,  the  use  of  the  magnifying  glass  is 
required.  These  main  constituents  are  shown  by  the  microscope  to  be  in 
most  instances  quartz,  actinolite,  liematite,  and  magnetite.  The  first  of 
these  is  occasionally  lacking,  but  such  pliases  are  unusual.  The  four  min- 
erals vary  greatly  as  to  relative  proportion,  each  in  turn  predominating 
over  the  others,  the  lighter  colored  phases  being  of  course  the  most  highly 
quartzose  ones,  while  the  darker  kinds  are  richer  in  iron  oxide,  which 
is  often  present  in  sufficient  quantities  to  give  the  rock  a  more  or  less 
distinct  metallic  luster.  Except  in  certain  thin  seams,  however,  which 
are  at  times  even  pure  hematite  and  magnetite,  the  content  is  not  so  great 
but  that  the  powder  of  the  rock  is  of  a  light  color.  In  the  more  detailed 
description  given  below  it  will  be  seen  that  in  one  unusual  phase  garnet  is 
a  prominent  constituent,  and  that  chlorite  and  biotite  occur  frequently  as 
somewhat  important  alteration  products.  The  lamination  of  these  rocks  is 
usually  more  or  less  strongly  marked  by  variations  in  color,  the  individual 
laminae  more  commonly  running  from  an  insignificant  thickness  to  about 
one-fourth  or  one-half  an  inch,  only  occasionally  exceeding  the  latter  figure. 
Parallel  to  the  lamination  there  is  generally  a  distinct  tendency  to  cleavage. 
Obliquely  transverse  to  this  bedding  cleavage  is  a  join;ting  which  renders  it 
exceedingly  difficult  to  get  from  the  ledge  large  sized  pieces  of  the  rock, 
they  usually  coming  out  in  small  lozenge-shaped  slabs.  The  large  amount 
of  magnetite  and  hematite  which  nearly  all  phases  of  this  rock  contain 
render  it  very  noticeably  heavy.  The  following  are  specific  gravity  deter- 
minations made  upon  samples  selected  with  a  view  to  illustrating  the  sev- 
eral pliases  of  these  actinolitic  rocks:  3-06,  3-37,  3-42,  3-43,  3-46,  3'50,  3-53, 
3"91,  4'31,  4'54,  5"01.  The  last  three  numbers  represent  thin  seams  unusu- 
ally rich  in  hematite  and  magnetite.  The  other  numbers  represent  large 
bodies  of  rock,  the  lowest  one  being  given  by  one  of  the  most  quartzose 
phases  met  with.  The  large  amount  of  magnetite  contained  in  the  rocks 
of  this  kind  produces  extraordinary  attractions  upon  the  magnetic  needle. 
In  this  connection  reference  should  be  made  to  the  magnetic  charts  given 
in  the  third  volume  of  the  Geology  of  Wisconsin,  and  in  the  atlas  accom- 
panying that  volume.^     While  the  magnetic  oxide  of  iron  is  the  greatly 

'  Volume  Pis.  xxiii,  xxiv,  xxv,  xxvi,  xxvii,  xxviii,  xxix,  and  xxx.    Atlas  Pis.  xxiii,  xxiv, 
XXV,  and  xxvi. 


.     196  THE  PENOKEB  IRON-BEAEING  SERIES. 

predominating  one  in  all  this  class  of  rocks,  nevertheless  analyses  show- 
that  some  of  the  sesquioxide  is  usually  mingled  with  it,  the  presence 
of  which  oxide  indeed  is  not  infrequently  evident  to  the  naked  eye  in 
the  more  ferruginous  varieties,  the  luster  of  specular  iron  and  a  red- 
dish or  purplish  tint  in  the  powder  both  testifying  to  its  presence.  In 
the  less  ferruginous  portions  of  these  phases  the  sesquioxide  is  less 
plentiful,  but  at  times  the  siliceous  seams  present  a  dull  reddish  or  jaspery 
appearance  from  the  presence  of  hematite.  In  one  vicinity  in  the  eastern 
portion  of  the  district  a  bright  red  jasper  is  thinly  laminated  with  a  typical 
actinolitic  magnetitic  slate,  but  this  is  tmusuaT.  The  ordinary  occurrence 
in  the  lake  Superior  region  of  the  bright  red  phase  of  nonfragmental 
silica,  commonly  spoken  of  as  jasper,  is  in  direct  association  with  the 
more  brilliantly  lustered,  steely,  specular  iron  ores.  Such  bright  colored 
jaspers  only  rarely  occur  in  immediate  association  with  the  actinolitic  and 
magnetitic  slates.^  The  following  analyses  of  the  rocks  of  this  type  are 
taken  from  the  Greology  of  Wisconsin.^  They  are  of  samples  selected  by 
the  senior  author  with  direct  reference  to  the  richness  of  the  rock  in 
metallic  iron;  they  therefore  represent  in  the  main  the  more  ferruginous 
portions  of  the  belt,  or  rather  the  more  ferruginous  portions  having 
any  considerable  thickness.  The  original  samples,  having  been  selected 
for  an  economic  purpose,  were  made  by  breaking  small  pieces  from  all 
across  the  thickness  sampled.     Had  the  analyses  been  made  more  especially 

^The  more  highly  siliceous  jihases  of  this  class  of  rocks  are  spoken  of  by  Irving  iu  the  Wis- 
consin Reports  as  quartzites  (Geol.  of  Wis.,  1880,  vol.  iii,  pp.  118,  119,  120,  etseq.),  the  adjectives 
magnetitic,  hematitie,  etc.,  heing  prefixed  to  the  word  quartzite  to  indicate  the  special  phases.  The 
name  quartzite  was  thus  used  in  ignorance  of  the  fandamental  distinction  which  we  now  know 
holds  between  these  rocks  and  the  genuine  quartzites,  the  latter  having  been  shown  to  be  always  in 
the  main  composed  of  original  fragmental  material,  while  the  siliceous  constituent  of  the  rock 
now  especially  under  consideration  is  always  of  a  nonfragmental  nature,  having  been  solidified  in 
situ.  The  term  quartzite  is  used,  then,  throughout  this  volume  and  all  other  later  writings  of  the 
authors  to  designate  only  a  genuine  fragmental  rock  indurated  by  means  of  interstiti.al  deposition. 
On  PI.  XXII  of  the  Atlas  of  the  Geology  of  Wisconsin,  and  on  page  119  of  vol.  iii  of  that  work,  the 
Iron-bearing  member  is  represented  as  made  up  of  three  subdivisions,  a  basement  quartzite  50  feet 
thick,  a  series  of  magnetitic  schists  and  quartzites  800  feet  thick,  and  a  garnetiferoiis  actinolitic  schist 
10  feet  thick.  The  quartz  at  the  base  of  the  series,  although  highly  vitreous,  we  now  know  to  be  of 
a  completely  fragmental  character,  and  to  belong  to  the  underlying  Quartz-slate  member  rather 
than  to  the  Iron-bearing  member,  the  so-called  quartzites  of  which  prove  to  be  wholly  nonfrag- 
mental rocks. 

3  Vol.  Ill,  pp.  156-160. 


Till!;  IRONBEAHING  MEMBER. 


197 


for  petroyrapliic  purijciscs,  the  .samples  would  of  cours(}  have  been  selected 
in  a  flifferent  fashion.  Nevertheless,  as  taken,  they  represent  quite  fairly 
the  more  ferruginous  portions  of  this  phase  of  rocks,  one  (No.  IV)  repre- 
senting- a  less  ferruginous  portion.  (If  these  analyses,  I  represents  a 
thickness  of  40  feet  exposed  in  the  SW.  \  of  Sec.  17,  T.  44  N.,  R.  3  W., 
Wisconsin ;  II,  41  inches  in  the  NE.  I  of  Sec.  15,  T.  44  N.,  R.  3  W.,  Wis- 
consin; III,  19  feet  exposed  on  the  west  cliff  at  Penokee  gap,  NW.  ^  of 
Sec.  14,  T.  44  N.,  R.  3  W.,  Wisconsin;  IV,  7  feet  6  inches  immediately 
overlying  the  rock  represented  by  III ;  V,  10  feet  from  the  same  cliff;  VI, 
50  feet  exposed  in  the  NE.  4  of  Sec.  14,  T.  44  N.,  R.  3  W.,  Wisconsin; 
VII,  58  feet  in  the  SW.  4  of  Sec.  10,  T.  44  N.,  R.  2  W.,  Wisconsin ;  VIII, 
25  feet  in  the  SW.  i  of  Sec.  1,  T.  44  N.,  R.  2  W.,  Wisconsin. 

Analyses  of  magnetiUc  slates. 


I. 

n. 

ni. 

IV. 

V. 

VI. 

VII. 

VIII. 

Iron  sesquioside 

Iron  protoxide 

Silica 

36-414 

15-767 

39-532 

0-110 

1-516 

2-516 

3- 120 

tr.ice. 

0-421 

0-543 

49-435 
8-460 
33-894 
1-151 
3-160 
2-403 
0-337 

none. 

none. 
1-500 

42-897 

19-173 

31-838 

0-384 

1-373 

1-293 

1-126 

none. 

none. 

0-378 

15-335 
7-851 

34-770 
15-819 
42-896 
none. 
1-.330 
2-623 
1-726 
tr.ice. 
trace. 
0-471 

67-064 
8-332 
18-472 
0-305 
2-483 
2-280 
1-050 
0-127 
none. 
0-450 

41-241 
16-797 

40-420 
12-331 

1-025           1-139 

Magiieaium  oxide 

Mangauous  oxide 

Phosphoric  acid 

2-150 
0-193 
trace. 
0-160 
1-078 

.1-890 
0-553 

trace. 

none. 
2-559 

Total             

99-939 

100-336 

98-462 

99-635 

100-613 

100-541 

99-436 

It  will  be  seen  that  several  of  these  analyses  show  a  proportion  of  iron 
sesquioxide  beyond  that  which  is  required  by  the  protoxide  contents  indi- 
cated to  make  up  the  mineral  magnetite.  In  fact  the  excess  is  even  slightly 
greater  than  is  indicated  at  first  sight,  since  a  certain  amount  of  the  iron 
protoxide  must  be  assigned  to  the  amphibolic  mineral  which  is  always  • 
present.  Disregarding  the  small  correction  to  be  made  on  this  account  and 
considering  all  of  the  iron  protoxide  indicated  by  the  analyses  as  contained 
in  magnetite,  we  find  that  the  following  respective  proportions  of  magnetite 
and  hematite  are  indicated  for  the  several  samples.  I,  50'80  per  cent  and 
1-37  per  cent;  II,  27-26  and  30-63  ;  III,  50-56  and  11-40;  IV,  23-38  per 
cent  of  magnetite  and  no  hematite;   V,  50-38  per  cent  of  magnetite  and  no 


198  THE  PENOKEE  IRON-BEARING  SERIES. 

hematite;  VI,  27  per  cent  of  magnetite  and  48 "43  per  cent  of  hematite.  For 
VII  the  figures  are  54-12  and  3-91;  for  VIII,  39-73  and  13-01;  for  IX,  16-27 
and  28-83. 

Distribution  of  the  three  types  of  rock. — In  all  that  portion  of  the  Iron- 
bearing  member  which  lies  to  the  west  of  the  passage  through  the  Penokee 
range  of  Tylers  fork,  in  Sec.  33,  T.  44  N.,  R.  1  W.,  Wisconsin,  the  rocks  of 
the  third  type  or  actinolitic  and  magnetitic  schists  prevail ;  in  fact,  except 
that  the  rock  of  certain  exposures,  much  more  highly  quai-tzose  than  usual, 
approaches  the  cherts  of  the  second  type,  it  may  be  said  that  these  rocks 
are  the  only  ones  met  with.  At  the  gorge  of  Tylers  fork,  however,  the 
actinolite  has  almost,  though  not  quite,  sunk  out  of  sight,  while  the  siliceous 
constituent  is  altogether  the  most  prominent  one.  At  the  same  time  hema- 
tite and  brown  iron  oxides  begin  to  prevail  over  the  magnetite. 

To  the  east  of  Tylers  fork  the  first  and  second  types  greatly  pre- 
dominate, though  phases  carrying  minute  quantities  of  actinolite  continue 
to  occur  as  far  east  as  the  Potato  river.  Nevertheless,  these  are  very  rare, 
and  nowhere  in  this  portion  of  the  range  has  any  highly  actinolitic  rock  yet 
been  met  with.  Just  where  these  actinolitic  rocks  cease  it  is  difiicult  to  tell, 
but  they  can  not  contiime  far  to  the  east  of  Tylers  fork,  not  far  from 
which  stream  the  cherty  and  carbonated  rocks  of  the  first  and  second  types 
have  increased  to  such  an  extent  as  to  occupy  the  whole  of  the  iron  belt, 
the  frequent  occurrence  of  bodies  of  hematite  ore  at  the  base  of  the  member 
beginning  at  the  same  time  with  this  change.  For  some  30  miles  now  to 
the  east,  or  as  far  as  near  the  east  side  of  T.  47  N.,  R.  4.5  W.,  Michigan, 
the  cherts  and  carbonates,  with  iron  ore  bodies  at  or  near  the  base-  of  the 
formation,  continue  neai'ly  to  the  exclusion  of  the  actinolitic  rocks.  In  this 
distance  is  included  with  few  exceptions  all  of  the  working  mines  of  the 
Gogebic  districts.  The  somewhat  magnetitic  and  actinolitic  rocks  just  west 
of  Tylers  fork  include  one  mine,  and  the  somewhat  actinolitic  and  magnetitic 
rocks  in  T.  47  N.,  R.  45  W.,  Michigan,  include  several.  A  rude  subordinate 
stratigraphic  ai'rangement  in  the  iron  belt  appears  to  hold  for  this  distance ; 
i.  e.,  the  purer  carbonates  are  characteristic  of  the  lipper  horizons,  the  fer- 
ruginous slates  of  the  middle  horizons,  while  the  ferruginous  cherts  and  ore 
bodies  lie  within  the  lower  horizons.     It  is  not  meant  to  indicate  by  this 


Tin;  IKONBHAUlNCi   MKMIJEH.  199 

stiitonu'iit   that   tlic  tniusition    Ix'twocii    the    clicrty   phases    unci   the    car- 
bonates   is    a    reg-ulai-    or    sharp    one,    as    the    carbonates    sink  to  lower 
horizons    in    s<inie    phices,  wliiU^    the    cherts    rise    to    higher   in    others, 
bnt  merely  that  exposnres  and  the  numerous  tc^st  i)it  explorations  show 
that  there  is  this  general  arrangenient.     It  is  thought  that  this  arrange- 
ment has  a  connection  with  the  origin  of  the  ore  bodies,  for  which  an  expla- 
nation is  attempted  on  a  subsequent  page.      It  sliould  be  said  that  this 
arrangement  is  much  more  d(3iinitely  made  out  through  T.  47  N.,  R.  4G  and 
R.  47  W.,  Michigan,  than  it  is  farther  to  the  west  in  Wisconsin,  where  the 
explorations  have  been  fewer.     So  far  as  exposures  liave  been  found  west  of 
the  state  line,  they  indicate  the  continuity  of  this  succession  westward  to 
beyond  the  Potato  river  crossing.     The  succession  as  seen  in  T.  47  N., 
R.  46  W.,  Michigan,  may  be  described  more  fully  as  follows:  Beginning 
with  the  uppermost  portion  of  the  belt,  we  find  the  finely  laminated,  little 
altered  siderite  rocks  to  prevail.     As  these  are  crossed  to  the  south,  more 
and  more  red  hematite  replaces  the  iron  carbonate,  until  finally  little  or  no 
unoxidized  carbonate  remains.     Here  the  rock  is  a  reddish  slaty  one,-  often 
preserving  quite  perfectly  the  original  regular  lamination,  but  the  only  con- 
stituents are  more  or  less  hydrated  hematite  and  sihca.     Still  farther  south- 
ward the  lamination  becomes  less  and  less  distinct  and  regular;  the  amount 
of  sihca  rapidly  increases,  and  the  rock  passes  into  the  ferruginous  chert 
of  the  second  tjqDC  in  which  the  iron  oxide  is  contained  in  irregular  blotches 
and  noncontinuous  bands.     Finally,  at  the  base  of  the  formation  the  iron 
oxide  is  collected  often  into  large  bodies,  which  generally  lie  directly  against 
the  quartzite,  the  uppermost  horizon  of  the  Quartz-slate  member. 

Following  the  iron  belt  now  farther  eastward  into  T.  47  N.,  R.  45  W., 
Michigan,  another  change  is  met  with— the  actinolitic  and  magnetitic  rocks 
of  the  third  type  coming  in  plentifully.  At  first  these  occur  along  with 
both  the  ferruginous  olierts  and  carbonates,  but  as  one  passes  farther 
eastward  no  injore  carbonates  are  met  with,  the  actinolitic  schists  and  ferru- 
ginous cherts  only  being  found.  The  latter  in  turn  lessen  in  amount,  the 
'actinolitic  schists  being  more  and  more  plentiful  until  in  the  exposures 
near  the  Little  Presque  Isle  river,  in  T.  47  N.,  R.  44  W.,  Michigan,  they  are 
the  only  kinds  met  with.     This  transition  from  the  predominance  of  the 


200  THE  PElSrOKBE  lEON-BEAKING  SERIES. 

ferruginous  cherts  to  that  of  the  actinoHtic  type  is  a  very  gradual  one, 
extending  through  a  distance  of  10  or  12  miles. 

To  one  visiting  the  Iron-bearing  member,  only  at  two  or  three  points 
distantly  removed  from  one  another,  without  Examining  the  intervening 
portions — say,  for  instance,  at  Penokee  gap  and  in  the  vicinity  of  the 
mines  at  Bessemer,  Michigan — the  difference  between  the  rocks  which 
compose  it  might  be  so  striking  as  to  give  rise  to  a  doubt  of  the  actual  con- 
tinuity of  the  belt  between  the  two  places;  but  by  one  who  follows  it 
throughout  its  whole  length  no  such  question  for  a  moment  can  be  enter- 
tained. Mining  developments  and  natural  exposures  make  the  belt  practi- 
cally continuous  and  one  finds  a  gradual  transition  between  the  three  types 
of  rock,  which  take  place  only  in  the  most  gradual  manner.  Further,  he 
finds  immediately  beneath  the  Iron-bearing  member  the  Quartz-slate,  and 
immediately  above  it  the  Upper  slate.^ 

Microscopical  character  of  the  clierty  iron  carbonates  (PI.  xxi). — This 
type  of  rock  has  two  chief  constituents,  siderite  and  chert.  In  composition 
the  siderite  is  an  iron  carbonate,  bearing  more  or  less  of  calcium  and  mag- 
nesium and  not  infrequently  passes  into  a  ferrodolomite.  The  siderite 
varies  in  its  character  from  earthy  to  well  crystallized.  When  crystallized 
the  small  individuals  perfectly  interlock.  It  is  usually  more  or  less  impure, 
including  at. times  green  chlorite,  carbonaceoiis  matter,  and  occasionally 
numerous  minute  crystals  of  magnetite.  The  chert  making  up  the  other 
part  of  this  type  of  rock  is  in  part  amorphous.  It  is  that  variety  of  silica 
or  quartz  which  has  in  the  polarized  light  a  minute  spotty  appearance, 
due  to  exceedingly  small  individuals  of  quartz,  mingled  with  more  or  less 
of  silica  which  is  apparently  amorphous.  Hornstone  or  flint  gives  in  thin 
section  the  same  appearance;  and  the  chert  of  the  iron  carbonates  and 
hornstone  have  a  very  close  microscopic  resemblance.  That  a  portion  of 
the  silica  is  really  amorphous,  as  indicated  by  the  a2Dpearance  of  the  section 

'  At  the  time  the  Wisconsin  geological  survey  of  this  district  was  made,  ,there  had  been  no 
prospecting  along  the  east  end  of  the  Wisconsin  part  of  the  range,  and  the  exposures  hetween  the 
Potato  and  Montreal  rivers  are  very  sparse ;  yet  with  these  few  exposures,  Ibehle  magnetic  attraction, 
and  the  assistance  of  topography,  the  survey  was  ahle  to  locate  so  accur.ately  the  Iron-bearing  mem- 
ber that  within  it  or  very  close  to  it  are  all  the  iron  mines  yet  discovered  upon  the  Wisconsin  side  of 
the  boundary.     (See  PI.  xxvi.  Atlas,  Geology  of  Wisconsin.) 


THE  IKON-BEARING  MEM  HER.  201 

under  the  mii-roscopo,  is  furtlici-  shown  \,y  its  mid}-  soliiljility  in  caustic 
alkalies.  Nowhere  iloes  this  elicit  present  a  concretionary  or  brecciated 
appearance. 

The  two  constituent  minerals  occur  sometimes  in  solid  hands,  alter- 
nating- with  each  other,  and  interstratified  with  these  are  other  bands  com- 
posed in  o-reater  or  less  proportion  of  the  chert  and  siderite.      Sometimes 
the  bands  of  siderite  (contain  a  little  chert  (PI.  xxi,  Fig.  3),  and  the  nearly 
piu-e  bands    of  chert  frequently  contain  more  or  less  of  sidente,  the  indi- " 
viduals  of  this  mineral  generally  being  in  a  regular  rliombohedral  form.    In 
the  unaltered  rocks  the  chert  can  no  more   be  said  to  be  a  background  ijj 
which  the  siderite  has  crystallized  than  the  reverse.     Apparently,  as  a 
result  of  changing  conditions,  regular  alternations  of  siderite  and  chert  and 
A'arious  mixtures  of  the  two  have  followed  one  after  the  other.     Sometimes 
the  layers  of  solid  siderite  are  of  considerable  thickness;  the  same  is  true 
to  a  less  degree  of  the  cherty  layers.     These  rocks  appear  to  be  in  essen- 
tially their  original  condition.     If  there  has  been  any  change  since  they 
were  formed,  there  is  no  evidence  of  it,  and  for  our  present  purposes  they 
must  be  regarded  as  original  rocks.     It  is  not  impossible  that  they  have 
undergone  great  changes,  but  if  so  we  are  unable  to  find  any  clew  as  to 
their  nature. 

From  these  apparently  unaltered  rocks  of  the  first  type  there  are  gra-^ 
dations  to  those  of  the  first  phase  of  the  second  type.  The  first  alteration 
to  which  these  rocks  are  subject  is  an  oxidation  of  the  carbonate  producing 
brown  hydrated  hematite,  red  hematite,  or  magnetite.  Very  frequently  the 
decomposition  of  the  iron  carbonate  has  not  changed  the  forms  of  the 
original  crystals,  and  thus  leave  the  various  oxides  as  perfect  pseudomorphs 
after  the  iron  carbonate.  (PI.  xxi,  Fig.  4.)  All  phases  of  this  change  to  each 
of  the  iron  oxides  is  exhibited  by  the  various  sections.  Frequently  upon 
one  side  of  the  same  section  is  unaltered  iron  carbonate,  and  upon  the  other 
side  iron  oxides  alone,  psendomorphous  after  the  iron  carbonate.  Between 
the  two  are  all  stages  of  the  change.  The  formation  of  magnetite  pseudo- 
morphs after  the  carbonate  is  much  less  common  than  that  of  the  brown 
hydrated  hematite  and  the  red  hematite.  In  the  siderites  which  were  moi'e 
nearly   pure,    this    alteration,    completely  carried   out,  produces  the  lean 


202  THE  PENOKBE  lEON-BEARING  SEEIBS. 

ferruginous  slates  wliicli  are  one  of  the  characteristic  rocks  of  the  second 
type  below  described.  Accompanying  this  oxidation  of  the  siderite  is 
generally  a  rearrangement,  and  apparently  often  an  introduction  of  silica 
from  extraneous  sources.  The  silica  in  these  altered  rocks,  instead  of  being 
the  spott)^  chert,  characteristic  of  the  unaltered  rock,  is  frequently  more 
coarsely  crystalline,  and  often  in  combination  with  the  iron  oxides  has  a 
concretionary  and  brecciated  appearance.  When  this  rearrangement  and 
introduction  of  silica  is  earned  to  the  extreme,  with  accompanying  changes 
in  the  iron  oxide,  the  cherty  iron  carbonate  passes  into  the  second  phase  of 
the  second  type  of  rocks — the  ferruginous  cherts. 

The  numerous  veins  and  fissures  which  cut  through  the  rock  form  a 
characteristic  feature  of  the  iron  carbonates.  These  veins  are  of  greatly 
varying  widths,  one  of  them  frequently  breaking  into  several  smaller  veins, 
and  in  some  cases  the  sections  are  cut  by  a  system  of  ramifying  veins. 
They  are  generally  composed  of  the  same  minerals  which  make  up  the 
section  in  which  they  are  contained;  that  is,  chert,  siderite,  and  some- 
times chlorite.  Upon  the  whole,  silica  is  nmch  the  more  abundant  filling. 
This  silica  is  frequently  easily  separated  from  the  remainder  of  the  silica 
in  the  sections  by  being  coarsely  crystallized,  oi*  by  having  the  radial 
fibrous  arrangement  of  chalcedony,  and  in  including  little  iron  oxide.  Not 
■unfrequentl}'^,  however,  the  veins  contain  a  large  amount  of  siderite.  This 
siderite,  like  the  silica,  is  usually  purer  and  more  coarsely  crystallized  than 
the  original  siderite. 

Microscopical  character  of  the  ferruginous  slates  and  ferruginous  cherts 
(PI.  xxii). — The  general  macroscopic  characters  of  this  type  of  rock  have 
been  indicated  above.  It  has  also  been  said  already  that  they  are  in  the 
main  composed  of  iron  oxides  and  silica  of  a  nonfragmental  nature ;  that 
the  rocks  of  this  type  present  a  gradual  transition  into  those  of  the  third 
type  by  an  increase  in  the  amount  of  actinolite  and  magnetite,  and  that 
similarly  they  present  transitions  into  the  carbonates  of  the  first  type  and 
into  the  hematites  of  the  iron-ore  todies  themselves.  In  additiou  to  the 
predominating  siliceous  groundmass,  there  is  always  present  more  or  less 
iron  oxide,  which  may  be  magnetite,  hematite,  or  the  hydrated  oxide  of 
iron,  or  two  or  all  three  of  these  together.     Accessory  to  these  two  prime 


TIIK   IKON-BEAKING  MKMBEK.  20'A 

constituents,  ;irc  (Ictcctcd  under  tlic  microscope  iron  ciirbonate,  wliich 
occurs  \('r\-  tViHjxiontly ;  actinolite,  which  is  of  less  frequent  occuiTence; 
(•iirhonaceous  matter,  and  chlorite,  these  two  being  of  nnnsual  occurrence. 
'I'lic  predominating-  silic{>ous  gronndmass  of  these  rocks  varies  from  very 
minutely  thougli  completely  crystalline  to  jjartl}'  amorphous  (PL  xxii,  Fig. 
2),  often  presenting  the  intermediate  condition  characteristic  of  chal(;edony. 
In  some  sections'none  of  the  chalcedonic  or  amorphous  phase  of  silica  is 
present,  but  in  others  all  three  of  these  phases  occur  together.  In  general 
there  is  in  these  rocks  a  much  greater  tendency  towai'd  the  crystalline 
kind  of  silica  than  in  those  of  the  first  type,  the  chert}'  iron  carbonates ; 
but  as  compared  with  the  silica  in  the  rocks  of  the  third  tjqoe,  or  actinolitic 
slates,  there  is  a  greater  tendenc}^  toward  the  noncrystalline  kinds.  It  has 
been  shown  that  the  actinolitic  slates  have  their  main  distribution  in  the 
western  portion  of  the  iron  belt,  occurring-  again  in  considerable  develop- 
ment far  to  the  east,  while  in  the  intervening  space  the  ferruginous  cherts 
and  carbonates  have  their  great  development.  Correspondingly  those 
phases  of  the  ferruginous  cherts,  which  in  tlieir  content  of  small  quantities 
of  magnetite  and  actinolite  present  us  with  a  gradation  into  the  actino- 
litic slate  type,  are  fouiid ,  particularly  toward  the  west  as  one  approaches 
the  region  of  actinolitic  slates  proper,  and  again  at  the  extreme  east  as  the 
actinolitic  rocks  of  that  area  are  reached.  Accompanying  the  presence 
of  magnetite  and  actinolite  in  these  gradation  phases  is  usually  found  the 
most  completely  crystalline  condition  of  the  siliceoixs  groundmass. 

The  ferruginous  cherts,  as  indicated  in  the  general  description,  pre- 
sent two  widely  different  phases  ;  the  ferruginous  slates  in  which  tlie  iron 
oxides  and  silicates  are  quite  uniformly  mingled,  and  rocks  in  which  the 
iron  oxide  is  concentrated  to  a  greater  or  less  degree  in  bands,  rings,  and 
shots,'  leaving  the  silica  comparatively  or  almost  wholly  free  from  ii-on 
oxide.  .  The  rocks  of  the  first  phase  are  composed  of  intimately  mingled 
chert  and  brown,  somewhat  hydrated  hematite,  red  Hematite,  and  occa- 
sionally magnetite.  In  the  ferruginous  slates  there  has  also  been  a  concen- 
tration of  the  iron  oxide  to  a  small  extent  in  layers.  At  times  these  layers 
are  very  regular  ones,  between  which  alternate  layers  of  chert,  containing 
comparatively  little  oxide  of  iron.     From  these  perfectly  laminated  phases, 


204  THE  PBNOKBB  IKON  BEARING  SERIES, 

wliicli  are  as  regular  as  any  of  the  slaty  iron  carbonates  of  the  first  type  of 
rock,  the  specimens  vary  to  those  in  which  the  thin  section  shows  appar- 
ently no  proper  lamination,  although  in  hand  specimen  there  is  always 
some  evidence  of  stratification.  In  these  laminated  phases  the  chert  may 
be  a  background  for  the  iron  oxide  or.  the  reverse,  depending  upon  which 
is  predominant. 

The  chert  in  the  ferruginous  slates  varies  from  finely  crystalline  to  the 
very  fine  spotty  quartz  mingled  with  amorphous  silica,  characteristic  of  the 
first  type  of  rock.  When  the  quartz  is  of  the  more  coarsely  crystalhne 
kind  the  sections  are  often  cut  by  veins  of  silica.  In  these  cases  the  sec- 
ondaiy  nature  of  a  portion  of  the  silica  at  least  is  indicated  by  the  fact  that 
it  does  not  always  lie  directly  parallel  to  the  lamination,  but  breaks  across 
the  more  ferruginous  bands  in  little  veinlets,  while  various  singular  depar- 
tures from  the  regularity  of  the  lamination  indicate  the  same  thing.  Usually 
the  quartz  shows  little  or  no  indication  of  a  concretionary  or  brecciated 
nature.  The  iron  oxide  is  generally  of  the  brown  somewhat  hydrated 
hematite.  Occasionally  the  hematite  is  bright  red,  when  the  rock  becomes 
a  jasper.  These  jaspery  portions  are  not  usually  in  any  great  thicknesses. 
Sometimes  the  rocks  are  quite  reguilarly  laminated,  but  often  the  jaspery 
parts  are  in  the  shape  of  noncontinuous  fine  laminaj.  Less  frequently  the 
oxide  of  iron  is  in  part  magnetite. 

The  iron  oxide  is  present  in  irregular  areas,  and  frequently  is  in  suffi- 
cient quantity  to  form  continuous  ramifjdng  areas  in  which  the  chert  is 
buried.  In  a  portion  of  the  specimens  little  or  no  iron  carbonate  remains,  or 
the  iron  oxide,  either  hematite  or  hydroxide,  may  present  itself  as  mere 
stains  in  the  carbonate,  replying  the  carbonates  in  varying  degrees,  until 
finally  an  entire  crystal  or  bunch  of  crystals  of  that  mineral  is  changed  to  the 
oxides.  Rhombic  crystal  sections,  composed  of  oxides  of  iron,  are  to  be 
.found  in  nearly  all  of  the  sections.  In  quite  a  good  many  cases  all  of  the 
iron  oxide  of  a  sectit)n,  or  nearly  all  of  it,  will  present  itself  in  these  rhombic 
shapes.  More  often  the  rhombs  will  be  perceptible  only  on  the  edges  of  the 
iron  oxide  areas,  the  middle  portions  of  these  aggregates  being  too  compact 
to  allow  of  their  recognition.  The  carbonate  itself  is  found  in  more  than 
half  of  the  sections  examined,  and  in  nearly  all  of  the  remainder  its  former 


THE  IRON-HEARING  MEMMER.  205 

presence  is  indicated  by  these  rhombic  crystal  sections.  Tlie  inag'netite, 
wlion  present,  is  commonly  rhombic  in  outline.  Such  rliombic  outlines 
may,  of  course,  ])e  produced  by  random  sections  of  the  ordinary  niagnetite 
octahedra,  l)ut  the  (piestion  has  su<>-<^ested  itself  as  to  whether  it  is  not 
])ossible  that  these  rhombic  magnetite  sections  express  in  their  shape  the 
outlines  of  carbonate  crystals.  In  some  cases  such  magnetite  crystals  with 
similar  shaped  sections  of  carbonate  and  hematite  are  found  in  the  same 
rock. 

In  one  direction  these  rocks  may,  then,  be  traced  into  those  of  the  first 
type.  In  the  reverse  direction  by  concentration  of  the  iron  oxides  and  by 
the  development  of  a  concretionary  and  brecciated  character  this  phase 
passes  into  the  second  phase  of  the  second  type  of  rock. 

In  the  ferruginous  cherts  the  iron  oxides  are  often  concentrated  more 
or  less  into  regular  bands,  but  besides  these  bands  there  are  many  oval  or 
spherical  bodies  of  iron  oxide,  so  that  the  specimens  are  best  described  as 
cherts  containing  bands  and  shots  of  ore.  These  shots  occur  in  cavities  in 
the  cherts  and  often  they  do  not  entirely  fill  them.  In  such  cases  the  iron 
oxide  is  usually  lined  with  crystals  of  quartz.  The  structure  is,  upon  a 
-small  scale,  that  of  a  geode,  the  cavity  of  which  has  a  layer  of  iron  oxide, 
and  within  this  quartz  crystals.  Even  when  the  cavities  are  completely 
filled  with  iron  oxide  the  same  similarity  to  a  geodic  structure  is  apparent. 
It  would  seem  that  the  cavities  formed  at  some  stage  in  the  development 
of  the  rock  (perhaps  by  a  solution  of  a  part  of  the  iron  carbonate  at  the 
time  another  part  was  oxidized,  or  else  by  solution  of  silica),  were  subse- 
quently partly  or  completely  filled  with  iron  oxide,  after  which,  if  space 
remained,  followed  silica.     (PI.  xxiii.  Figs.  1  and  2.) 

In  this  phase  of  rock  the  arrangement  of  the  constituent  particles  is 
often  closely  similar  to  that  which  has  been  described  on  a  preceding  page 
as  characterizing  the  chert  of  the  Limestone  member;  tliat  is,  it  often  pre- 
sents a  more  or  less  perfect  concretionary  arrangement,  but  in  the  limestones' 
and  cherts  this  is  at  times  exceedingly  vague.  However,  the  concretions 
are  so  numerous  as  to  be  one  of  the  most  important  characteiistics  of  this 
phase  of  rock.  (PL  xxii,  Figs.  1  and  2.)  The  concretionary  structure  affects 
both  the  iron  oxide  and  chert,  although  it  is  most  clearly  made  out  by 


206  THE  PENOKEE  IRON-BEARING  SERIES. 

means  of  the  iron  oxide.  In  some  cases  the  iron  oxide  which  marks  these 
concretionary  areas  is  so  plentiful  as  to  render  them  nearly  or  quite  opaque. 
From  such  extreme  cases  there  is  every  gradation  to  those  sections  in 
which  only  an  exceedingly  minute  amount  of  iron  oxide  remains  to  separate 
these  areas  from  the  interstitial  silica,  while  there  are  not  unfrequent  cases 
in  which  even  a  minute  amount  of  iron  oxide  is  absent.  The  outlining  of 
the  areas  is  then  perceived  only  in  the  polarized  light,  its  silica  being  either 
nearer  to  or  farther  from  the  amorphous  condition  than  that  portion  of  the 
matrix  immediately  in  contact  with  it. 

The  iron  oxide  which  designates  the  concretions  from  the  matrix  pre- 
sents itself  in  the  shape  of  a  general  stain,  composed  of  minute  particles, 
distributed  in  the  shape  of  a  mere  border  or  in  concentric  rings.     It  may 
be  any  of  the  three  oxides — magnetite,  hematite,  or  a  brown,  somewhat 
hydrated  oxide,  or  a  mixture  of  two  or  three  of  them  together.     In  cases 
where  the  iron  oxide  is  magnetite,  as  in  the  cherts  which  occur  between 
Tylers  fork  and  Sec.  10,  T.  45  N.,  R.  1  E.,  Wisconsin,  it  may  present  itself 
either  in  the  shape  of  an  exceedingly  fine  dust  or  as  sharply  outlined  crys- 
tals of  some  little  size,  and  these  crystals  are  not  unfrequently  arranged 
around  the  edges  of  the  concretionary  area,  their  sharp  angles  projecting 
from  its  outline.     (PI.  xxviii.  Fig.  2.)     It  is  not  to  be  understood  that  the 
iron  oxides  are  completely  lacking  in  the  interstitial  material;  on  the  con- 
'  trary,  they  are  often  present  either  in  minute  stains  or  aggregations  of  par- 
ticles; but  the  rule  is  that  they  are  more  plentiful  in  concretions  than  in  the 
matrix,  while  in  many  cases  the  matrix  appears  to  be  almost  wholly  devoid 
of  them.     A  concretion  will  often  be  sharply  defined  only  along  a  portion  of 
its  outline,  the  remainder  being  exceedingly  vague.     This  arises  at  times 
from  lack  of  sufficient  iron  oxide  stain  to  diff'erentiate  the  concretions  from 
the  matrix,  while  the  silica  of  both  may  be  so  closely  of  the  same  degree 
of  crystallization   as   not   to   help  in  the  definition  when  the   section  is 
examined    with   the  polarized  light.     In  some  sections  the  outlines  of  the 
concretions  are  of  such  a  nature  as  to  suggest  very  strongly  their  having 
been  partly  dissolved  away,  while  in  many  cases  veinlets    of   purer  and 
differently  crystallized  silica  of  the  matrix  enter  into  the  mass  of  the  concre- 
tions.    (PI.  xxii,  Fig.  3;  PI.  XXIII,  Figs.  1  and  2;  PI.  xxv,  Fig.  3.)     These 


THE  IRON-BE AKING  MEMBER.  207 

veinlots  have  very  varying  extents,  and  have  often  severed  concretions  into 
several  frag'uients.  In  the  concretions  the  iron  oxide  is  arranged  in  concentric 
oval  or  spherical  bands,  and  this  is  efjually  trne  whether  the  iron  oxide  is 
limonite,  hematite,  or  magnetite.  This  iron  oxide  is  arrang-ed  without  the 
slio-htest  reference  to  the  individuals  of  quartz  cuttiaig  tln-ouyh  them  in  the 
most  indiscriminate  manner.  The  concretionary  structure  aifects  the  silica 
both  in  concentric  arrano-ement  and  coarseness  of  individuals.  It  is  quite 
common  for  the  individuals  of  quartz  to  be  larger  in  the  concretions  than  in 
the  cherty  matrix.  (PI.  xxii,  Figs.  1  and  2.)  Rarely  these  concretions  have 
nuclei  from  extraneous  sources,  as,  for  instance,  small  particles  of  fragmental 
quartz,  of  a  comparatively  large  individual  or  a  cluster  of  individuals 
of  quartz,  or  else  of  iron  oxide,  but  ordinarily  they  have  none.  Some- 
times the  concretions  are  so  closely  clustered  that  in  their  growth  they 
interfere,  and  in  such  cases  two  or  more  concretions  are  used  as  a  nucleus 
about  which  the  bands  of  iron  oxide  arrange  themselves  concentrically, 
thus  forming  compound  concretions.     (PI.  xxvii,  Fig.  3.) 

A  less  prevalent  characteristic  of  these  rocks  than  the  concretions 
consists  in  the  extraordinary  brecciated  appearance  which  they  present. 
(PI.  XXII,  Figs.  2  and  3  ;  PI.  xxiii.  Figs.  1  and  2.)  A  similar  appearance  has 
already  been  noted  as  characterizing  the  chert  of  the  limestone.  The  out- 
lines of  the  fragmental  areas  are  very  commonly  more  or  less  sharply 
angular,  while  frequently  convexities  and  projections  in  the  outline  of  a 
detached  fragmental  area  correspond  to  concavities  and  recessions  of  out- 
line of  another  fragment  in  such  a  manner  as  to  demonstrate  a  former 
continuity  of  the  two.  The  concretionary  areas,  in  which,  it  should  be 
remembered,  silica  is  still  the  main  ingredient,  occiu-  along  with  the  frag- 
ment-like areas,  while  between  the  two  there  is  found  such  a  complete  series 
of  gradations  that  it  is  impossible  to  resist  the  conclusion  that  in  many 
cases  both  are  of  the  same  origin,  only  the  brecciated  phases  have  been 
shattered  by  dynamic  movements. 

The  great  vai-iety  of  forms  presented  by  the  different  sections  fortu- 
nately relate  the  history  of  these  concretionary  and  brecciated  areas.  (PI. 
xxvii.)  In  the  first  place,  entirely  unaltered  areas  of  iron  carbonate  are 
found  associated  with  these  concretions.     Other  iron  carbonate  areas  lying 


208  THE  PENOKEB  IRON-BBAEING  SERIES. 

within  the  usual  sihceous  grounclmass  may  be  cut  by  ramifying  veinlets  of 
sihca.  Again,  areas  are  seen  in  which  curving  Unes  of  iron  oxide  have  been 
developed  at  or  near  their  edges.  These  lines  may  or  may  not  complete 
a  loop.  If  the  outline  of  the  original  carbonate  area  is  more  or  less  irreg- 
ular and  angular,  the  iron  oxide  curve  cuts  off  the  irregularities.  In  other 
cases,  again,  several  such  complete  or  partial  iron  oxide  lines,  concentric 
with  one  another,  have  been  developed.  Subsequent  tct  or  alternate  or  sinml- 
taneous  with  this  process  silica  appears  within  the  concretion,  filling  the 
spaces  between  the  rings  of  iron  oxide,  the  iron  carbonate  being  previously 
removed.  At  other  times  the  space  occupied  by  the  iron  carbonate  is  left 
vacant,  or  else  only  partially  filled,  whence  arise  the  frequent  geodic  arid 
other  cavities  found  in  this  phase  of  rock.  Finally,  no  iron  carbonate 
remains,  its  place  being  taken  by  the  concentric  rings  of  iron  oxide  and  the 
minutely  crystalline  to  amorphous  silica.  Between  the  unaltered  iron  car- 
lionate  areas  and  the  perfectly  formed  concretions  there  are  at  times,  often  in 
a  single  section,  every  possible  gradation.  If  the  iron  oxide  developed  from 
the  carbonate  was  very  plentiful  but  little  room  was  left  for  the  entering 
silica,  and  these  areas  appear  now  nearly  or  quite  opaque  with  oxide,  but 
generally  the  iron  oxide  has  been  developed  in  the  partial  or  complete 
ovals  referred  to,  and  the  substitution  of  silica  for  the  remaining  carbonate 
left  these  lying  within  a  siliceous  background. 

Quite  similar  has  evidently  been  the  process  by  which  the  pseudo- 
fragmental  areas,  such  as  shown  in  PI.  xxiii,  Figs.  1  and  2,  have  been 
produced.  In  such  cases  the  silica  of  the  background  has  plainly  also  been 
rearranged,  so  that  it  resembles  and  merges  into  the  silica  of  the  concre- 
tionary areas,  thus  giving  the  vague,  irregular  outlines  which  make  these 
forms  so  closely  resemble  fragments.  More  rarely  the  fragmental  areas 
are  tabular  in  form,  or  else  show  a  subordinate  parallel  lamination  which 
appears  to  be  lines  of  original  deposition.  The  laminae  of  these  areas 
frequently  abruptly  terminate  at  the  exteriors  of  the  apparent  fragments. 
A  number  of  such  areas  sometimes  occur  in  a  single  section,  now  lying 
within  the  siliceous  groundmass  in  such  positions  as  to  have  the  laminae  of 
the  different  areas  make  all  sorts  of  angles  with  each  other.     In  such  cases 


THE  IRON-BEARIXG  :\rE^rBER.  209 

the  ureas  must  Ix-  lidicvcd  lo  \h;  real  iVaoinciUs  wliidi  Ii.lvc  IoihumI  citlifr 
111  siiii  liy  ail  actual  local  brecciation  of  tlii^  rock  caused  hv  clicinical  or 
inwJiHuicMl  action,  or  tlicy  may  he  cousiderod  as  liaviii<i'  'h'<'Ii  derived  from 
tlie  iiuiuediakdy  underlyiiif>'  layer  of  nonfraj^-iiieiital  rock  by  a  tem))orary 
miii.iiliii.i-- of  fra^iiieiital  and  iioufrag-iucutal  de[)o.sition.  Tliat  tlie  former  is 
the  true  ex[)lauatioii  of  tlu'ir  oriyiu  we  liave  litth-  d<)\\\)t. 

It  is  exeeediui^ly  diilicult  to  convey  in  any  general  description  a  o'ood 
idea  of  the  nniltituihnous  phases  presented  by  this  second  type  of  rock,  ))ut 
some  ftn-ther  and  more  accurate  conception  may  be  obtained  Ijy  an  exami- 
nation of  the  accompanying  carefully  described  plates  and  study  of  the 
detailed  descriptions  of  individual  sections  given  below. 

In  connection  with  the  second  type  of  rock,  it  is  necessary  to  allude  to 
some  peculiar  phases  which  resemble  on  the  one  hand  the  breceiated  rocks 
described  above,  and  upon  the  other  the  true  fragmental  rocks  of  the 
underlying  Quartz-slate  member.  In  them  there  has  evidently  been  a 
mingling  of  mechanical  and  chemical  sedimentation.  They  contain  simple 
grains  of  quartz  which  have  been  enlarged,  the  fragmental  character  of 
which  can  not  be  doubted.  These  same  sections  also  contain  rounded  chert 
areas,  which  include  sometimes  little  iron  oxide,  sometimes  abundant  red  . 
hematite,  making  them  jasper,  and  sometimes  both  hematite  and  magnetite. 
Often  the  areas  containing  little  or  no  iron  oxide  and  those  containing  it 
abundantly  are  in  juxtaposition.  Simple  fragmental  grains  of  quartz  and 
the  chert  areas  are  generally  arranged  with  their  longer  axes  in  a  common 
direction.  The  fragmental  character  of  the  simple  quartzes,  the  likeness  to 
them  in  form  of  the  chert  areas,  and  their  an-angement  with  longer  axes  in 
a  common  direction,  are  almost  conclusive  proof  that  these  parts  of  the  rocks 
are  genuine  mechanical  sediments.  Tlie  fragments  are  cemented  by  a 
matrix,  which  is  a  ferruginous  chert  in  every  respect  similar  to  the  matrices 
of  the  ferruginous  cherts  of  the  second  type  of  rock. 

These  rare  occurrences  of  semifragmental  rocks  are  interstratified 
with  others  which  are  typical  forms  of  the  Iron-bearing  member.  The 
meaning  of  this  interlamination  of  the  two  classes  of  sediments  will  be  con- 
sidered later. 

MON  XIX 14 


210  THE  PENOKEE  IKON  BEARING  SERIES. 

Microscopical  character  of  the  actinolitic  slates  (PI.  xxiii,  Figs.  3  and  4; 
PL  XXIV,  Figs.  1  and  2 ;  PI.  xxviii). — As  already  stated,  the  actinolitic  rocks 
of  the  third  type  above  macroscopically  described  have  as  their  four  main 
constituents,    quartz,    hematite,    magnetite,  and    actinolite.     In  the   large 
exposures  all  of  these  minerals  are  invariably  found.     In  hand  specimens 
any  one  of  them  may  be  found,  to  the  almost  complete  exclusion  of  the 
others,  and  also  all  possible  combinations  of  any  two  or  three  of  them,  and 
of  all  together.     The  order — quartz,  iron  oxide,  and  actinolite — expresses 
their  usual  i-elative  abundance,  taking  the  exposures  as  a  whole.    However, 
in  the  thin  section,  as  would  of  course  follow  from  the  foregoing,  any  one 
of  the  three  may  at  times  be  the  predominant  constituent,  while  also  any 
one  or  two  of  them  may  be  almost  or  (juite  wanting.     In  o;-der  of  time  of 
crystallization,  the  iron  oxide  is  always  the  earliest  of  these  constituents, 
next  the  actinolite,   and  finally  the  quartz.     Yet  while  this    is  true  as  a 
general  statement,  it  is  not  meant  to  imply  that  the  crystallization  of  any 
one  constituent  is  finished  before  that  of  the  second  one  begins,  for  they 
are,  to  a  certain  extent,  simultaneous.     The  areas  of  iron  oxide  are  the 
freest  from  inclusion  of  the  other  two  chief  constituents.     The  actinolite 
contains  large  quantities  of  iron  oxide,   but    everywhere  penetrates  the 
quartz,  a  single  needle  of  actinolite  commonly  cutting  two  or  more  indi- 
viduals of  quartz,  and  finally  the  quartz  individuals  include  much  magnetite 
and  actinolite.     However,   each  one  of  the  minerals  is,  in  certain  cases, 
found  to  include  both  of  the  other  two.   In  addition  to  these  three  main  con- 
stituents, chlorite  and  biotite  are  found  as  extensive  alteration  products  of 
the  actinolite.     The  alteration  products,  chlorite  and  bio,tite,  are  always  in 
minute  flakes,  several  or  many  individuals  of  these  minerals  forming  from 
a  single  needle  or  blade  of  actinolite.     The  magnetite  and  hematite  are 
directly  associated  and  probably  formed  at  the  same  time.     Occasionally 
the  hematitic  ingredient  is  manifest  macroscopically  by  the  joeculiar  steely 
luster  characteristic  of  specular  iron,  or  by  a  purplish  tint  to  the  powder  of 
very  highly  ferruginous  varieties;  but  the  analyses  quoted  above  show  that 
this  ingredient  is  at  times  contained  when  neither  microscopic  nor  macro- 
scopic characters  would  suggest  its  presence.     It  appears  to  be  intimately 
associated  with  the  magnetite,  and  occurring  in  aggregates  of  very  minute 


THE  IKON-BEARING  MEMBER.  211 

metallic  lustered  |iai'ticli's,   ii    fails  lV('(|iicnlly  to  <>;'\vi'  the  reddish  translu- 
cency  ordinarily  dcscrilxMl  as  a  characteristic  of  it.' 

Of  the  tlircc  i)rinci|)al  mineral  constitnents  named,  tjie  ((nartz,  or 
silica,  presents  itself  in  a,  j^'cneral  way  as  a  very  minntely  crystalline 
i;ronndinass  or  hackf^^-round  in  which  the  other  minerals  are  contained. 
Only  rarely  is  the  crystallization  of  this  (^nartz  so  coarse  as  indicated  in  PI. 
XXVIII,  Fig-.  4,  in  which,  however,  the  individuals  are  still  small,  being 
greatly  magnified.  On  the  other  hand,  it  only  rarely  sinks  to  that  exces- 
sive degree  of  fineness  which  is  cliaractei'istio  of  the  chalcedoiiic  form  of 
silica.  Still,  a  few  sections  have  been  found  in  which  it  does  reach  this 
excessively  tine  condition,  and  even  becomes  quite  amorphous;  but  tliese 
conditions  are,  as  seen,  far  more  cliaractei-istic  of  the  cherty  rocks  of  the 
type  previously  described.  Since  these  two  types,  however,  manifestly 
grade  into  one  another,  it  is  evideiat  enough  that  all  of  the  silica  of  the 
three  types'  is  of  the  same  origin;  that  is  to  say,  is  all  water  deposited. 
Even  where  least  minutely  crystalline,  as  in  some  of  the  actinolitic  slates, 
the  individuals  of  quartz  interlock  with  one  another  in  such  a  fashion  as 
to  place  their  deposition  in  situ  quite  beyond  question.  Nowhere  is  there 
any  indication  of  a  fragmental  origixi  for  any  of  the  siliceous  groundmass. 
Cases  have  been  noted  of  the  occurrence  of  genuine  fragments  of  quartz 
within  the  nonfragmental  mass  of  the  rocks  of  the  first  and  second  types, 
but  these  fragments  are  always  sharply  defined  and  wholly  distinct  in 
character  from  the  prevalent  nonfragmental  silica. 

* 

The  magnetite  and  hematite  occur  partly  in  the  form  of  an  excessively 
fine  dust  contained  within  the  quartz  and  actinolite  individuals,  partly  in 

'A  single  individual  of  hematite  is  cliaracterized  in  the  thin  section  ordinarily  by  a  reddish 
color  in  transmitted  light,  and  lack  of  metallic  luster  in  reflected  light;  but  wheu  the  individuals  are 
exceedingly  small,  an  aggregate  of  hematite  may  readily  present  the  metallic  luster,  and  unless  thin- 
ner than  the  ordinary  thin  section  is,  may  at  the  same  time  appear  perfectly  oiiaque  in  transmitted 
light,  in  which  case,  if  mingled  with  magnetite,  its  presence  is  difficult  to  detect  with  (lie  microscope. 
In  order  to  test  this  question  a  number  of  thin  suctions  were  made  of  specular  hematites  from  various 
places  in  the  lake  .Superior  region.  Some  of  these,  after  powdering,  yielded  nothing  at  all  to  the 
magnet,  while  others  contained  more  or  less  magnetite  intermingled  with  the  hematite.  In  all  cases, 
however,  where  the  grain  is  sufficiently  fine,  it  was  found  that  the  heiuatite  presented  the  same 
metallic  luster  in  the  reflected  light  and  opacity  in  transmitted  light  as  the  magnetite.  Many  of 
these  thin  sections,  when  the  ore  contained  no  magnetite  whatever,  presented  in  the  transmitted 
light  a  mixture  of  red  trauslucent  and  black  opaque  material,  tMs  opaque  niaterial  giving  the  metaj- 
]ic  luster  in  the  reflected  light, 


212  THE  PENOKEE  IKON-BEAEING  SERIES. 

the  shape  of  single  crystals,  or  groups  of  crystals,  of  moderately  large  size, 
and  partly  in  the  irregular,  oval,  opaque  bunches  of  crystals  above  men- 
tioned.     The   outlines   of    the  single   crystals   are   usually   very   sharply 
defined,  square,  hexagonal,   and  rhombic  outlines  presenting  themselves 
constantly.     On  the  whole  the  rhombic  outline  is  the  most  common.     In 
the  most  regularly  slaty  phases  of  the  actinolitic  schists  the  iron  oxides  are 
found  to  be  aggregated  much  more  plentifully  in  certain  bands  than  in 
others,  the  individual  laminse  of  these  slaty  rocks  presenting  in  turn  each  of 
the  three  main  constituents  as  the  predominant  one.     PI.  xxviii.  Fig.  4, 
shows  the  arrangement  of  the  minerals  in  one  of  the  more  quartzose  laminse 
of  such  a  slaty  rock,  while  PL  xxiii,  Figs.  3  and  4,  from,  photographs,  show, 
with  a  nuich  smaller  degree  of  enlargement,  portions  of  tlu-ee  lamiufe,  the 
central  one  composed  maiidy  of  quartz  and  actinolite,  while  the  other  two 
are  richer  in  magnetite,  but  tliese  last  two  figures  represent  by  no  means  a 
highly  magnetitic  phase.     The  rocks  of  the  second"  or  ferruginous  chert 
type  have  been  described  as  presenting  very  commonly  a  most  singular 
occurrence  of  the  oxides  of  iron,  the  particles  of  these  oxides  being  distrib- 
uted upon  the  quartz  background  in  rude  circles  or  ovals,  or  in  two  or 
more  concentric  circles  or  ovals.     In  these  cases  the  curving  lines,  made 
up  of  particles  of  one  or  more  of  the   three  oxides,  present  no  relation 
whatever   to    the    silica  individuals,    traversing   a  number  of  these  indi- 
viduals   indifferently,   and    looking    as    though    painted    upon    the    silica 
background.     While    this    occurrence  is    far    more    common    among   the 
cherty  rocks  of  the  second  type,  it  is  also  met  with  among  some  of  the 
highly  actinolitic  slates,  in  which  case  the  actinolite  is  usually  associated 
with  the  curving  lines   of  iron  oxide.     In  some  cases  actinolite  and  mag- 
netite are  associated  in  a  bimchy  fashion,  as,  for  instance,  in   PI.  xxiv, 
Fig.  2,    and   are    at    times    dissevered  from    one    another    by  the    intru- 
sion of  silica,  this  silica,  however,  presenting  the  appearance  of  passing 
about    the    several    detached    areas    where    these  are    sufficiently  trans- 
lucent to  allow  such  a  relation  to  be  seen.     These  relations  are  still  better 
shown  by  certain  rocks  from  the  Animikie  series.    (PI.  xxix.  Fig.  2.)     The 
existence  of  this  peculiar  structure  in  the  highly  actinolitic  rocks  is  of  very 
considerable  interest,  since  it  furnishes  one  more  link  between  them  and 


'rilK  lUON-HKAltINC  iMIO.M  i:i:i{.  '2U] 

tln'  I'ciTiiii'iiKiiis  clifrts  ol'  the  last  t\  |)c,  (iiic  iikiic  tiling  \^>  indicate  that  all 
ai'f  ilci'i\ril  t'loiii  ('ss('iitiall\  the  saiiir  (irjoinal  material.  Tlic  actiiKilitic 
injiTcdifiit  nt'  tlicsc  i-dcks  iircscuts  itscll'  iisiialK  in  liin\clics  of  iniiuitc 
radiatiuji'  i)lades  oia  pale  greenish  tint.  OccasionalK-  the  iiidixidiial  l)ladcs 
;iiv  largo  enough,  as  indicated  previoush',  for  them  to  be  seen  with  the 
naked  eye  or  with  the  luagnit'ying  g-lass,  but  more  usually  they  are  micro- 
scopic. Kven  these  radiating  bunches  of  actinolite  are  aggregated  into 
certain  laiuiuiv  to  the  approximate  exclusion  of  the  other  mineral  ingredi- 
ents. They  occur  also  in  single  blades  scattered  through  the  thin  section, 
many  minute  blades  being  not  infrequently  inclosed  in  a  single  grain  of 
quartz,  while  other  Ijlades  or  bunches  of  blades  often  traverse  a  number  of 
quartz  individuals. 

One  unusual  ^ihfise  of  the  aetinolitic  rocks  deserves  especial  description. 
At  Penokee  g-ap,  both  on  the  west  and  on  the  east  side  of  the  fault  which 
there  occurs  (see  PI.  xxxvi),  the  uppermost  layers  of  the  Iron-bearing 
jnember  are  composed  of  a  thin  belt  of  peculiar  slate,  in  which  garnet  is  a 
prominent  constituent.  This  slate  is  nearly  aphanitic,  of  a  dark  color, 
banded  vaguely  with  lighter  streaks,  and  cleaves  readily  parallel  to  the 
lamination.  Minute  individuals  of  garnet  may  at  times  be  detected 
with  a  strong  ■  magnifying  glass  in  the  lighter  colored  streaks,  but 
usually  the)'  require  the  microscope  for  their  detection.  Under  the 
microscope  this  rock  is  seen  to  be  almost  wholly  composed  of 
actinolite,  magnetite,  and  garnet.  The  garnet  is  in  small  and  very 
numerous  individuals,  presenting  a  pale  pinkish  tint.  In  the  ordinary 
light  many  of  these  garnets  are  to  be  seen  provided  with  crystal  out- 
lines, but  in  the  polarized  light  this  becomes  less  evident,  because  many 
of  the  garnets  are  penetrated  by  numerous  needles  of  actinolite,  which  are 
frequently  very  plentiful  in  the  outer  portions  of  the  garnet  individtials. 
The  actinolite  also  makes  up  most  of  the  interstitial  portions  of  the  thin 
sections,  while  the  magnetite,  in  fine  particles,  at  times  provided  with  sharp 
crystal  outlines,  is  scattered  uniformly  throughout  all  portions  of  the  sec- 
tion, being  included  within  each  of  the  other  minerals.  Numerous  minute 
flakes  of  biotite  occur  always  in  such  relation  to  the  actinolite  as  to  suggest 
their  possible  derivation  from  it.     Chlorite  occurs  also  in  some  sections 


214  THE  PENOKEE  IftON  BEARING  SERIES. 

quite  plentifully  as  an  alteration  product  of  the  actinolite.  In  some 
cases  brown  iron  oxide  stains  the  actinolite  blades.  A  separation  of 
the  garnet  (specimen  9553)  by  W.  S.  Bayley  having  been  made,  the 
following  analysis^  of  the  separated  material  was  made  by  Dr.  Thomas 
M.  Chatard,  of  the  laboratorj^  of  tlie  U.  S.  Greological  Survey:  Silica, 
39-31;  alumina,  12-86;  iron  sesquioxide,  10-21;  iron  protoxide,  32-81; 
manganous  oxide,  1-03  ;  calcium  oxide,  1-88  ;  magnesium  oxide, 
1-90  =  100-00.  Dr.  Chatard  observes  -that  this  analysis  is  close  to  that 
of  an  average  almandite.  At  one  time  in  our  study  of  this  rock 
it  was  supposed  that  the  actinolitic  ingredient  might  be  the  result  of  a 
secondary  alteration  of  the  garnet,  but  the  abundance  of  the  same  actino- 
lite in  the  underlying  inagnetitic  and  actinolitic  schists  with  which  this 
rock  is  so  intimately  associated,  and  in  which  the  actinolite  is  surely  not  an 
alteration  product  of  garnet,  seemed  to  render  such  an  origin  improbable. 
A  further  study  of  the  sections  appears  to  show  that  the  actinolite,  even  in 
the  garnetiferous  rock  itself,  is  independent  of  the  garnet,  having  been  in 
part  a  simultaneous  crj^stallization  and  in  part  an  earlier  crystalliza- 
tion. This  conclusion  is  borne  out  by  the  analysis  above  quoted,  from 
which  it  appears  that  the  composition  of  this  garnet  is  such  that  the  actino- 
lite could  not  readily  have  l^een  derived  from  it.  Immediately  over- 
lying this  garnetiferous  rock  is  found  a  black  fragmental  slate.^  At  the 
junction  between  the  two  the  garnets  are  found  to  occur  within  the  frag- 
mental material,  an  occurrence  which  suggests  the  possibility  that  this  frag- 
mental material  furnished  some  of  the  ingredients  which  subsequently 
made  up  the  garnet. 

'  Witli  regard  to  thisi  analysis  Dr.  Chatard  says  tliat  the  amount  of  material  furnished  was 
only  1.2  grams,  and  that  owing  to  the  loss  of  a  portion  there  remained  but  0.54  gram  for  the 
determinatioa  of  the  absolute  amount  of  iron  protoxide.  This  reiuuant  lieing  too  small  for  the  deter- 
mination, the  irou  protoxide  was  calculated  from  the  excess  over  100  in  the  summation  of  the  entire 
analysis  after  estimating  all  tlie  iron  as  the  sesquioxide.  Dr.  Chatard  also  says  that  this  method  of 
calculation  "gives  a  result  close  to  the  aver.age  analysis  of  almandite  garuet,  which  the  mineral 
undoubtedly  is." 

''For  an  early  description  of  this  garnetiferous  slate,  which,  thou.gli  not  frequently  met  with  in 
the  Penokee  region,  is  closely  allied  to  certain  garnetiferous  actiu(ditic  schists  occurring  moi-e 
widely  in  the  Marquette  region  of  Michigan,  see  Geol.  of  Wis.,  vol.  HI,  pp.  123,  124;  see  also  for 
rude  colored  drawings  of  the  thin  sections  of  the  same  rocks,  Figs.  1  and  2  of  PI.  xva  of  the  same 
volume. 


Till-;  llfON  r.K.MMNd   MKMl'.IOi;.  215 

111  tilt'  (letailod  (Icscrlptioii  ol'  tlic  nctiiiulitic  mikI  iii:i<jnetitic  schists 
below,  the  iron  oxide  is  spoken  dl'  ;is  il'  wliolly  iiiaf4-iietite,  unless  tlie  lieinatite 
can  be  microscopically  (lisciMniinatcd.  However,  as  shown  b}'  the  analyses, 
pao-e  197,  the  niaj^netitc  is  always  accompanied  b}-  hematite.  The  actino- 
lite  is  so  named  because  it  has  the  microsc(tpical  properties  of  that  mineral. 
No  separations  of  it  have  been  attempted,  but  the  universal  presence  of  coii- 
siderable  quantities  of  magnesium  and  calcium  in  these  rocks,  as  shown  by 
the  analyses  referred  to,  renders  it  probable  that  the  mineral  is  actinolite 
rather  than  grunerite;  since  no  other  silicate  is  common  and  the  only 
remaining  important  minerals  are  quartz  and  iron  oxide. 

TABULATION    OF    PETROGRAPHICAL    OBSERVATIONS. 

From  the  exposures  in  the  SE.  i  of  Sec.  26,  T.  44  N.,  R.  (J  W.,  Wisconsin. 

1.  Magnetitic  actiuolite-schist.  Specimens  20li  Wr.,'  20.'i  Wr.,  from  350  N.,  (550 
W.,  Sec.  26,  T.  44  N.,  R.  6  W.,  Wisconsin. 

A  dark  colored  slaty  rock,  in  which  bands  about  one-fourth  of  an  inch  wide  are 
mainly  comijosed  respectively  of  magnetite  and  actinolite,  the  latter  arranged  in  quite 
large,  dark  green  radiating  blades.  In  some  cases  the  magnetitic  bands  are  wider, 
however,  and  show  the  minute  actinolites  mingled  with  tlie  magnetite. 

The  thin  sections  are  both  cut  from  the  more  actiuolitic  seams.  In  both  sections 
the  actinolite  is  in  very  large  and  interlocking  blades,  which  are  often  altered  to 
chlorite  and  have  in  places  given  rise  to  a  secondary  biotite.  Included  within  tlie 
actinolite  blades  are  numerous  small  particles  of  magnetite,  often  with  crystal 
outlines. 

2.  Magnetitic  actinolite-schist.  Specimen  9698  (slide  3171) ;  from  480  N.,  520  W., 
Sec.  26,  T.  44  N.,  E.  6  W.,  Wisconsin. 

A  dark  gray,  nearly  aphanitic  rock,  with  faint  lamination  lines,  and  narrow 
seams  parallel  to  the  lamination,  which  are  quite  rich  in  magnetite.  The  magnifying 
glass  reveals  the  presence  in  the  body  of  the  rock  of  numerous  minute  actinolite 
blades,  mingled  with  quartz  particles  and  some  magnetite.     Sp.  gr.,  3-50. 

In  thin  section  interlaced  blades  and  needles  of  actinolite  compose  the  larger 
part  of  the  rock.  This  actinolite  is  partly  altered  to  chlorite  and  includes  numerous 
small  particles  of  magnetite. 


'The  numbers  of  specimeus  aud  slides  are  usually  those  of  the  collection  of  the  lake  Superior 
division.  Specimens  with  Wr.  after  the  numbers  are  from  the  collection  of  the  late  Mr.  Charles  E. 
Wright.  Specimens  with  Wis.  after  the  numbers  are  from  the  coUectiou  of  the  Wisconsin  Geological 
Survey.    Locations  are  given  from  the  southeast  corner  of  the  sections,  in  steps  of  2,000  per  mile. 


2l6  THE  PENOKEE  IRO:N"-BEARI]srG  SERIES. 

3.  Magnetitic  actinolite-scliist.  Specimen  9696  (slide  3170) ;  from  620  N.,  380  W., 
Sec.  26,  T.  44  K,  R.  G  W.,  Wisconsin. 

This  rock  is  closely  similar  to  2. 

The  thin  section  shows  a  backgronnd  of  minntely  crystalline  quartz.  Traversing 
this  backgronnd  are  numerous  blades  of  actinolite,  which  are  often  arranged  in  rosette 
form.  The  individual  blades  of  actinolite  each  frequently  traverse  several  quartz 
individuals,  but  there  are  also  seen  included  within  single  quartz  grains  a  number  of 
minute  actinolite  needles.  Magnetite  occurs  in  minute  particles  within  the  actinolite. 
But  little  biotite  and  chlorite,  secondary  to  the  actinolite,  are  present. 

From  the  Atkins  lake  iron-  range. 

4.  Actinolitic  and  magnetitic  quartz-schist,  from  a  lower  horizon  than  5.  Spec- 
imen 9680  (slide  3167) ;  from  625  N.,  685  W.,  Sec.  20,  T.  44  N.,  R.  5  W.,  Wisconsin. 

This  rock  is  very  closely  like  2,  differing  only  in  being  still  finer  in  grain  and 
more  quartzose. 

The  thin  section  shows  a  background  of  intricately  interlocking  completely 
crystalline  quartz,  within  which  are  included  numerous  concretionary  areas  composed 
of  magnetite,  actinolite,  and  quartz,  the  former  at  times  being  predominant.  Within 
these  areas  the  magnetite  and  actinolite  particles  show  a  tendency  to  an  arrangement 
in  curving  concentric  lines  which  traverse  the  quartz  individuals  indifferently.  The 
minerals  between  the  concretions  are  not  different  from  those  that  compose  them,  only 
the  (quartz  is  greatly  predominant  and  more  coai-sely  crystallized.  Many  of  the  larger 
mostly  interstitial  grains  of  quartz  run  over  into  adjacent  concretions  just  as  though 
they  were  not  there,  then  including  the  bands  of  actinolite  and  magnetite.  The  con- 
cretions have  strongly  the  appearance  of  being  painted  on  a  quartzose  background. 
No  section  could  present  better  evidence  that  the  quartz  was  the  last  mineral  to 
crystallize  than  does  that  of  this  rock.     (PI.  xxiv,  Fig.  2.) 

5.  Magnetitic  actinolite-schist,  from  a  higher  horizon  than  4.  Specimen  9684 
(slide  3168) ;  from  1340  IST.,  330  W.,  Sec.  20,  T.  44  N.,  R.  5  W.,  Wisconsin. 

The  rock  resembles  2,  except  in  being  darker  in  color,  richer  in  magnetite,  and 
finer  in  grain.     Sp.  gr.,  3-43. 

The  thin  section  is  like  that  of  2. 

0.  Magnetitic  and  actinolitic  quartz-schist,  from  a  higher  horizon  than  5.  Speci- 
men 9692  (slide  3169);  from  400  5^.,  1000  W.,  Sec.  16,  T.  44  N.,  R.  5  W.,  Wisconsin. 

A  very  compact  rock,  com]>osed  of  vaguely  defined  lighter  and  darker  colored 
bands,  the  latter  so  rich  in  magnetite  as  to  give  a  metallic  luster.  The  lighter  colored 
ones  are  seen  with  the  magnifying  glass  to  contain  more  quartz.     Sp.  gr.,  3'46. 

The  groundmass  is  comiiosed  of  finely  crystalline  quartz.  Magnetite  particles  are 
scattered  generally  through  the  groundmass,  but  are  also  often  grouped  in  irregular 
outlined  clusters.     Actinolite,  with  its  alteration  product,  chlorite,  occurs  as  in  2. 


TlIK  lliON-15RARrN(}  IsriOMI'.KK'.  217 

From  the  Marenno  rircr  iron  runtji;  Sit.  SI,  '/'.  //  .V.,  A'.  ■;   IT.,  Wisconsin. 

7.  Magimtitic  iiftiiiolitic  scliist.  Siu^ciiiicn  ISC  \Vr. ;  IVoin  l!t(i(l  X.,  13lir»  VV.,  Sec. 
2:5,  T.  44  N.,  It.  r>  W.,  VVis( sin. 

An  aplianitic,  dark  gray,  slaty  rock,  sn  excessively  line  grained  that  the  mag- 
nifying- glass  detects  none  of  its  constituents. 

The  thin  section  is  mainly  made  up  of  interlaced,  often  radiating,  blades  of  actiuo- 
lite,  within  which  are  inclnded  niimei'ous  crystals  and  particles  of  magnetite.  The 
particles  of  the  latter  mineral  are  also  at  times  aggregated  into  bunches  of  some  size. 

8.  Magnetitic  actinolite-scbist.  Specimen  180  Wr.;  from  500  N.,  500  W.,  Sec.  23 
T.  44  N.,  R.  5  W.,  Wisconsin. 

A  dark  gray,  nearly  aplianitic  rock,  showing  on  the  weathered  portions  very  thin 
and  regular  lamination.  The  magnifying  glass  can  barely  detect  the  magnetite 
particles  and  minute  needles  of  actinolite. 

The  thin  section  is  similar  to  that  of  7,  save  that  the  actinolite  is  in  larger  blades 
and  that  the  rock  contains  a  larger  pi'oportion  of  magnetite. 

From  the  exposures  in  See.  24,  T.  44  m,  R.  4  IF.,  Wisconsin. 

9.  Actinolitic  and  magnetitic  quartz-schist,  from  a  low  horizon.  Specimen  9649 
(slide  3159);  from  1637  N.,  0  W.,  Sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin. 

This  rock  closely  resembles  2  and  4,  being  composed  of  dark  colored,  strongly 
magnetitic  bauds  and  light  colored,  quartzose,  ones. 
The  thin  section  of  this  rock  is  similar  to  that  of  4. 

From  the  exposures  in  Sec.  9,  T.  44  If.,  R.  3  W.,  Wisconsin. 

10.  Actinolitic  and  magnetitic  quartz-schists,  from  a  middle  horizon.  Specimens 
9641  (slide  3151),  from  0  N.,  400  W.,  and  9642  (slide  3152),  from  0  N.,  500-600  W., 
Sec.  9,  T.  44  N.,  R.  3  W.,  Wisconsin. 

These  rocks,  again,  are  closely  like  2  and  4,  having  tlie  same  interlamination  of 
lighter  and  darker  materials.  The  darker  colored  bauds  are  very  rich  m  magnetite, 
quite  aphauitic,  and  have  a  conchoidal  fracture.  Sp.  gr.  of  magnetite  seam  in  9042, 4-54. 
The  thin  sections  show  a  groundmass  of  minutely  crystalline  quartz,  the  indi- 
viduals for  the  most  part  niterlocking  with  one  another  more  or  less  deeply,  thouo'h 
some  of  the  more  minute  ones  are  provided  with  crystal  outlines.  This  groundmass 
in  slide  3152  composes  two-thirds  of  the  rock;  in  3151  less  than  half.  Magnetite  is 
scattered  through  the  groundmass  in  numerous  minute  grains  and  crystals,  which  are 
included  within  the  quartz  individuals.  These  are  so  abundant  in  3152  as  to  make 
u])  ahalf  of  the  whole  section.  Actinolite  occurs  in  a  few  blades  which  are  heavily 
stained  with  iron  oxide  and  traverse  the  groundmass  in  such  a  manner  that  each 
blade  cuts  a  number  of  quartz  individuals. 


218  ^HE  PEI^OKEE  lEON-BEARING  SEHIES. 

From  the  PenoJcee  gap  section. 

11.  Magnetitic  actinolitic  quartz-schists,  from  middle  horizons.  Specimens  9556 
(slide  3142),  from  1900  N.,  1500  W.;  9557  (slide  3096),  from  1850  N.,  1500  W.;  9558 
(slide  3191),  from  1800  N.,  1500  W.,  Sec.  14,  T.  44  N.,  E.  3  W.,  Wisconsin. 

These  specimens  represent  the  composition  of  the  west  cliff'  at  Penokee  gap. 
This  cliff  is  made  up  of  alternating  bands  of  lighter  and  darker  colors,  individual 
bands  varying  very  greatly  in  width  and  in  the  amount  of  distinctness  of  definition. 
The  darker  bands,  often  in  themselves  thinly  laminated,  are  very  rich  in  magnetite, 
at  times  so  much  so  as  to  present  a  distinct  metallic  luster.  The  lighter  colored  por- 
tions are  highly  quartzose  and  carry  relatively  little  magnetite.  The  bands  which 
are  richest  in  magnetite  are  usually  found  running  from  a  fraction  of  an  inch  to 
several  inches  in  thickness,  but  thicknesses  of  a  number  of  feet  are  met  with  in  which 
there  is  as  much  as  40  per  cent  of  metallic  iron.  The  darker  colored,  very  highly 
magnetitic  phases  are  quite  aphanitic,  but  the  lighter  color.ed  portions,  though  still 
very  fine  grained,  show  distinctly  under  the  magnifying  glass  a  mixture  of  minute 
actinolitic  blades  and  quartz.     Sp.  gr.  of  9556,  3-37;  of  9557,  4-31;  of  9558,  3-91. 

The  thin  sections  are  composed  of  magnetite,  actinolite,  and  quartz,  named  in 
order  of  time  of  crystallization.  The  only  difference  between  the  thin  sections  lies  in 
the  varying  coarseness  of  grain  and  the  varying  proportions  of  the  three  constituents, 
each  one  of  which  is  in  turn  the  preponderating  ingredient,  and  again  insignificant 
in  quantity.  The  quartz  forms  a  groundmass  of  closely  fitted  or  interlocked  grains, 
the  smaller  ones  of  which  are  often  provided  with  crystal  outlines.  Nowiiere  do  these 
grains  show  any  traces  of  fragmental  cores,  the  whole  appearance  forbidding  any 
thought  of  a  detrital  origin  for  any  portion  of  the  section.  The  actinolite  is  in  color- 
less to  pale  green,  feebly  dichroic  needles  and  blades,  at  times  arranged  in  radiatiug 
aggregations  which  vary  very  greatly  in  size.  Numbers  of  the  more  minute  needles 
occur  often  within  single  quartz  grains,  while  the  larger  needles  and  blades  extend 
through  a  number  of  quartz  individuals.  The  arrangement  of  the  actinolite  has  plainly 
been  withoxrt  any  reference  whatever  to  the  quartz,  which  appears  therefore  to  be  of 
subsequent  origin.  The  magnetite  occurs  in  minute  dust-like  particles  within  both 
quartz  and  actinolite,  and  also  in  quite  large  aggregations  of  irregular  outline.  At 
times  these  aggregates  of  magnetite  include  particles  of  actinolite  and  quartz,  but 
such  cases  seem  rather  to  be  esxjlicable  on  the  idea  that  the  actinolite  and  quartz 
have  entered  into  cavities  within  the  magnetite  aggregations  subsequent  to  their 
solidification.  N"o  evidence  is  found  that  these  two  minerals  ever  are  included  within 
or  traverse  single  individuals  of  magnetite.     (PI.  xxiv.  Fig.  1.) 

12.  Magnetitic  and  actinolitic  quartz-schists,  from  near  the  summit  of  the  Iron 
member.  Specimens  9555  (slide  3190),  from  0  N.,  1625  W.,  Sec.  11;  and  9567  (slide 
3195),  from  1675  N.,  1100  W.,  Sec.  14,  T.  44  N.,  E.  3  W.,  Wisconsin. 


TIIK  llt()N-i;i;Ai;lN(!  I\lKMltKI{.  219 

These  specimens  are  from  very  ncur  thr  siiiiiniil  dl'  ilio  lidii  1(c;irino-  Imrizdii, 
haviiii;- lu'cii  lakfii  immf<li;ilfly  Ix-iit'iiMi  the  very  (liiii  mid  |Mu-iili:ir  j4iuiictiri'iiiiis  rock 
wliich  lies  at  tlie  top  of  tliis  member.  Tliey  are  tiiinly  iaiiiiiiated  in  li^lil  ami  dark 
gray  shades,  aplniiiitic,  and  \ery  cU)sely  resemble  the  rocks  last  described.     Sp.  gr. 

of  drt'i't,  ;{•(»"). 

The  thill  sections  of  those  specimens  ]iresent  appearances  like  tluise  of  II.  (I'l. 
XXIII,  Figs.  .?,  4;  I'l.  xxviii,  Fig.  4.) 

i;5.  Magiietitic  and  garnctiferons  actiiiolite-scliists,  from  the  summit  of  the  Iron 
member  on  the  west  side  of  i'eiiokee  gap.  Specimens  955.^  (slide  3188),  from  0  N., 
1800  W.;  1404,  Wis.  (slide  241),  from  (»  N.,  1!H)(>  W.;  1444,  Wis.  (.slide  263),  from 
0  N.,  1600  W.,  Sec.  11,  T.  44  N.,  \i.  3  W.,  Wisconsin. 

These  are  dark  colored  aphanitic  rocks,  banded  vaguely  with  lighter  streaks, 
having  a  pronounced  coiichoidal  cross  fracture,  but  cleaving  readily  parallel  to  the 
lamination.  In  some  places,  with  a  strong  magnifying  glass  minute  individuals  of 
garnet  may  be  detected.  These  apiwar  most  abundant  in  the  light  colored  streaks. 
Sp.  gr.  of  !t5.53,  3-42. 

The  thin  sections  are  almost  wholly  composed  of  magnetite,  actinolite,  and 
garnet.  The  garnet  is  in  rather  small  and  very  numerous  individuals,  which  present 
a  pale  pinkish  tint  in  the  thin  sections.  In  the  ordinary  light  many  of  them  are  seen 
to  be  provided  with  crystal  outlines;  iu  the  polarized  light  this  becomes  less  evident, 
because  many  of  the  garnets  are  penetrated  by  numerous  needles  of  actinolite,  which 
are  ft'equently  more  plentiful  in  the  outer  portions  of  the  garnet.  The  actinolite 
makes  up  most  of  the  interstitial  portions  of  the  sections,  but  also  penetrates  and  is 
included  within  the  garnet  individuals.  It  was  at  first  supposed  that  this  actinolite  is 
an  alteration  product  of  the  garnet,  but  the  fact  of  the  abundance  of  the  actinolite  in 
the  immediately  underlying  maguetitic  and  actinolitic  schists  with  which  this  garnetif- 
erous  rock  is  so  intimately  associated,  and  in  which  the  actinolite  is  surely  not  an 
alteration  product  of  garnet,  seeming  to  render  snch  an  origin  improbable,  this  view 
■was  abandoned.  A  further  study  of  the  section  seems  to  show  that  the  bulk  of  the 
actinolite  is  rather  independent  of  the  garnet,  having  been  in  part  of  a  simultaneous 
crystallization  with  it  and  iu  part  of  a  x^revious  erystallization.  The  magnetite,  in  fine 
particles,  at  times  crystal  outlined,  is  scattered  uniformly  through  all  portions  of  the 
section,  being  included  within  all  of  the  other  minerals.  Biotite  occurs  in  numerous 
mimrte  flakes,  which  appear  in  all  cases  to  be  secondary  to  the  actinolite  individuals. 
14.  Altered  garnetiferons  actinolite-schists,  from  the  summit  of  the  Iron  member 
on  the  west  side  of  Penokee  gap.  Specimens  1501,  Wis.  (slide  276),  from  1670  N., 
1100  W.;  1502  Wis.  (slide  277),  from  1670  N.,  1100  W.,  Sec.  14,  T.  44  N.,  R.  3  W., 
Wisconsin. 

Macroscopically  these  rocks  resemble  13. 


220  THE  PENOKEE  lEON-BEAEING  SERIES. 

This  resemblance  holds  iu  thethiu  sections,  save  that  the  garnet  is  less  abund- 
ant and  that  the  actiuolite  has  been  much  more  largely  altered.      The  alteration 
■  products  are  biotite  and  chlorite,  but  more  or  less  brown  iron  oxide  accompanies  the 
biotite. 

Frotn  the  Penokee  range,  Sec.  9,  T.  44  N.,  R.  2  TF.,  Wisconsin. 

15.  Maguetitic  and  actinolitic  quartz-schist,  from  a  low  horizon.  Specimen  2064 
Wis.  (slide  300),  from  0  N.,  1000  W.,  Sec.  9,  T.  41 IST.,  R.  2  W.,  Wisconsin. 

The  thin  section  of  this  rock  does  not  dift'er  iu  any  essential  respect  from  those 
of  the  Penokee  gap  actinolitic  schists,  11  and  12.  The  actinolite  is  arranged  in  the 
usual  radiatiug  bundles  and  is  more  or  less  stained  witli  brown  and  red  iron  oxides. 
The  quartz,  as  usual  in  these  rocks,  is  very  minutely  crystalline,  and  wholly  without 
any  appearance  of  a  fragmental  origin. 

From  ihe  (jorge  of  Tylers  forh,  Wisconsin. 

lis.  Ferruginous  quartz  schists,  from  a  low  horizon.  Specimens  9620  (slide  3147), 
from  1170  N.,  160  W.;  9624  (slide  3149),  from  1235  N.,  193  W.,  Sec.  33,  T.  45  N.,  R.  1 
W.,  Wisconsin. 

The  exposures  of  the  Iron-bearing  member  at  the  gorge  of  Tylers  fork  present 
rocks  having  a  geueral  resemblance  to  those  of  the  Penokee  gap  section,  including 
darker  colored,  more  maguetitic,  and  lighter  colored  more  quartzose  phases;  but  one 
is  imjiressed  at  once  with  the  relatively  small  proportion  of  magnetite  contained  in 
the  Tylers  fork  section  where  the  light  colored,  more  quartzose  kinds  very  greatlj' 
predominate,  and  where  the  thin  banding  which  characterizes  the  Penokee  section  is 
only  developed  in  one  or  two  places.  Sj)ecimens  9620  and  9624  are  mainly  made  up 
of  the  lighter  colored  phase,  which,  it  should  be  said,  is  still  lighter  colored  than  any 
of  the  Penokee  gap  rocks.     jSTo  actinolite  is  perceptible  with  the  magnifying  glass, 

m 

while  the  appearance  of  the  quartz  is  quite  peculiar,  suggesting  some  sort  of  a  con- 
cietionary  arrangement.  The  lean  ferruginous  seams  are  exceedingly  irregular  in 
thickness  and  in  course,  branching  and  running  in  all  sorts  of  irregular  fashions 
instead  of  lying  i^arallel  to  the  general  bedding  of  the  rock.  Sp.  gr.  of  the  quartzose 
portion  of  9620,  2-93;  of  maguetitic  banding  seams,  5-01. 

The  thin  sections  are  composed  of  quartz,  actinolite,  and  magnetite,  and  so  far 
are  analogous  to  the  actinolitic  and  magnetitic  schists  of  Penokee  gap  above  described, 
but  there  are  some  important  differences.  In  the  first  place,  the  actinolite  iu  the  Tylers 
fork  rocks  is  very  unich  less  plentiful,  and  as  a  rule  is  in  much  more  minute  blades. 
A  much  m,ore])ronounced  difference,  however,  consists  in  the  striking  tendency  toward 
a  concretionary  development  that  these  Tylers  fork  rocks  show.  This  is  brought  out 
particularly  in  the  arrangement  of  the  individuals  of  the  greatly  predominating 
quartz  of  the  groundmass  and  comes  out  with  special  prominence  in  the  polarized 
light,   when  there  is    seen   a  tendency  of  the  coarser    individuals  of  the   quartz  to 


TFIE  IK'OX-I'.KAlv'rXd  MEMRETt.  221 

arrange  tliemsclvos  intlio  cenkMS  of  areas  whoso  outer  portions  are  made  up  of  more 
iiiimitc  piirticlcs.  The  iiiaHiu^titc,  liowcvor,  also  shows  some  sliglit  tciidcMicy  to  a  cou- 
crctioiiarv  arraiij;ciiicut,  wliilc  the  tiiiiiiitc  actiiiolitc  blades  arc  af^yrcgated  into  little 
dusters,  which  appear  in  the  main  to  lie  in  tiic  spaces  between  the  concretionary 
areas  or  in  the  outer  portions  of  these  areas.  TIk^  niaf^netite  o(!(Mirs  in  bunches  of 
crystals,  and  in  curvilinear  ajij-rcjuations,  and  also  in  single  individuals  scattered  in- 
discriminately, thongh  sparsely,  throughout  the  section.  Another  noticeable  difler- 
eiice  between  these  rocks  and  those  from  Penokee  gap  (ionsisLs  in  their  lack  of  the 
highly  developed  lamination  which  the  latter  rocks  show,  both  in  the  thin  section 
and  in  the  hand  specimen.  There  is  no  trace  of  evidence  that  the  quartz  of  these 
rocks  is  in  any  measure  of  a  fragmental  uatuie.  In  fact,  its  concretionary  arrange- 
ment and  the  interlocking  of  the  diflerent  individuals  puts  such  an  origin  entirely 
out  of  the  question.  In  slide  3147  stains  of  brown  iron  oxide  occur  here  and  there, 
usually  mingled  with  the  clusters  of  the  minute  actinolite  blades.  lu  the  section  of 
this  specimen  there  is  seen  also  a  single  rhombic  crystal  section,  which,  on  examina- 
tion with  a.  high  power,  proves  to  be  made  up  chiefly  of  a  number  of  actinolite  blades, 
which  on  the  border  of  the  crystal  are  deeply  stained  by  brown  iron  oxide.  The 
shape  of  this  piece  suggests  the  i)robability  that  it  was  originally  a  single  individual 
of  iron  carbonate.     (PI.  xxviii,  Pig.  1.) 

17.  Ferruginous  quartz-schist,  from  a  low  h(n-izon.  Specimens  9617  (slide  3100) 
9623  from  1175  N.,  I'M  W.,  9618  (slide  3148),  from  1245  X.,  185  W.,  Sec.  33,  T.  45  N.' 
E.  1  W.,  Wisconsin. 

These  si)eoimeus  resemble  those  of  16  in  all  respects,  except  that  numerous  spots 
of  reddisli  jasper  are  seen  upon  them.  These  stand  out  in  a  prominent  fashion  and  in 
such  a  way  as  to  suggest  a  fragmental  origin  for  them.  In  the  same  ledge,  however, 
are  large  bands  of  the  same  jasper  (9618),  directly  interlaminated  with  the  other 
materials;  Specimen  9617  shows  the  peculiar  concretionary,  or  partly  fragmental 
character  described  under  16.     Sp.  gr.,  of  9617,  2-76;  of  9618,  2-92. 

The  thin  sections  show  that  these  rocks  differ  from  16  in  containing  no  actinolite- 
but  a  more  interesting  difference  lies  in  the  very  nuich  more  striking  deveioi)ment 
which  they  show  of  a  concretionary  structure.  This  appeaiance  shows  m)t  merely  in 
the  quartz  background  itself,  as  seen  in  the  i)olarized  light,  but  in  the  arrangement  of 
the  particles  of  iron  oxides,  which  include  both  hematite  and  magnetite,  and  in  smaller 
quantity  the  brown  oxide.  In  some  cases  the  magnetite  and  liematite  are  aggiegated 
into  opaque,  round  or  oval  areas,  the  edges  of  which  show  the  projecting  corners  of 
numerous  individual  crystals  of  magnetite;  but  there  are  more  plentiful  aieas  in  which 
these  oxides  are  seen  to  be  disposed  in  concentric  bands,  made  u]>  of  niore  or  less  sep- 
arated particles  of  the  oxides,  and  having  between  them  the  ordinary  quartz  back- 
ground of  the  rock.  When  these  areas  are  exainined  in  the  polarized  light  they  are 
found  to  lie  in  the  quartz  background  in  sueh  a  fashion  as  to  suggest  their  haying 


222  THE  PEl^OKEE  lEOK-BEAEING  SERIES. 

been  painted  upon  it,  a  single  band  of  the  iron  oxide  traversing  multitudes  of  quartz 
individuals,  to  whose  arrangement  it  bears  little  or  no  relation.  As  usual,  the  quartz 
in  these  rocks  is  plainly  the  last  separated  ingredient,  and  it  seems  that  its  concre- 
tionary form  had  something  to  do  with  the  concentric  arrangement  of  the  iron  oxide 
also.  From  what  follows  later,  it  is  probable  that  the  bands  of  iron  oxide  are  the 
remains  of  iron  carbonate  areas  whicli  have  altered  to  iron  oxide  in  part,  but  have 
partly  been  replaced  by  silica,  these  processes  produciug  the  concretions.  Besides 
these  concretionary  areas,  there  are  other  areas  whose  outlines  in  the  polarized  light 
suggest  their  being  fragments.  These  are  mainly  made  up  of  silica  like  that  of  the 
groundmass,  except  that  in  some  cases  they  are  possibly  finer  grained  and  have  the 
iron  oxide  particles  scattered  through  them  irregularly.  This  is  particularly  noticeable 
in  slide  3148,  which  appears  with  a  low  power,  as  if  it  might  have  been  a  fragmental 
rock  whose  fragments  had  all  been  silicified,  the  same  silica  tilling  the  spaces  between 
the  fragments.  In  this  sectiou  also  some  of  the  apparent  fragments  are  jaspery — that 
is  to  say,  a  mixture  of  red  iron  oxide  and  quartz — and  appear  therefore  quite  different 
from  the  rest  of  the  rock.  Of  particular  interest  is  the  occurrence  in  this  slide  of  quite 
an  amount  of  iron  carbonate.  This  substance,  which  is  usually  more  or  less  stained 
with  brown  iron  oxide,  but  is  very  i)lainly  recognizable  as  a  carbonate,  appears  both 
in  single  individuals  scattered  through  the  groundmass  and  in  clusters  of  individuals. 
These  clusters  often  lie  in  groups,  between  which  are  areas  of  the  groundmass,  in  such 
a  fashion  as  to  render  it  evident  that  they  were  once  connected  with  one  another  and 
have  since  been  separated  by  the  insertion  of  the  quartz.  In  a  few  cases  some  of  the 
apparent  fragments  above  alluded  to  are  mainly  made  up  of  this  brown  stained  car- 
bonate, little  veins  and  streaks  of  the  quartzose  groundmass  entering  into  them  or  even 
entirely  traversing  them.  This  occurrence  of  the  carbonate  is  of  great  interest, 
because  it  seems  to  indicate  that  all  of  the  apparently  fragmental  and  concretionary 
areas  which  are  now  made  up  of  quartz  and  uon  oxide  were  originally  composed  of 
this  substance.  Whether  the  fragmental  appearance  which  these  rocks  have  is  due  to 
a  real  fragmental  character,  or  is  caused  by  the  same  processes  which  have  formed  the 
rounded  and  oval  concretions  (i.  e.,  is  of  a  secondary  nature),  will  be  discussed  later. 
18.  Ferruginous  quartz-sclust,  from  a  middle  horizon.  Specimen  9625  (slide  3150); 
from  1285  N.,  230  W.,  Sec.  33,  T.  45  N".,  R.  1  W.,  Wisconsin. 

A  nearly  black  aphanitic  flinty  rock  in  which  the  only  ingredient  recognizable 
with  the  magnifier  is  hematite  in  minute  metallic  lustered  scales.    Sp.  gr.,  2-90. 

The  thin  section  of  this  rock,  examined  with  a  low  power  and  iu  the  ordinary  light, 
presents  a  much  more  striking  irregular  appearance  than  those  immediately  above 
described  (except  slide  3148),  being  made  up  of  round  or  oval  areas,  whicli  differ  from 
the  whiter  quite  sparse  matrix  in  being  dotted  over  with  a  fine  black  dust.  These 
fireas  are  usually  outlined  by  a  border  of  magnetite  crystals,  but  otherwise  there  is 


THE  IHON-BEAHING  MEMBER.  223 

none  of  the  concretionary  appearance  nlxiiit  llicni  tliat  characterizes  the  apparent  frag- 
ments of  17.  Examined  with  a  higher  power  in  tlic  polarized  li;;li(,  some  of  these 
areas  are  found  (o  lie  nnule  up  mainly  of  (puut/,,  whicli  for  the  most  pait  is  (|iiiteliiie 
that  of  the  interspaces,  though  tlie  latter  is  occasionally  coarser  than  is  seen  within  the 
apparent  fragments.  Tlie  black  particles  alluded  lo  appear  to  be  wholly  iron  o.xide 
and  in  the  main  magnetite,  which  mineral,  however,  occturs  also  in  (piitc  large  sized 
crystals,  arranged  in  bauds,  as  already  stated,  bordering  the  apparent  fragments,  but 
also  sometimes  scattered  Ihrough  them,  and  again  in  clusters  within  the  interstitial 
quartz.     (PI.  xxvui,  Pig  2.)  ^ 

From  tlie  Pcmlcee  range  in  Sec.  34,  T.  45  N.,  R.  1  W.,  Wisconsin. 

19.  Maguetitic  (luartz-schist.  Specimeu  9081  (slide  4206) ;  from  140  N.,  0  W., 
Sec.  24,  T.  45  N.,  R.  1  W.,  Wisconsin. 

The  rock  is  a  very  highly  inagnetitic,  fine  grained,  dark  gray  schist,  in  which 
the  magnifier  reveals  otdy  quartz  and  magnetite,  certain  irregular  bauds  being  much 
more  highly  maguetitic  than  the  rest  of  the  rock. 

The  thin  section  of  this  rook  is  very  closely  like  those  of  the  Tylers  fork  rocks, 
particularly  17,  presenting  the  singular  concretionary  and  apparent  fragmental 
appearance.  In  the  case  of  the  present  rock  there  is  more  of  the  red  and  brown 
oxides  of  iron  than  of  the  magnetite.  In  many  places  the  iron  carbonate  is  found  in 
the  concretions.  Here  at  times  the  iron  carbonate  extends  beyond  the  outlines  of  the 
concretions  and  apparent  fragmental  areas  in  such  a  manner  as  to  make  it  probable 
that  the  iron  carbonate  was  the  original  material,  and  that  the  minerals  contained 
ill  it — iron  oxide,  actinolite,  and  quartz,  making  up  concretionary  and  apparently 
fragmental  areas — have  formed  from  in  part  and  replaced  in  part  the  original  iron 
carbonate.  In  places  the  rhombic  shai)es  of  the  brown  and  red  iron  oxides  make  it 
evident  that  these  are  the  direct  results  of  oxidation  of  siderite  individuals.  (PI.  xxiii. 
Figs.  1  and  2.) 

From  the  Potato  river  section. 

20.  Maguetitic  quartz-schist.  Specimen  9104  (slide  4214);  from  1050.  K.,  250 
W.,  Sec.  19,  T.  45  N.,  R.  1  E.,  Wisconsin. 

A  fine  grained,  nearly  white,  cherty  rock,  dotted  with  magnetite  particles,  and 
irregularly  banded  with  thin  seams  of  the  same  mineral.     Sp.  gr.,  3-21. 

The  thin  section  of  this  rock  resembles  those  of  the  actinolitic  and  maguetitic 
quartz-schists  of  Peuokee  gap  (11  and  12),  i>articularly  those  in  which  there  is  the 
minimum  of  parallelism  in  arrangement  of  the  several  ingredients.  The  arrangement 
of  the  quartz  individuals  and  the  bunchiness  of  the  magnetite  and  actinolite  suggest 
a  vague  concretionary  arrangement,    The  actinolite  is  scattered  about  through  the 


224  THE  PENOKEB  lEOI^-BEAEHSTG  SERIES. 

quartz  grouudmass,  but  occurs  very  plentifully  also  among  the  magnetite  aggrega- 
tions. As  compared  with  the  last  rocks  described,  and  others  like  it,  this  rock  lacks 
the  distinct  separation  into  a  relatively  pure  quartzose  matrix  and  dark  colored  areas 
in  which  the  iron  oxides  are  particularly  abundant,  and  which  are  outlined  so  dis- 
tinctly as  to  suggest  their  having  been  fi-agments.  However,  after  having  seen  the 
sections  in  which  this  arrangement  is  very  pronounced,  one  realizes  that  even  in  a 
case  like  the  present  one  there  is  a  faint  indication  of  the  same  structure.  More  or 
less  of  the  iron  oxide  occurs  in  the  hematitic  and  hydrated  forms. 

21.  Magnetitic  quartz-schist.  Specimen  9183  (slide  4213) ;  from  1050  N.,  270  W., 
Sec.  19,  T.  45  N.,  R.  1  E.,  Wisconsin. 

The  rock  is  a  fine  grained,  dark  gray  magnetitic  schist,  closely  resembling  19. 
Sp.  gr.,  3-39. 

The  thin  section  resembles  the  last  described  as  to  its  constituents,  but  differs 
from  it  in  having  the  concretionary  and  pseudo-fragmental  structure  more  largely 
developed.  Since  the  two  come  from  the  same  rockmass,  it  is  evident  that  the  faint 
indication  of  this  peculiar  structure  seen  in  slide  4214  (20)  points  to  a  similar  origin  for 
the  two  rocks,  Quite  a  little  quantity  of  carbonate  remains  in  some  of  the  apparent 
fragments.  Minute  actinolite  blades  occur  here,  as  in  other  similar  sections,  aggre- 
gated in  minute  needles  about  the  clusters  of  magnetite. 

From  the  PenoJcee  range  in  Sec.  16,  T.  45  W.,  B.  1  E.,  Wisconsin. 

22.  Ferruginous  quartz-schist.  Specimen  12831  (slide  5495) ;  from  513  N.,  1647 
W.,  Sec.  16,  T.  45  N.,  R.  1  B.,  Wisconsin. 

The  rock  is  line  grained,  dark  gray  schist,  banded  with  seams  of  black  iron 

oxide. 

The  thin  section  of  this  rock  resembles  those  of  the  several  peculiar  concretion- 
ary or  pseudo-fragmental  rocks  above  described,  particularly  20.  As  seen  in  the  ordi- 
nary light,  it  shows  a  colorless  background,  which  is  thickly  studded  witli  distinctly 
outlined  areas  that  differ  from  the  background  merely  in  containing  numerous  minute 
dark  colored  particles.  Some  of  these  areas,  however,  are  surrounded  by  a  rim  of 
the  several  iron  oxides  and  occasionally  these  iron  oxides,  particularly  the  red  and 
brown  oxides,  cover  the  whole  of  one  of  these  areas.  In  the  polarized  light  the  spotted 
areas  lose  much  of  their  deiiniteness  and  are  seen  to  be  mainly  made  up,  like  ttie  matrix, 
of  a  very  minutely  crystalline  cherty  quartz.  Some  of  the  areas  are  quite  angular  in 
outline,  but  most  of  them  are  more  or  less  rounded,  while  at  times  a  number  of  the 
areas  are  so  related  to  one  another  as  to  seem  to  have  been  once  continuous,  having 
been  severed  by  insertion  of  the  matrix  material  along  irregular  rifts.  Here  and 
there  these  areas  are  seen  to  have  not  merely  a  single  ring  of  iron  oxide,  but  to  have 
a,lso  one  within  and  concentric  to  the  outer  one, 


TllK  I KOX-BEARING  MEMBER.  225 

From  the  Pemkce  range  in  Sec.  10,  T.  15  N.,  Ji.  I  K.,  Wisconsin. 

23.  Fernij;iiioii,s  iiiid  actiiiofitic  vhart,  or  quartz  aiid  rC-hert.  Specimen  9185 
(slide  4-*ll);  lioiii  0  N.,  1200  W.,  Sec.  10,  T.  45  N.,  K.  I  E.,  Wisconsin. 

Tilt!  r(H!k  is  a  lino  sriii"C<l  cliert-scList,  much  stained  with  brown  iron  oxide. 
On  close  examination  it  is  seen  to  be  minutely  i)orous.  The  fresh  fracture  is  studded 
with  little  clusters  of  bright  lustered  magnetite,  of  wliich  mineral  the  weight  shows 
that  there  is  a  considerable  portion  present.     Sp.  gr.,  3*2(). 

The  section  is  a  rather  confused  one,  on  account  of  the  large  amount  of  brown 
iron  oxide,  but  carefiil  examination  shows  that  it  consists  of  a  number  of  more  or  less 
thoroughly  detached  areas,  between  which  is  a  background  of  minutely  crystalline 
silica.  The  areas  referred  to  arc  of  an  angular  to  roundish  outline,  and  in  them  tlie 
oxides  of  iron,  including  the  brown  oxide  and  magnetite,  are  mainly  aggregated. 
When  the  iron  oxide  is  least  thick  they  show  sheaf-like  aggregations  of  actinolite 
blades  in  a  background  of  minutely  crystalline  silica.  Actinolite  needles  also  occur 
in  the  groundmass. 

From  the  Penokee  range  in  Sec.  13,  T.  45  N.,  R.  1  H.,  Wisconsin. 

24.  Ferruginous  chert-schist.  Specimen  9189  (slide  2930) ;  from  1920  N.,  1825  W., 
Sec.  12,  T.  45  N.,  E.  1  E.,  Wisconsin. 

A  very  iiue  grained  chert,  analogous  to  9185,  deeply  stained  with  brown  and 
red  iron  oxides,  which  minerals  occur  also  in  little  bands  of  some  thickness. 

The  thin  section  is  made  from  some  of  the  less  ferruginous  portions.  It  presents 
essentially  the  same  appearance  as  that  of  26,  below  described.  The  silica  individuals 
are,  however,  more  generally  of  somewhat  larger  size  than  in  the  latter  section, 
although  still  quite  small,  and  the  concretionary  structure  is  not  so  strongly  devel- 
oped. Neither  is  there  so  much  iron  oxide  present.  The  concretionary  areas  and 
apparent  fragments  seen  in  26  are  less  evident  in  the  present  case  because  they  have 
less  iron  remaining  in  them,  but  many  are  readily  seen  in  ordinary  light,  minute  films 
of  iron  oxide  still  outlining  them,  while  their  interiors  are  commonly  composed  of  a 
somewhat  more  minutely  divided  silica  than  that  of  the  matrix. 

25.  Hematite  iron  ore  in  thin  seams  directly  interstratifled  with  24.  Specimen 
9187  (slide  3105) ;  from  1950  N.,  1825  W.,  Sec.  12,'  T.  45  K,  E.  1  E.,  Wisconsin. 

The  rock  is  a  dark  jjurplish  red  slaty  hematite  iron  ore. 

The  thin  section  of  this  rock  differs  from  that  of  24  ouly  in  .that  it  is  mainly  com- 
posed of  a  nearly  opaque  mass  of  particles  of  the  red  and  brown  oxides  of  iron. 
Small  areas  of  minutely  crystalline  silica  are  seen,  and  a  single  vein  of  quartz  of  later 
origin  than  the  iron  oxide  traverses  the  section.  The  quartz  of  this  vein  is  in  quite 
large  individuals  compared  with  those  of  the  groundmass  proper. 
MON  XIX 15 


22o  THE  PENOKEE  IROlSr-BEAEmG  SEEIES. 

From  the  Penohee  range  in  See.  6,  T.  43  If.,  B.  2  U.,  Wiseonsin. 

36.  Ferruginous  chert-schist,  from  the  base  of  the  Iron-bearing  member.  Speci- 
men 9190  (slide  2931);  from  1825  N.,  325  W.,  Sec.  6,  T.  45  N.,  E.  3  E.,  Wisconsin. 

The  rock  is  a  brown  stained  porous  chert. 

The  greater  part  of  the  section  is  made  up  of  a  concentrically  arranged  silica, 
quite  analogous  in  its  appearance  to  the  concretionary  cherts  already  described  as 
belonging  to  the  Limestone  member  of  the  Penokee  series.  This  silica  is  in  the  main 
divided  off  into  minute  interlocking  quartz  individuals,  which  vary  somewhat  in 
degree  of  minuteness,  the  hnest  material  at  times  passing  over  into  chalcedony. 
Here  and  there  through  the  section  are  seen  what  appear  in  the  ordinary  light  to  be 
areas  of  some  different  material  from  the  rest  of  the  section.  These  are  stained  a 
light  brown  color,  and  have  smooth,  curved  outlines,  concentric  with  which  there  is  at 
times  near  the  edges  of  them  a  curvilinear  arrangement  of  the  particles  comijosing 
the  iron  oxide  stain.  In  the  polarized  light  these  areas  are  seen  to  be  made  up  of  a 
minutely  divided  silica  analogous  to  that  forming  the  bulk  of  the  rock,  but  usually 
defined  from  that  portion  of  the  matrix  with  which  it  is  directly  in  contact  by  being 
much  liner  grained.  Some  of  the  areas,  however,  lack  definiteness  of  outline,  and 
appear  as  if  invaded  and  dissolved  away  by  the  surrounding  matrix.  In  one  or  two 
cases  remnants  of  iron  carbonates  were  detected  within  them.  Scattered  through 
the  chert  matrix  in  irregularly  outlined  aggregates  and  curving  lines  is  a  brown 
iron  oxide  in  which  may  be  seen  at  times  remnants  of  iron  carbonate.  The  cavities 
seen  macroscoj^ically  in  the  specimen  appear  in  the  thin  section  as  holes  with  ragged 
outlines,  which  are  brought  into  particular  prominence  by  a  coating  of  iron  oxide. 

37.  Flint  or  chert,  from  near  the  base  of  the  Iron-bearing  member.  Specimen 
mu  (sUde  2799);  from  1150  K,  1625  ^¥.,  Sec.  6,  T.  15  N.,  E.  2  B.,  Wisconsin. 

The  rock  is  an  aphanitic,  white,  chalcedouy-like  flint  or  chert,  translucent  on 
the  edges,  and  possessed  of  a  conchoidal  fracture.  Irregular  bands  of  a  nearly  black 
color  also  occur.     Sp.  gr.  of  white  portion,  2-67 ;  of  black  bauds,  2-69. 

The  thin  section  is  composed  almost  wholly  of  a  colorless  silica,  which  for  the 
most  part  x)olarizes  only  as  an  aggregate  and  very  feebly.  High  powers,  however, 
bring  out  distinctly  minute  polarizing  particles,  which  are  mingled  with  much  mate- 
rial which  seems  to  be  quite  amorphous.  Portions  of  the  section  show,  running 
through  this  silica,  cloudy  bands  and  streaks  arranged  in  general  parallelism  to  one 
another;  a  number  of  these  streaks  appearing  as  if  once  continuous  bands  which 
have  been  separated  by  the  intrusion  of  siUca.  These  bands  owe  their  cloudiness,  in 
l^art  at  least,  to  the  presence  of  a  minutely  divided  brown  iron  oxide.  In  the  main, 
however,  their  substance  seems"  now  to  be  made  iip  of  silica  more  thoroughly  amor- 
phous than  is  that  of  the  bulk  of  the  rock.  In  some  of  these  streaks  are  apparent 
remnants  of  a  carbonate,  in  aggregates  of  minute  individuals.    This  section  is  of 


THE  1U()N-15EA1UNG  MEMHIOK.  227 

particular  interest  as  bearing  upon  the  question  as  to  whether  such  an  excessively 
fine  fjTiiined  silica  as  that  seen  hero  is  of  the  same  origin  with  the  more  completely 
crystalline  material  more  generally  characteristic  of  the  viirious  ferruginous  schists. 
The  ai)pearance  of  tliis  section  would  seem  to  make  it  very  probable  that  some  of 
the  fine  grained  silica  is  of  a  secondary  origin  and  not  in  the  nature  of  an  original 
deposition. 

28.  Magiietitic  sidcrite,  from  a  lower  middle  horizon.  Specimen  9191  (slide 
2996);  from  1S50  K,  325  W.,  Sec.  6,  T.  45  N.,  R.  2  E.,  Wisconsin. 

A  dark  gray,  aphauitic,  very  heavy  rock,  x)ossessed  of  a  very  thin  lamination, 
the  laminae  being  alternately  light  gray  and  dark  gray.  Some  of  the  darker  colored 
lamina',  contain  enough  magnetite  to  malic  them  attractable  by  the  magnet,,  but  tli« 
high  specific  gravity  is  plainly  not  wholly  due  to  the  presence  of  this  magnetite  in  the 
darker  colored  bands.  Sp.  gr.,  3*50.  Composition:  Silica,  15-62 ;  alumina, 4'27 ;  ferric 
oxide,  844;  ferrous  oxide,  32'85;  mangauous  oxide,  5-06 ;  calcium  oxide,  0-81;  magne- 
sium oxide,  2-66;  carbon  dioxide,  30-32;  water,  0-68^100-41. 

The  thin  section  is  from  the  predominating  grayish  part  of  the  rock — none  of 
the  black  magnetitic  bands  being  represented  in  it — which  seems  to  be  composed 
almost  entirely  of  a  gray  felted  mass  of  siderite  individuals.  These  are  known  to  be 
siderite  fi'om  analysis.  Mingled  with  this  siderite  is  a  small  proportion  of  an  exceed- 
ingly minutely  crystalline  and  even  amorphous  silica,  and  probably  clay;  and  scat- 
tered sparsely  through  it  are  minute  crystals  and  irregularly  outlined  particles  of 
magnetite.  Both  in  the  hand  specimen  and  in  the  thin  section  the  rock  presents 
every  appearance  of  being  in  an  unaltered  original  condition.     (PI.  xxi,  Fig.  3.) 

From  the  section  on  the  West  Branch  of  the  Montreal  river. 

29.  Ferruginous  chert  or  flint,  from  a  low  horizon.  Specimen  9048  (slide  2886); 
from  400  N.,  1165  W.,  Sec.  27,  T.  46  K,  R.  2  E.,  Wisconsin. 

The  rock  is  a  compact  chert  or  flint  of  a  dark  brown  color,  mottled  irregularly 
with  white  spots.    Sp.  gr.  2-65. 

The  thin  section  is  made  up  mainly  of  a  silica,  which  ranges  from  very  minutely 
crystalline  through  a  chalcedouic  jjhase  to  an  entirely  amorphous  material.  Staining 
this  background  of  silica,  in  irregular  patches  and  particularly  in  strongly  marked 
concretionary  areas,  is  a  pale  brown  hydrated  oxide.  The  concretionary  areas  are 
most  striking,  being  composed  of  concentric  bands  of  the  iron  oxide,  which  lie  in 
appearance  as  mere  stains  in  the  siliceous  background.  These  concretionary  areas 
are  closely  similar  to  those  described  above  as  characterizing  the  thin  sections  of  4, 17, 
21,  22,  24,  26,  with  this  difference,  however,  that  instead  of  a  black  magnetite  or  a 
nearly  opaque  hematite,  the  iron  oxide  concerned  is  now  a  pale  brown  hydrated  kind. 
Whatever  may  be  the  origin  of  these  peculiar  concretionary  areas,  it  is  certainly  the 
same  in  this  case  as  in  the  case  of  the  other  rocks  referred  to.    The  siliceous  back- 


228  THE  PE3!fOKEE  lEON-BEAEING  SEEIES. 

groiind  of  this  rock,  as  examined  in  the  polarized  light,  doe>s  not  appear  to  have  been 
much  affected  by  concretionary  action,  except  that  here  and  there  the  larger  indi- 
viduals are  bunched  in  irregular  oval  areas.  Two  or  three  slender  veins  of  quartz 
traverse  the  section.  In  these  the  individuals  of  quartz  are  larger  than  in  the  general 
background  of  siUca.  Here  and  there  the  nuclei  of  the  concretionary  areas  referred 
to  are  seen  to  be  composed  of  a  number  of  unusually  large  sized  quartz  individuals 
(PI.  XXII,  Figs.  1  and  2.) 

30.  Ferruginous  chert,  from  a  low  horizon.  Specimen  9047  (slide  2775) ;  from 
405  F.,  1095  W.,  Sec.  27,  T.  46  K,  E.  2  E.,  Wisconsin. 

'  The  rock  is  a  brown  chert  similar  to  29,  but  having  a  darker  color  and  contain- 
ing some  spots  which  are  bright  red  and  jaspery. 

The  thin  section  is  composed  of  a  finely  divided  but  still  whoUy  crystalline 
silica,  the  larger  sized  individuals  being  much  more  plentiful  than  in  29.  Irregularly 
blotching  this  background  are  areas  of  nearly  opaque  hematite,  which  are  without 
any  perfect  concretionary  arrangement,  which  arrangement  is  also  only  faintly  indi- 
cated in  a  portion  of  the  siliceous  background  itself. 

From  the  Germania  mine. 

31.  Ferruginous  chert  or  flint,  from  a  very  low  horizon.  Specimen  9015  (slide 
3103) ;  from  200  N.,  1575  W.,  Sec.  24,  T.  46  N.,  E.  2  E.,  Wisconsin. 

A  dark  brownish  gray,  aphanitic,  cherty  rock,  carrying  irregular  seams  of 
hematite. 

This  section  shows  an  almost  pure  chert,  quite  closely  resembling  that  which 
forms  the  background  of  29.  A  few  concretionary  areas  only  faintly  marked  with 
iron  oxide  are  seen. 

From  the  Montreal  river  section. 

32.  Ferruginous  chert  or  flint,  at  the  base  of  the  Iron-bearing  member,  Ashland 
mine.  Specimen  7619  (slide  2308) ;  from  1965  N.,  1925  W.,  Sec.  27,  T.  47  N..  E.  47  W., 
Michigan. 

The  thin  section  is  made  up  almost  wholly  of  silica  in  an  exceedingly  finely 
divided  and  even  amorphous  state,  at  times  showing  faintly  the  radial  chalcedonic 
arrangement.  Brownish  and  reddish  iron  oxides  occur  here  and  there  in  irregular 
patches  and  spots.  They  also  appear  in  v^ry  distinctly  outlined  rhombic  sections, 
which  suggest  a  derivation  from  the  oxidation  of  an  iron  carbonate;  a  suggestion 
which  is  borne  out  by  the  presence  in  some  of  the  rhombic  sections  of  areas  of  unal- 
tered carbonate. 

33.  Black  banded  flint  and  siderite,  from  a  high  horizon.  Specimens  9007  (slide 
2916),  9009  (slide  2765) ;  from  160  N.,  100  W.,  Sec.  21,  T.  47  N.,  E.  47  W.,  Michigan. 

A  fresh  fracture  of  these  rocks  shows  an  interbanding  of  dark  gray  and  black  layers 
ranging  from  about  one-eighth  to  one-half  inch  in  width.    The  black  bands  are  com- 


Till']  IK'ON-lJEAlMNt}  MEMHKit.  229 

l)oso(l  ofiui  ai)1iiuiiti('  Hint ;  llic  dnrk  j;r;iy  l);ni(ls,  wliilc  contaiiiint^-  more  or  li^ss  oCtliis 
Mint,  apjjoar  to  hc^  iiiiult'  ii|)(>('a  crystiilliiic  ag^jrc^'iiUoii  of  soiiHM-arhonate,  Mie  crystal- 
lizatioii  heiiigf  coarsti  oiioujili  for  tlic  iiakod  eye  to  detect  tlio'cleiivaROS  of  tlie  indi- 
vidual crystals.     S]t.  f;r.  of  !t(»Oi),  .'$-24. 

Ill  thin  sections  the  black  bands  of  the  rock  represented  by  tliese  specimens  prove 
to  bo  made  up  mainly  of  an  amorphous  and  an  exceedingly  finely  crystalline  silica. 
In  this  siliceous  background  arc  included  numerous  minute  black  particles,  to  which 
evidently  the  black  color  of  the  bands  is  due.  These  pai'ticles,  when  examined  with 
a  high  power,  are  found  to  be  exceedingly  iiregular  in  sha]ie,  though  often  they  are 
aggregated  into  thin  belts,  which  are  so  identifiil  as  to  give  a  strongly  laminated 
ap]iearance  to  these  i)ortions  of  the  sections.  They  ap]iear  to  be  quite  without  crys- 
tal outlines,  and  from  the  macroscopic  a.i)pearance  of  the  rock  and  its  close  resem- 
blance to  others  in  which  carbonaceous  matter  has  been  detected  on  analysis  it  is 
supposed  that  these  particles  are  of  that  nature  also.  Very  possibly  they  may  be 
mingled  witli  more  or  less  of  pyrite  and  iron  oxides.  A  few  pieces  of  what  ajipears 
to  be  a  fragmental  quartz  are  met  with  in  those  portions  of  these  sections  which 
represent  the  black  bands,  and  also  a  few  irregular  areas  of  iron  carbonate  which 
are  analogous  in  structure  and  alterations  to  those  spoken  of  under  .32.  The  gray 
bands  seen  microscopically  have  a  background  or  groundmass  similar  to  the  nmterial 
which  composes  the  bulk  of  the  black  bands.  But  in  this  case  there  are  contained  in 
this  groundmass,  so  plentifully  as  to  constitute  the  larger  portion  of  these  parts  of 
the  sections,  irregular  areas  of  the  iron  carbonate  in  various  stages  of  alteration  and 
others  less  plentiful  of  a  greenish  chlorite.  These  greenish  areas  have  often  an  oval 
form.  The  viridite  or  chlorite  wliich  makes  them  up  is  arranged  in  fan-like  aggre- 
gations. The  areas  of  iron  carbonate  vary  greatly  in  size,  the  smaller  ones  being 
almost  perfect  single  rhombohedra;  while  the  larger  ones,  although  aggregates  of  a 
number  of  individuals,  show  around  their  borders  the  projecting  edges  of  rhom- 
bohedra. Much  of  this  iron  carbonate  has  altered  more  or  less  thoroughly  to  iron 
oxides,  including  both  the  brown  oxide  and  hematite.  In  many  cases  where  the 
alteration  has  been  complete  the  secondary  iron  oxides  are  plainly  perfectly  jiseudo- 
morphous  after  the  siderite,  i.  e.,  they  preserve  still  the  rhombic  outlines  of  the 
siderite  crystals.  More  usually,  however,  the  alteration  has  been  only  partial,  and 
in  such  cases  the  secondary  oxides  are  either  in  irregular  bunches  or  (and  this  is 
very  much  more  commonly  the  case)  are  arranged  in  very  irregular  concentric 
spherical  rings,  of  which  there  are  generally  several  in  one  area,  though  m  other 
cases  a  single  ring  is  seen  or  a  single  ring  with  the  beginnings  of  one  or  more 
others.  These  rings  of  iron  oxide,  which  are  so  plainly  the  result  of  a  secondary  oxi- 
dation of  the  carbonate,  occur  quite  without  any  reference  to  the  individuals  of  the 
latter  mineral;  that  is  to  say,  a  single  ring  or  a  set  of  rings  traverses  an  area 
made  up  of  a  number  of  carbonate  individuals.     In  some  of  the  most  comi)letely 


230  THE  PBNOKEE  lEON-BEAEING  SEEIBS. 

altered  areas  of  carbonate  there  is  contained  more  or  less  of  the  finely  divided  silica 
of  the  matrix,  and  in  Sl^ch  cases  we  appear  to  have  an  intermediate  phase  between  the 
unaltered  carbonate  and  those  cases  above  described  as  characterizing  other  sections 
where  simple  rings  of  iron  oxide  are  within  a  siliceous  background.  In  other  words, 
the  iron  carbonate  remaining  after  formation  of  rings  of  oxide  has  been  more  or  less 
removed  and  silica  deposited  in  its  place.  As  has  been  noted  above,  these  concen- 
tric areas  of  iron  oxide  and  iron  oxide  and  silica  are  found  in  every  stage  of  forma- 
tion until  the  iron  carbonate  disappears.     (PI.  xxvii,  Fig.  4.) 

34.  Ferruginous  and  sideritic  chert,  from  a  higli  liorizon,  interstratified  with  33. 
Specimen  7622  (shde  2072);  from  160  N.,  100  W.,  Sec.  21,  T.  47  K,  R.  47  W.,  Michigan. 

The  rock  is  from  another  layer  of  the  ledge  from  AVliich  33  comes.  It  has  a  most 
striking  and  jjeculiar  character,  being  grayisli,  mottled  with  black,  and  suggests  even 
to  the  naked  eye  a  fragmental  texture,  blackish  angular  fragments  being  imbedded 
in  a  grayish  cherty  mass.  - 

In  thin  section,  a  minutely  crystalline  to  amorphous  silica  forms  a  grouudmass 
which  occupies  a  relatively  small  i^ortion  of  the  whole  area.  Contained  in  tliis  ground- 
mass  are  small  crystals  and  aggregates  of  crystals  of  siderite,  oval  or  sjilierical 
concretions  of  mingled  flinty  silica  and  Iron  oxides,  particles  of  a  blackish,  probably 
carbonaceous,  material,  with  or  without  accompanying  siderite,  and  also  irregularly 
outlined  and  even  sharply  angular  areas  or  fragments  composed  of  a  mixture  of  ilinty 
silica,  iron  oxide,  and  carbonaceous  material  with  or  without  siderite.  The  iron  car- 
bonate, except  that  it  is  more  often  fresh,  appears  as  in  33.  Where  altered,  its  alter- 
ations are  the  same'  as  in  those  sections,  the  peculiar  beautiftil  concretionary  forms 
there  described  occurring  here  also  and  are  evidently  of  the  same  origin.  The  more 
irregular  outlines,  and  especially  the  angular  fragment-like  areas  that  appear  in 
this  groundmass,  are  entirely  similar  in  composition  and  structure  ^o  the  jnaterial 
which  has  been  described  as  composing  the  blacker  bands  in  33.  These  are  at  times 
plainly  fragments,  as  may  be  seen  from  their  angular  outlines,  sharp  definition  from 
the  matrix  of  the  rock  and  from  the  fact  that  the  lamination  lines  abruptlj-  terminate 
■  at  the  extremities  of  the  areas.  The  lines  of  lamination  in  these  fragments,  when 
they  are  perceptible,  are  never  parallel  for  any  two  fragments. 

It  seems  evident  that  the  history  of  the  rock  exposed  where  Nos.  9007,  9008,  9009 
and  7622  were  obtained  has  been  about  as  follows:  It  was  first  a  stratiform  car- 
bonate of  iron,  including  apparently  more  or  less  carbonaceous  matter,  as  such  strati- 
form carbonates  so  generally  do,  and  perhaps  more  or  less  of  silica.  By  a  pr(#ess  of 
subsequent  silicification,  accompanied  by  oxidation,  the  structures  now  apparent  were 
produced.  At  times  the  substitution  of  the  silica  and  the  solution  and  oxidation  of 
the  carbonate  went  on  so  as  not  seriously  to  break  up  the  continuity  of  the  original 
layers;  but  in  other  cases  the  rock  had  become  shattered  and  the  silica  entered 
into   the    minute    cracks    and  interstices   of   the    rock,   and    this   shattering    was 


THE  IKON-lJEAUINd  MEMBEH.  231 

often  produciul  after  a  certain  amount  of  alteration  liad  been  elfectert,  inasmuch  as 
tlu^  fragments  thoniselvos  have  evidently  been  in  part  altered  before  beingf  torn  ajiart. 
The  prDcess  continniiig,  tiiese  disrupted  pieces,  so  far  as  they  were  still  pretty  pure 
carbonate,  h.id  developed  in  them  by  oxidation  the  concretionary  structure,  and 
liiially  were  often  replaced  more  or  less  completely  by  siliceous  material.  luto  many 
of  the  fragmental  areas  ramifyiuf;'  veinlets  of  the  siliceous  groundmass  extend,  so 
that  the  various  appearances  iJicsented  by  the  section  seem  all  expli(!able  by  a  contin- 
uing process  of  solution,  oxidation,  and  silicification,  accompanied  by  a  certain 
amount  of  dynamic  movement. 

While  the  above  process  explains  a  part  of  the  fragmental  character  of  the  rock, 
it  seems  probable  that  it  has  been  to  some  extent  actually  shattered  by  erosion. 
The  layers  here  contain  some  fragmental  quartz;  they  are  at  a  high  horizon, 
which  represents  probably  the  beginning  of  the  change  from  nonclastic  to  clastic 
sedimentation.  The  nonclastic  sediments  immediately  after  deposition  were  perhai^s 
broken  to  a  greater  or  less  extent,  forming  detritus,  which  was  mingled  with  the 
same  kind  of  sediments  which  continued  to  form  at  favorable  times.  (PI.  xxii, 
Figs.  3  and  4;  PI.  xxvn,  Figs.  2  and  3.) 

From  the  section  on  the  Mount  Hope  property. 

35.  Henatitic  flint,  from  a  middle  horizon.  Specimen  7610  (slide  2306);  from 
1150  N.,  0  W.,  Sec.  23,  T.  47  N.,  R.  47  W.,  Michigan. 

The  thin  section  shows  seams  composed  entirely  of  a  very  minutely  crystalline 
to  qtiite  amorphous  silica,  interbanded  with  others  in  which  the  red  and  brown  iron 
oxides  are  mingled  with  more  or  less  of  the  silica.  There  is  no  brecciation  apparent 
in  the  section.  Here  and  there  are  minute  iiatches  of  iron  carbonate  from  whose 
oxidation  it  may  be  supposed  the  iron  oxides  came. 

From  the  section  on  the  Puritan  property. 

36.  Ferruginous  cherts  or  flints,  from  lower  and  middle  upper  horizons.  Speci- 
mens 7604  (slide  2304);  from  70  N.,  1000  W.;  7605  (slides  2011,  2069);  from  150  N., 
1050  W. ;  7006  (slide  2305) ;  from  276  N.,  1175  W.  All  from  Sec.  18,  T,  47  K.,  R.  46 
W.,  Michigan. 

The  rocks  are  grayish  to  whitish  chferts,  banded  by  reddish  seams. 

The  sections  of  these  rocks  are  made  up  of  minutely  crystalline  chalcedonlc 
and  amorphous  silica,  mingled  with  bauds  and  irregular  patches  of  brown  and  red 
iron  oxides.  Occasional  remnants  of  iron  carbonate  may  be  detected,  and  at  times 
the  background  presents  vaguely  the  concretionary  appearance  described  in  foregoing 
sections. 

37.  Hematitic  flint  or  chert,  from  a  low  horizon.  Specimen  7601  (slide  2302); 
from  275  N.,  0  W.,  Sec.  18,  T.  47  N.,  E.  46  W.,  Michigan. 

The  prevailing  siliceous  grotindmass  is  in  large  proportion  of  exceedingly  finely 
crystalline  spotty  quartz  mingled  with  amorphous  material.     In  irregular  areas  and 


232  THE  PENOKEE  IRON-BEAEING  SERIES. 

bands  in  this  groundmass  are  opaque  aggregations  of  red  iron  oxide.  They  are 
in  part  plainly  derived  directly  from  an  iron  carbonate,  since  the  rhombic  portions  of 
the  crystalline  sections  are  plainly  visible,  particularly  around  the  borders  of  the 
opaque  aggregations.  There  is  also  abundantly  in  the'  groundmass  vaguely  outlined 
areas  representing  probably  the  silicilied  areas  of  the  carbonate.  These  are  analo- 
gous to  those  occurring  in  above  described  sections,  but  are  of  rather  small  size. 

38.  Hematitic  ilint  or  chert,  from  a  low  horizon.  Specimen  7600  (slide  2009); 
from  300  N.,  1500  W.,  Sec.  17,  T.  47  K,  E.  46  W.,  Michigan. 

In  thin  section  the  chert  background  is  completely  though  minutely  crystal- 
line. It  otherwise  closely  resembles  the  last  section  described.  The  hematite  aggre- 
gates, however,  show  somewhat  larger  remnants  of  unaltered  iron  carbonate. 

39.  Hematitic  flint  or  chert,  from  a  middl6  horizon.  Specimen  7596  (slide  2301); 
from  453  N.,  1300  W.,  Sec.  17,  T.  47  N.,  R.  46  W-.,  Michigan. 

This  section  differs  from  the  two  last  described  only  in  the  very  much  larger 
proportion  of  iron  oxide  contained  and  that  the  silica  is  intermediate  in  crystalliza- 
tion between  the  silica  of  them.  The  iron  oxides  appear  in  the  same  sort  of  aggre- 
gates in  which  are  often  to  be  seen  the  rhombic  crystal  outlines  of  the  original  carbon- 
ate. The  texture  of  the  chert  in  this  case  also  is  somewhat  open,  numerous  cavities 
being  contained  by  the  section. 

From^  the  Colby  mine  section. 

40.  Siliceous  iron  carbonate,  from  a  high  horizon.  Specimen  12508  (slide  5522); 
from  1125  N.,  175  W.,  Sec.  16,  T.  47  N.,  E.  46  W.,  Michigan. 

A  very  fine  grained  rock,  with  an  irregular  division  into  light  and  dark  gray 
laminffi.  Scattered  along  these  laminae  are  minute  shining  facets  of  a  carbonate, 
which  show  however,  more  plentifully  in  much  larger  individuals  irregularly  blotched 
all  over  the  specimen  without  any  reference  to  the  arrangement  of  the  bands.  The 
appearance  of  this  carbonate  alone  is  sufficient  to  identify  it  as  siderite,  an  identifi- 
cation which  is  confirmed  by  the  deep  brown  weathering  that  the  specimen  shows. 

This  section  differs  from  many  of  those  previously  described  in  the  very  large 
amount  of  fresh  iron  carbonate  which  it  contains.  Tliis  mineral  occurs  not  only  in 
single  intUviduals,  but  in  compact  aggregations  of  individuals,  which  sometimes 
occuijy  quite  extensive  areas  in  the  section.  Ii'regularly  interwoven  with  the  carbon- 
ate areas  are  areas  of  silica  which  are  largely  amorphous.  In  this  siliceous  back- 
ground are  seen,  however,  in  i)laces  the  peculiar  concretionary  areas  described  as 
characterizing  some  of  the  above  sections.  These  areas  very  plainly  originate  from  a 
change  of  the  carbonate  aggregates.  Dotted  throiigh  the  section  are  particles  of 
quartz  of  varying  size.  These  are  partly  in  the  background  and  partly  within  the 
masses  of  carbonate,  the  larger  ones  often  appearing  as  if  of  fraguiental  origin.  (PI. 
XXVII,  Fig.  1.) 


TllK   lltON-l'.KAl{IN(l  MKMKKlt.  233 

From  the  seefioii  on  I  be  Tilden  mine  property. 

41.  ITciiiiititic  Hint  or  choit,  from  a  middle  horizon.  Si)ocinuMi  7571  (slide  19!>;j); 
from  15(M»  N.,  121;!  W.,  Sec.  15,  T.  47  N.,  It.  Ki  W.,  Mi.hjoai,. 

Tiie  thill  section  is  ii  clicrt  containinj--  much  liiiely  divided  iiematite. 

12.  lEematitic  flint  or  chert,  from  a  middle  horizon.  Specimen  7505  (slide  2064); 
from  1450  N.,  1185  W.,  Sec.  15,  T.  47  N.,  R.  46  W.,  Michigan. 

The  section  is  a  hematitic  Hint  analogous  to  that  of  41.  The  hematite  in  the 
larger  part  of  the  section  is,  however,  more  uniformly  contained  and  in  more  minute 
particles.  It  is  arranged  so  as  to  show  faint  but  perfect  concretions,  lu  parts  of  the 
section  are  concentrated  large  areas  of  hematite. 

From  the  section  on  the  Palms  property, 

43.  Cherty  and  altered  iron  caibonate,  from  a  middle  horizon.  Specimen  7573 
(slide  1996) ;  from  1600  IST.,  1955  W.,  Sec.  14,  T.  47  N.,  R.  40  W.,  Michigan. 

About  half  the  area  of  the  thin  section  is  a  light  gray  and  the  other  half  a  red- 
dish brown.  The  light  colored  portion  is  composed  of  uniformly  mingled  minutely 
crystalline  quartz  and  gray  siderite.  The  siderite  occurs  within  the  siliceous  ground- 
mass  both  in  detached  individuals  and  in  complex  areas,  the  single  individuals 
showing  usually  very  distinctly  the  rhombic  outlines.  The  brown  portion  of  the  slide 
is  like  the  other  portion  so  far  as  the  siliceous  groundmass  is  concerned,  but  here  the 
brown  and  red  oxides  of  iron  take  the  place  of  the  siderite  of  the  lighter  colored  part 
of  the  slide.  Since  these  iron  oxides  show  often  the  same  rhombic  outlines  as  seen  in 
the  siderite;  since  they  occupy  precisely  the  same  relation  to  the  chert;  since  there  is 
a  gradation  and  not  a  sharp  definition  between  the  two  iiortions  of  the  section ;  and, 
finally,  since  the  single  individuals  of  siderite  may  be  seen  partly  changed  to  iron 
oxide,  it  is  evident  that  the  latter  mineral  is  a  secondary  product  of  the  former. 

From  the  section  on  BlacTc  river. 

44.  Jasper  and  ferruginous  chert,  from  very  near  the  base  of  the  Iron-bearing 
member.  Specimens  9508  (slide  2983),  from  1665  N.,  1915  W.;  9509  (slide  3137),  from 
1600  N.,  1900  W.;  9510  (slide  3087),  from  1580  N,,  1900  W.  All  in  Sec.  13,  T.  47  N., 
R.  46  W.,  Michigan. 

Specimen  9508  is  a  bright  red  jasper,  banded  with  very  thin  seams  of  brilliantly 
metallic  lustered  hematite.  Specimen  9509  is  a  dark  brownish  gi'ay  chert.  Specimen 
9510  is  similar  to  9508  but  is  mottled  with  irregular  blotches  of  red  iron  oxide. 

The  thin  section  2983  consists  of  finely  crystalline,  mingled  with  some  amor- 
phous silica,  in  which  are  included  very  numerous  minute  particles  of  bright  red 
hematite,  which  are  aggregated  more  thickly  along  certain  bands.  Many  of  the 
hematite  particles  have  rhombic  crystal  outlines,  and  appear  therefore  to  have  orig- 
inated from  the  oxidation  of  iron  carbonate. 


234  THE  PENOKBE  lEON-BEARING  SERIES. 

In  slides  3137  and  3087  tlie  quartz  is  more  coarsely  crystalline,  and  the  iron 
oxide  is  mainly  the  brown  hydrated  variety.  The  iron  oxides  in  them  are  arranged 
in  concretionary  and  brecciated  forms,  and  the  silica  is  affected,  although  less  plainly, 
by  the  same  arrangement.  Some  of  the  larger  concretions  are  complex,  one  of  them 
perhaps  including  several  smaller  concretions.  The  areas  in  3137  are  very  irregular, 
and  suggest  a  mechanical  brecciation  or  a  brecciation  caused  by  the  processes  of 
alteration. 

45.  Ferruginous  chert,  from  a  low  horizon.  Specimens  7554  (slide  2298),  7555 
(slide  1983);  both  from  1680  N.,  1900  W.,  Sec.  13,  T.  47  N.,  E.  46  W.,  Michigan. 

Slide  2298  is  an  exceedingly  finely  and  regularly  laminated  ferruginoiis  chert, 
quite  analogous  in  general  character  to  the  black  bands  of  33;  while  1983  is  a  brec- 
ciated chert  closely  analogous  to  34,  which  in  turn  is  directly  interstratifled  with  33. 
In  other  words,  we  have  here  a  representation  of  the  phenomena  presented  by  the 
exposure  oh  the  Montreal  river,  from  which  33  and  34  came,  except  that  here  all  of 
the  iron  carbonate  has  been  removed. 

46.  Siliceous  siderite,  from  a  lower  middle  horizon.  ,  Specimen  9504  (slide  3186), 
from  1725  N.,  1900  W.,^ec.  13,  T.  47  N.,  E.  46  W.,  Michigan. 

A  very  fine  grained,  dark  gray,  evenly  and  finely  laminated  earthy  rock.  Sp. 
gr.,2-07. 

The  thin  section  shows  an  intimate  mixtiire  of  minutely  crystalline,  with  perhaps 
some  amorphous  silica,  with  minute  grayish  rhombohedra  of  siderite  only  slightly 
altered  here  and  there  by  oxidation.  These  rhombohedra  are  generally  single,  but 
are  at  times  aggregated  closely  into  bunches. 

47.  Ferruginous  chert  or  jasper,  from  an  upjjer  middle  horizon.  Specimen  9500 
(slide  3182),  from  1840  N.,  1975  W.,  Sec.  13,  T.  47  N.,  R.  40  W.,  Michigan. 

The  rock  is  an  aphanitic  chert  or  jasper,  the  colors  varying  in  irregular  blotches 
through  dark  gray  to  bright  red. 

The  thin  section  is  a  brecciated  concretionary  chert  like  others  above  described; 
rounded  areas  of  a  finely  crystalline  and  amorphous  silica,  mingled  with  more  or  less 
brown  and  red  iron  oxides,  being  embedded  in  a  silica  which  is  usually  more  coarsely 
crystalline.  Occasionally,  instead  of  the  rounded  areas,  there  are  long  tabular  pieces 
showing  the  lamination  of  the  original  chert.  In  both  kinds  of  areas  there  are 
numerous  places  where  the  rhombohedral  outlines  of  the  original  carbonate  reproduced 
in  the  iron  oxide  may  be  seen.  Besides  this  are  to  be  seen  within  the  chert  frag- 
ments rhombic  areas,  often  of  large  size,  comitosed  of  silica  similar  to  the  rest,  but 
outlined  distinctly  by  brown  iron  oxide.  These  again  are  taken  to  be  substitutions 
for  original  carbonate  crystals. 

48.  Ferruginous  chert,  from  an  upper  middle  horizon.  Si^ecimens  9501  (slide 
3183),  9502  (slide  3184);  both  from  1840  N.,  1975  W.,  Sec.  13,  T.  47  N.,  E.  46  W., 
Michigan. 


TIIK  IKON-HliAKINC  MEMHKII,  235 

The  rocks  arc  diirk  red  cluirts  or  jaspi-rs,  wliii'li  lor  flic  must  part  luck,  however, 
tilt'  apliaiiitic  lliiity  appcaiance  of  tine  jasjier.  Tliey  arc  Iciiiilcd  (|iiifc  lesulaily  witli 
lifilitci'  colored  seams. 

Tlic  Ihiii  section  ;>ls;!  shows  a  backiii-oiiiid  of  exceedingly  liiiely  crystalline,  min- 
gled probably  with  iiniorplious  silica,  t liiMtnj^li  whicli  arc  scattered  a  few  small  appar- 
ently fragnn'iital  iiurticles  of  the  same  material  and  nnmerons  minute  stains  of  brown 
iiydrated  iron  oxide  which  occasionally  present  riiombic  ontlines.  The  thin  section 
3184  difl'ers  from  MKi  simply  in  containing  much  larger  and  even  ])redominiiting 
quantities  of  iron  oxide,  mucli  of  which  is  hematite.  Rhond)ic  outlines  to  the  parti- 
cles of  iron  oxide  are  frequently  to  be  seen.  The  iron  oxiih^  is  aggregated  especially 
into  certain  bands,  so  as  to  give  the  section  a  laminated  appearance. 

•49.  A  ferruginous  chert,  from  an  upper  middle  horizon.  Specimen  950.'1  (slide 
3185),  from  1775  N.,  1925  W.,  Sec.  13,  T.  47  iST.,  R.  40  W.,  Michigan. 

A  light  gray,  evenly  laminated  earthy  rock,  banded  with  seams  of-  dark  red 
hematite. 

The  thin  section  is  a  ferruginoirs  chert  very  closely  resembling  3183  in  48. 

50.  Iron  carbonate  or  siderite  rock,  from  an  upper  horizon.  Specimens  9481 
(slide  3177),  9482  (slide  3178),  9483  (slide  3348).  '  All  from  250  N.,  1000  W.,  Sec.  12,  T. 
47  N.,  R.  40  W.,  Michigan. 

An  aphanitic,  dark  gray,  earthy  looking,  stratiform  rock,  banded  in  some  por- 
tions with  nearly  black  bands,  which  are  at  times  Ijroken  and  the  detached  portions 
imbedded  in  a  lighter  colored  material.     Sp.  gr.,  from  3-22  to  3-40. 

The  thin  sections  are  composed  mainly  of  a  felted  mass  of  iron  carbonate,  which 
in  some  of  the  darker  bauds  is  mingled  with  a  dark  colored  material  in  line  particles, 
presumably  of  a  carbonaceous  nature.  Certain  bands,  as  seen  in  section  3177, 
contain  a  finely  divided  silica  which  has  a  distorted  lamination,  the  carbonate  being 
separated  into  more  or  less  detached  areas.  At  the  same  time  these  bands  show 
large  sized  iiatchjes  of  a  greenish  chlorite.  The  carbonate  is  also  altered  to  a  consider- 
able extent  to  iron  oxide,  tbe  latter  being  largely  arranged  in  rings.  This  is  another 
good  illustration  of  the  formation  of  concretions. 

51.  Ferruginoirs  chert,  from  an  ui^per  horizon.  Specimen  9485  (slide  3179) ;  295 
N.,  980  W.,  Sec.  12,  T.  47  N.,  E.  40  W.,  Michigan. 

A  very  dark  colored,  nearly  black  aphanitic  rock,  banded  irregularly  with  ill 
defined  dark  red  seams. 

The  thin  section  is  evidently  from  one  of  the  dark  reddish  brown  seams.  It 
presents  a  confused  admixture  of  chlorite,  brown  iron  oxide,  and  magnetite,  the  latter 
mineral  being  in  distinctlj^  outlined  crystals,  all  in  a  minutely  crystalline  and  amor- 
phous siliceous  background.  Many  of  the  brown  particles  also  present  rhombic  out- 
lines, and  are  taken  to  have  arisen  from  an  oxidation  of  the  carbonate.  Siderite  is 
not,  however,  recognizable  in  the  section,'biit  may  be  present  plentifully  in  the  dark 
colored  earthy  looking  bauds  mentioned  above. 


236  THE  PENOKEE  IRON-BEAEING^  SEEIES. 

52.  Black  chert,  from  the  siiinniit  of  the  Iron-bearing-  member.  Specimen  7534 
(slide  19G8),  from  500  N.,  1075  W.,  7535  (slide  1969),  from  535  N.,  1050  W.,  Sec.  12, 
T.  47  ]Sr.,  E.  40  W.,  Michigan. 

The  thin  sections  are  concretionary,  brecciated,  and  ferruginous  cherts,  analo- 
gous to  a  number  above  described.  Slide  19G8  shows  a  predominating  groundmass  of 
exceedingly  finely  crystalline  and  amorphous  silica,  in  which  are  strewn  small  parti- 
cles of  quartz  which  are  certainly  fragmental.  The  fragmental  grains  of  quartz  stand 
out  in  the  background  in  a  wonderfully  distinct  way.  They  vary  from  well  rounded 
to  angular;  some  of  them  are  distinctly  enlarged.  This  section  illustrates  well  the  great 
difterence  in  appearance  between  fragmental  quartz  and  the  nonfrag mental  quartz  of 
the  iron  formation.  Here,  as  in  previously  described  rocks,  as  an  upper  horizon,  is  a 
mingling  of  chemical  and  mechanical  sedimentation;  the  beginning  of  the  transition  to 
the  upper  fragmental  member  of  the  series.  There  are  also  present  magnetite,  hema- 
tite, and  irt>n  carbonate,  all  of  which  are  arranged  in  a  semiconcretiouary  fashion,  and 
in  such  a  way  as  to  suggest  the  derivation  of  the  whole  from  an  original  carbonate. 
Slide  1969  differs  from  1968  only  in  containing  large  areas  of  what  seems  to  be  a 
secondary  calcite. 

From  the  section  on  the  Miner  &  Wells  property. 

53.  Oherty  iron  carbonate,  from  near  the  base  of  the  Iron-bearing  member. 
Specimens  12885  (slide  5507),  12886  (slide  5508) ;  Sec.  13,  T.  47  N".,  E.  46  W.,  Michigan. 

An  aphanitic  ro(;k,  showing  a  very  thin  and  for  the  most  part  regular 
lamination,  though  in  certain  layers  these  lamiuie  are  somewhat  bent.  The  laminiB 
range  in  thickness  from  that  of  a  sheet  of  jiaper  to  as  much  as  a  quarter  or  half  an 
inch.  They  range  in  color  from  black  thi-ough  various  shades  of  brownish  gray  and 
greenish  gray  to  a  very  light  gray.  All  save  the  black  bands  show  a  very  earthy, 
compact  look,  and  the  whole  appears  at  first  sight  as  that  of  some  banded  or  earthy 
limestone;  but  the  high  specific  gravity  of  the  rock  proves  at  once  the  presence  of 
much  iron.  The  surfaces  of  some  of  the  black  laminae  glisten  brightly  as  though  con- 
taining a  carbonaceous  or  graphitic  material.  Composition  of  12885:  silica,  4G'01; 
titanic  oxide,  0-12 ;  alumina,  0-83 ;  iron  .sesquioxide,  1-35 ;  iron  protoxide,  26-CO ;  man- 
ganous  oxide,  2-09;  calcium  oxide,  0-63;  magnesium  oxide,  2-86;  carbon  dioxide, 
17-72;  phosphoric  acid,  0-07;  iron  sulphide,  0-11;  waterat  red  heat,  1-71  =  99-50. 

In  the  thin  sections  the  light  colored  bands  are  seen  to  consist  of  an  almost  solid 
aggregate  of  minute  rhombohedra  of  iron  carbonate,  whose  outlines  are  particularly 
well  observed  on  the  borders  of  the  bands  where  separated  slightly  from  the  rest  of 
the  mass  by  the  silica,  which,  while  constituting  the  main  constituent  of  the  darker 
colored  bands,  penetrates  the  siderite  in  irregular  tongues  and  seams.  This  silica  is 
exceedingly  finely  crystalline  and  perhaps  in  part  amorphous.  Mingled  with  it  in  the 
darker  colored  bands  are  films  of  chlorite,  detached  rhombohedra  of  iron  carbonate, 
and  dark  colored  seams  lying  parallel  to  the  geueral  lamination  of  the  rock,  but  non- 


TilK   IK()N-liEAliIN(i  MEMIJEU.  237 

continuous.  Those  consist  mainly  of  chlorite,  l.u(  miiy  iirobably  also  include  some, 
carbonacemis  material.  Similar  films  ajjpear  also  in  the  Ii};lit  colored  bands.  The, 
section  is  cut  by  small  veins  running  in  various  directions,  which  are  lilled  as  often 
with  siderite  as  (luartz,  and  sometimes  a  single  vein  contains  both  minerals;  also 
occasionally  chlorite  is  contained. 

ol.  Cherty  iron  carbonates,  from  middle  horizons.  Specimens  0473  (slide  .'5135), 
9473  (slide  3081),  0-174  (slide  30S2);  0475  (slide  .'iOSo),  047<J  (slide  3()S4),  0477  (slide 
3085);  all  from  1400  N.,  180  W.;  0470  (slide  3175),  0480  (slide  317G);  both  from  1325 
N.,  180  W.;  also  7548  (slide  2001),  7549  (slide  2062);  from  1450  to  1500  N.,  150  W. 
All  in  Sec.  13,  T.  47  IST.,  K.  40  VV.,  Michigan. 

These  specimens  represent  a  large  precipitous  exposure  of  a  ferruginous  slaty 
rock  in  the  bed  and  on  the  south  side  of  the  outlet  of  Sunday  lake.  In  the  main 
the  exposures  show  a  dark  colored  platy  look  with  an  earthy  fi-actui'e  and  the  general 
appearance  of  an  earthy  carbonate.  luterbedded  with  these  earthy  portions,  which 
are  themselves  finely  laminated,  are  nearly  black  flinty  seams.  These  again  show  a 
fine  banding  of  lighter  aud  darker  shades.  In  certain  portions  there  are  very  often 
black  seams,  which  upon  their  surfa(;es  show  a  graphitic  luster.  The  exposure  is  in 
general  heavily  stained  with  red  and  brown  iron  oxides,  the  fresh  fractures  showing 
these  oxides  often  arranged  along  certain  of  the  lamimc,  which  they  have  at  times 
entirely  replaced,  but  in  other  cases  along  irregular  cracks.  Some  of  these  streaks 
of  iron  oxides  reach  as  much  as  a  foot  in  width,  in  which  case  they  form  a  moderately 
rich  hematite  iron  ore  with  a  porous  texture  and  slight  metallic  luster.  Some  more 
minute  cracks  have  been  filled  with  brilliantly  lustered  specular  iron.  The  sp.  gr.  of 
the  more  compact  aud  less  siliceous  portions  varies  from  3  to  3-50.  The  chemical 
composition  of  9472,  which  is  little  altered  and  includes  a  number  of  minute  silicified 
bands,  is:  silica,  28-86;  titanic  oxide,  0-20;  alumina,  1-29;  iron  sesquioxide,  1-01;  iron 
protoxide,  37-37;  manganous  oxide,  0-97;  calcium  oxide,  0-74;  magnesium  oxide, 
3-64;  water,  0-68;  carbon  dioxide,  25-21;  phosphoric  acid,  trace;  organic  matter, 
undetermined  =  99-97. 

The  thin  sections  from  these  specimens  show  a  rock  closely  resembling  that  of 
43.  The  sections  difi'er  from  one  another  only  in  relative  proportions  of  minerals 
contained,  and  in  that  some  of  them  contain  a  little  chlorite.  The  relation  of  the 
silica,  iron  carbonate,  aud  of  the  blackish  supposedly  carbonaceous  seams  are  all  as 
in  43.  -Some  of  the  sections  include  portions  of  the  hematite  seams,  in  which  case  one 
side  of  each  section  shows  the  unaltered  carbonate  rhombohedra,  the  middle  of  the 
section  showing  these  rhombohedra  partly  changed  to  hematite,  while  on  the  other 
side  of  the  slide  they  are  completely  replaced  by  the  hematite.     (PI.  xxi.  Fig.  4.) 

From  the  exposures  in  Sees.  7  and  18,  T.  47  N.,  M.  45  W.,  Michigan. 

55.  Cherty  iron  carbonate,  fi'om  a  low  horizon.  Specimen  12543  (slide  5336); 
from  1600  N.,  1076  W.,  Sec.  18,  T.  47  IS".,  E.  45  W.,  Michigan. 


238  THE  PENOKEE  IRON-BEARmG  SERIES. 

A  rock  closely  similar  to  13886  in  53.  Sp.gr.,  3-29.  Compositiou:  silica,  36-73; 
titanic  oxide,  0-19;  alumina,  0-38;  iron  sesquioxifle,  0-98;  iron  protoxide,  3-4-74;  man- 
ganous  oxide,  0-52;  calcium  oxide,  0-48;  magnesium  oxide,  2-74;  carbon  dioxide, 
22-44;  phosphoric  acid,  0-009;  iron  sulphide,  0-12;  water  at  105°,  0-12;  water  above 
1-05°,  1-40  =  100-84. 

The  thin  section  shows  bands  of  an  almost  compact  siderite  with  very  slight 
admixture  of  silica,  which  very  gradually  grade  into  and  alternate  with  others  com- 
posed mainly  of  a  very  minutely  crystalline  and  amorpljoas  silica.  In  the  latter  are 
included  numerous  rhombohedra  of  iron  carbonate.  The  differences  noted  between 
this  rock  and  53  lie  in  the  somewhat  larger  size  of  the  siderite  rhombohedra,  the 
presence  of  only  a  small  proportion  of  chlorite,  and  in  exhibiting  the  transition 
between  siderite  and  iron  oxide.  The  section  is  cut  by  veins,  which  are  filled  with 
quartz,  siderite,  and  actinolite.  The  quartz  and  siderite  are  in  larger  individuals  here 
than  in  the  remainder  of  the  section. 

56.  Oherty  iron  carbonates,  from  middle  horizons.  Specimens  7526  (slide  1962), 
from  0  ]:^.,  1730  W.;  7527  (slide  2297),  from  0  N.,  1000  W.;  7528  (slide  2059),  from  0  S., 
735  W.,  Sec.  7,  T.  47  N.,  R.  45  W.,  Michigan. 

The  thin  sections  all  contain  a  considerable  quantity  of  iron  carbonate.  In 
slide  2059  it  is  the  chief  constituent,  in  1962  it  about  equals  in  quantity  the  flinty 
background,  while  in  2297  it  is  subordinate  in  quantity.  Slide  1962  is  essentially  like 
53.  The  alteration  of  the  iron '  carbonate  in  2059  has  resulted  in  the  formation  of 
abundant  hematite  and  numerous  small  crystals  of  magnetite.  In  2297  the  alteration 
^  of  the  carbonate  and  the  introduction  of  silica  has  resulted  in  forming  numerous 
beautiful  concretions,  most  of  which  contain  several  concentric  belts  of  iron  oxide. 
In  quite  a  large  number  of  these  concretions  iron  carbonate  varies  in  quantity  from 
little  to  a  chief  constituent. 

57.  Ferruginous  chert  and  jasper,  from  a  middle  horizon.  Specimen  12665 
(slide  5389),  from  200  N.,  1475  W.,  Sec.  7,  T.  47  N.,  R.  45  W.,  Michigan. 

A  fine  grained,  dark  purplish  rock  carrying  vaguely  outlined  brigbt  red  spots 
which  have  the  appearance  of  jasper. 

The  thin  section  is  one  of  the  brecciated  mixtures  of  chert  and  silica,  differing 
from  a  number  above  described  only  in  having  the  iron  oxide  mainly  bright  red 
hematite  instead  of  magnetite  or  the  brown  oxide.  A  greenish  chloritic  material  is 
present  in  a  number  of  the  concretionary  areas.  The  silica  ranges  from  very  finely 
crystalline  to  amorphous.  Remnants  of  iron  carbonate  are  apparent  in  some  of  the 
concretionary  areas.  The  very  large  proportion  of  iron  peroxide  present  in  this 
section  disguises  its  structure  somewhat,  but  enough  is  perceptible  to  make  it  evident 
that  we  have  here  to  do  with  a  phase  of  one  of  the  brecciated  rocks. 

58.  Magnetic  cherty  iron  carbonates,  from  high  horizons.  Specimens  7530 
(sUde  1964),  from  840  N.,  500  W.;  7524  (slide  2296),  from  700  N.,  100  W.,  Sec.  7,  T.  47 
N.,  R.  45  W.,  Michigan. 


THE  mON-BEAlUNG  MEMW5R.  239 

Till-  iiiiaKcu'd  iMniioiisof  llic.  thin  sections  arc  iiiiide  up  iiiaiiily  of  grayish  aggre- 
gates of  siilerito  individuals  mingled  with  more  or  less  amori)li()ns  Dr  minutely  crys- 
talline, silica,  and  some  darkening  material  in  certain  hands  wliidi  is  ])orha|)s  in  the 
nature  of  carbonaceous  nuitter.  Mximerous  minute  black  crystals  of  magnetit(!  are 
scattered  through  the  siderite.  Erom  these  less  altered  forms  there  are  grades  of 
transition  to  an  opaque  mass  of  red  iron  oxide,  which  is  aggregated  particulai  ly  along 
certain  seams.  In  slide  '2'2m  there  is  mingled  with  the  other  ingredients  a.  consider- 
able proportion  of  greenish  chlorite  and  a  few  small  particles  of  fragmental  quartz. 

From  the  exposures  in  Sees.  10  and  11,  T.  17  N.,  Ji.  -15  W.,  Michigan. 

59.  Actinolitic  and  magnetitic  quartz-schist,  from  aloM'  horizon.  Specimen  7512 
(slide  2294),  from  570  N.,  873  W.,  Sec.  10,  T.  17  N.,  E.  15  W.,  Michigan. 

The  thin  section  of  this  rock  shows  magnetite  and  hematite  predominating,  and 
for  the  most  part  aggregated  into  opaque  masses.  Within  the  masses,  however,  are 
irregular  areas  and  streaks  of  lighter  colored  materials  which  are  in  ])art  minutely 
divided  crystalline  quartz,  and  in  part  a  greenish  material  which  includes  both  actin- 
olite  and  chlorite,  the  latter  apparently  derived  from  the  actinolite.  Quite  often  the 
small  detached  particles  of  the  iron  oxide  are  seen  to  have  a.  rhombic  outline.  In  the 
section  are  a  number  of  small  rounded  grains  of  ft'agmental  quartz.  These  quartz 
grains  are  often  enlarged,  and  are  all  readily  distinguishable  from  the  nonfragmental 
finely  crystalline  interlocking  quartzose  background. 

60.  Cherty  iron  carbonate,  from  a  nuddle  horizon  and  just  beneath  a.  greenstone 
of  the  Keweenaw  series.  Specimen  7516  (slide  2295),  from  815  JST.,  1170  W.,  Sec.  10, 
T,  47  ]Sr.,  E.  15  W.,  Michigan. 

The  thin  section  is  comi)osed  mainly  of  a  compact  grayish  mass  of  siderite, 
slightly  stained  along  certain  lines  with  iron  oxide  and  containing  here  and  there 
particles  of  jnagnetite.  On  one  side  of  the  section  is  a  band  of  very  minutely  crystal- 
line to  amorphous  silica  containing  numerous  particles  of  iron  carbonate,  which  are 
separated  from  the  main  mass  by  the  silica. 

61.  Ferruginous  chert  and  jasper,  from  a  middle  horizon,  and  only  a  short  dis- 
tance beneath  a  greenstone  of  the  Keweenaw  series.     Specimens  12791  (slide  5177) 
12791  (sUde  5178),  12794a  (slide  5479).     All  from  the  SW.  {-  of  the  SE.  ^,  Sec.  11,  T.  47 
If.,  E.  45  W.,  Michigan. 

These  specimens  represent  the  several  laminations  seen  in  the  crosscut  at  the 
bottom  of  a  test  pit.  Specimen  12791  shows  a  thin  lamination  of  bright  red  jasper 
with  black  seams.  Specimen  12794  shows  nearly  the  same,  exce^jt  that  the  jaspery 
portions  take  on  a  more  cherty  appearance  and  whitish  color,  and  the  dark  colored 
bands  now  and  then  present  the  metallic  luster  of  magnetite.  Specimen  12794a.  is 
mainly  composed  of  a  thinly  laminated  dark  red  hematite,  being  rich  enough  in  iron 
to  constitute  an  iron  ore. 


240  THE  PENOKEE  IRON-BEAEING  SEKIES. 

In  tliiu  sections  the  black  seams  above  mentioned  appear  to  owe  their  dark  color 
mainly  to  the  presence  of  magnetite,  which  is  in  sharply  outlined  crystals,  the  outlines 
being"  nearly  always  rhombic.  In  some  cases  the  magnetite  appears  to  constitute  tlie 
main  ingredient  of  the  dark  colored  bands,  while  in  others  it  is  mingled  with  nune 
or  less  hematite  and  actinolite.  The  red  bands  are  an  admixture  of  hnely  divided 
bright  red  hematite  and  A'ery  minutely  crystalline  to  amorphous  silica.  Tliese  two 
minerals  are  arranged  in  such  a  fashion  as  to  produce  a  general  laminated  appearance, 
the  hematite  being  aggregated  in  thin  belts.  There  are  portions  of  tliese  jasper  bands 
whii'h  show  relatively  little  hematite,  being  made  up  mostly  of  silica.  The  belts  of 
silica  follow  the  lamination  in  a  general  way,  now  and  then  breaking  across  it,  while 
a  single  seam  of  the  silica  often  branches.  In  some  of  the  black  bauds  a  considera- 
ble quantity  of  actinolite  arranged  in  aggregates  of  diu'k  green  blades  is  visible.  The 
red  iron  ore  seams  above  referred  to  are  small  portions  richer  than  usual  in  the  sesqui- 
oxide  of  iron.     (n.  xxviii,  Fig.  3.) 

From  the  test-pit  near  the  center  of  See.  18,  T.  -17  N'.,  li.  4.1  W.,  Michigan. 

62.  Hematitic  siderite,  from  a  low  horizon.  Specimen  127SS  (slide  563S) ;  from 
near  the  center  of  Sec.  IS,  T.  -iT  N.,  11.  ii  W.,  Michigan. 

An  aphanitic,  earthy,  light  reddish  rock.     Sp.  gr.  2.8(i. 

Iron  carbonate,  in  part  well  crystallized,  makes  up  the  larger  part  of  the  thin 
section.  It  is  as  usual  mingled  Mith  some  minutely  crystalline  to  amorphous  quartz 
and  particles  of  red  and  brown  iron  oxides. 

From  the  test-pit  in  Sec.  17,  T.  4.7  If.,  B.  44  W.,  Michigan. 

63.  Oherty  siderite,  from  a  low  horizon.  Specimens  127S3  (slide  5471),  12784 
(slide  5472);  from  550  N.,  1100  W.,  Sec.  17,  T.  47  K,  E.  44  W.,  Michigan. 

The  rocks  are  composed  of  alternate  bauds  of  an  aphanitic,  earthy  looking,  dark 
gray  material  and  of  reddish  clierty  and  jaspery  looking  material,  in  which  are  per- 
ceptible numerous  i)ieces  of  a  translucent  quartz,  red  jasper,  and  black  Hint.  Sp. 
gr.  of  gray  bauds,  3.20;  of  cherty  bands,  2.34. 

In  thin  sections  the  dark  colored  bands  are  composed  of  a  mixture  of  grayish 
siderite  and  tinely  crystalline,  with  perhaps  anun-phous,  silica.  Irregular  dark  colored 
streaks  traverse  these  bands  in  a  direction  parallel  to  the  lamination  of  the  rock. 
These  streaks  sire  made  up  of  minute  dark  colored  particles,  which  are  taken  to  be 
mainly  the  oxides  of  iron,  but  may  be  partly  of  carbonaceous  material.  Crystals  of 
magnetite  are  seen  scattered  here  and  there,  and  the  relations  of  the  silica  and  carbon- 
ate are  such  as  have  been  repeatedly  described  above.  In  the  cherty  belts  a  very 
finely  crystalline  quartzose  background,  including  little  oxide,  with  perhaps  some 
amorphous  silica,  contains  many  well  rounded  large  simple  grains  of  fi-agmental 
quartz  which  have  undergone  a  second  growth,  and  very  many  more  rounded  areas 
of  ferruginous  chert.    The  chert  areas  comprise  those  with  little  iron  oxide,  with 


THE  lEON-BEARING  MEMBER.  241 

abuiulaiit  umI  ln'inalitf  iiiakiiii;  lliciii  jasi.i  r,  uilli  iiiaunctilt',  ami  (iiially  with  liiTiiatilc 
and  magnetite.  Ottm  tlic  areas  wliiili  foniaiii  lilflr  m-  mi  inm  iixiilc  ami  tlidsc  con 
tainini;- the  largest  amoiiiils  are  in  Jiixtaposiliim.  Tlic  simph'  .mains  nl'  t'ia,uniental 
.(|nart/.  ami  Hie  chcil  areas  aliUc^  arc  jjcncrally  arranged  wiih  ihcir  limucr  axes  in  a 
eoaiinon  (lii'cci  inn.  The  unniislakaliic  fra.unn'iilal  cliara<'lcr  n\'  Ihc  sinipic  i|narl/,  Ihc 
essential  likeness  to  thcni  of  tin'  chert  ai-eas  in  ontlines.  ami  I  heir  arran.^enienl  willi 
longer  axes  in  a  coninion  direction,  are  eoncliisi\c  |)rool's  that,  they  arc  all  nieehanical 
sodiment.s — not  I'rauinent  sea  used  by  a  later  intnisicni  of  silica,  or  to  coneretioiniiy  action 
iu  connection  with  such  introduction  of  silica.  In  lacl.  these  areas  show  an  entire 
absence  of  any  concretionary  structure,  except  lieyoml  iheii-  outer  borders.  Here 
such  a  structure  is  at  times  seen,  l)ut  the  saiiu'  Ihiufi-  is  true  of  the  eidar.ncd  fraji- 
nieutal  qtuirtzes,  and  is  in  all  eases  plainly  iu  the  cementing  silica.  In  this  rock  is 
therefore  a  shai-p  alternation  of  clastic  ami  nom-lastie  sedinuMitatiou,  the  interlami 
nated  belts  being  at  tiiiu's  not  more  than  an  inch  broad. 

(U.  Sideritie  cherts,  from  low  horizons.  Specimens  0306  (slide  4l!132),  from  (?()0 
2s.,  1085  W.;  0307  (slide  4210),  from  615  N.,  1085  W.  Sec.  17,  T.  47  N.,  K.  44  W, 
Michigan. 

A  thiidy  laminated  rock,  in  which  fine  grained,  dark  gray  baiuls  alternate  with 
redjaspery  and  cherty  ones.     Sp.  gr.  of  the  grayish  baiuls,  .i.Ol. 

Iu  the  thin  .sections,  narrow  bands  are  composed  of  a  nearly  ])ure  aggregate  of 
iron  carbonate,  but  these  grade  off  into  other  portions  which  are  mainly  composed  ot 
minutely  divided  silica,  Avhich  contains  a  good  deal  of  hematite  and  magnetite.  In 
slide  i'2'2'2  these  reddish  bands  are  in  the  main  of  the  peculiar  type  described  in  63, 
although  containing  less  fragmental  (juartz  in  simple  grains.  In  4210  a  fragmeutal 
character  is  less  plain. 

65.  Ferruginous  chert  or  flint,  Ixoui  a  lower  middle  horizon.  Specimens  126S6 
(slide  5408),  12687  (slide  5400);  from  850  X.,  1650  W.,  Sec.  17,  T.  47  N.,  E.  44  W., 
Michigan. 

The  rocks  are  aphauitic,  dark  reddish  brown  cherts,  iu  whii'h  are  seen  aggre- 
gated in  certain .  portions  numertius  grains  of  limpid  quartz.  Sp.  gr.  of  the  more 
highly  hematitic  iwrtions,  3.62. 

The  thin  sections  are  in  all  essential  respects  like  the  fragmeutal  ]>arts  of  63. 
The  amount  of  simple  fragmental  qufirtz  iu  slide  5400  is  less  than  that  iu  5408  and  iu 
63.  The  iron  oxide  iu  the  fragments  aiul  matrix  is  mostly  hi'iuatite,  although  mag- 
netite is  i)resent.  The  grains  of  simple  tpiartz  have  been  well  rouiuled,  and  are  often 
widely  enlarged.  They  quite  often  contain  within  their  cores  crystals  of  hematite 
and  magnetite.  In  this  respect  they  are  closely  like  the  ferruginous  ([uartzitc  12680, 
described  page  171.  In  fact,  the  chief  difference  between  the  two  rocks  is  that  in 
12680  the  fragmental  material  is  more  abundant, 
jviON  XIX 16 


242  THE  PENOKBE  lEON-BE  A.RIIirG  SERIES. 

66.  Jaspery  and  cherty  siderite,  fiom  nu  upi:)er  middle  horizon.  Specimens  12683 
(slide  5405),  12684  (slide  5406),  12685  (slide  5407).  All  from  1260  N.,  1350  W.,  Sec.  17, 
T.  47  ]Sr.,  E.  44  W.,  Mieliigaii. 

This  is  a  laminated  rock,  composed  of  an  ai:)hanitic  to  dark  gray  earthy  looking, 
material,  which  is  miuntely  handed  in  itself  with  lighter  and  darker  shades,  and 
alternates  with  bands  of  very  bright  red  jasper,  which  range  from  the  thickness  of  a 
sheet  of  pai)er  to  an  inch  in  width.     Sp.  gr.  of  12685,  2.9'< . 

The  jaspery  bands  in  thin  section  5405  are  seen  to  be  mainly  composed  of  a 
uniform  intermixture  of  minutely  crystalline  quartz  and  bright  red  liematite,  with 
which  are  mingled  soiiie  nuignetite  particles.  Occasionally  the  hematite  shows 
rhombic  outlines,  but  these  are  not  generally  iierceptible,  i^erhaps  because  of  the  close 
aggregation  of  the  particles.  The  dark  colored  seams  prove  in  the  thin  section  to  be 
in  part  a  mixture  of  crystals  of  magnetite  and  minutely  crystalline  quartz.  Such  bands 
as  this  are  found  to  be  directly  interlaminated  with  the  biight  red  jasper.  In  other 
cases  the  darker  colored  bands  owe  their  dark  coloi'  to  a  luixtnre  of  minute  particles, 
which  aio  iu  part  probably  of  a  carbonaceous  nature.  Such  bands  as  this  are  found 
interlaminated  more  directly  with  those  portions  in  which  carbonate  of  iron  is  abun- 
dant (5407).  Tlic  lighter  colored  portions  of  5407  are  in  part  a  mixture  of  iron 
carbonate,  brown  and  red  iron  oxides,  magnetite  particles,  actinolite  Jieedles,  and 
minutely  crystalline  silica,  the  latter  mineral  being  tLic  least  plentiful.  Still  other 
bands  are  made  uj)  mainly  of  a-  silica  which  r-anges  from  very  minutely  crystaJliue  to 
nearly  o"  quite  amorphous.  In  this  flint  or  chert  are  found  remains  of  bands  of  iron 
carbonate,  single  rhombohedra  of  the  same,  and  scattering  crystals  of  magnetite. 

07.  Actinolitic  magnetite-slate,  from  a  high  horizon.  Specimens  12703  (slide 
5420),  from  1180  IST.,  655  W.;  12704  (slide  5421);  from  1200  N.,  655  W.,  Sec.  17,  T.  47 
N.,  R.  44  W.,  Michigan. 

A  very  heavy  aphanitic  slaty  rock,  made  up  of  )uinute  alternating  laminaj  of 
darlc  gray  and  black  shades.  Portions  of  the  si^ecimens  sh(jw  a  distinct  metallic 
luster,  particularly  on  the  black  lamiu;e,  and  (xuite  large  ineces  are  lifted  by  the  mag- 
net.    Interlaminated  with  this  material  are  bands  of  a  much  lighter  color. 

In  thin  section,  the  lighter  colored  bands  (5420),  last  referred  to,  are  seen  to  have 
a  minutely  crystalline  siliceous  groundmass,  in  which  are  included  numerous  minute 
actinolite  needles  and  particles  of  iron  oxide,  including  the  magnetic  oxide.  The 
actinolite  is  on  the  whole  quite  as  plentiful  as  the  silica,  and  in  portions  of  the  section 
is  aggregated  into  felted  masses.  The  earthy  black  i)ortions  of  the  rock  present  a 
section  which  differs  from  that  just  described  mainly  iu  the  relatively  great  abundance 
of  iron  oxides,  particularly  magnetite;  this  last  mineral  being  more  especially  aggre- 
gated into  irregular  lanuiue.  Actinolite  is  very  abundant,  constituting  in  considerable 
portions  of  the  section  an  iron-stained  felted  mass  of  minute  needles. 


THE  IKON-BEARING  MEMBER.  243 

68.  Magnetitif  iictiiHililc  schist,  tVimi  a  vciy  lii^li  iK.ii/.on  iiuiiicdiatcly  be.iuMtli 
the  Kewi-eiia Willi  fiiccnstniic.  Siu'cinHMi  10402  (slide  r,:;-2l);  t'nmi  l.>10  N.,  KiOO  W., 
Sec.  17,  T.  -17  N.,  K.  41  W.,  .Micliif;an. 

.V  ft'Ited  mass  of  actinolilc  needles  eoiii|i(ises  tlie  backf;roiuid  of  the  section. 
This  mass  is  eoiiiiiioiily  stiiiiied  red  and  hrowii  liy  iioii  oxides,  and  joiitaiiis  besides, 
opaque  aggregations  ol'  Mia.uiiclite  crystals  which  iiiak.'  iiii  as  much  as  half  the  section. 

From  the  expomircs  tind  trst-pifs  in  IScck.  I',,  id  and  :.>1,  T.  17  N.,  U.  J  J  W.,  Michigan. 

60.  Actiiiolitic  magnetite-schist,  from  ;i  low  horizon.  Specimens  1278(i  (slide 
5474),  12787  (slide  5475);  from  1625  N.,  650  W.,  Sec.  21,  T.  -17  N.,  R-.  44  W.,  Michigan. 

An  aphaiiitic,  laminated,  very  dark  gray  rock,  analogous  to  those  described 
under  OS.     Sp.  gr.  of  12787,  o.53. 

The  thin  sections  are  typical  actinolitic  magnetite  schists,  such  as  have  been 
described  already  as  occurring  at  Penokee  gap,  etc.  The  gronndmass  is  a  finely  crys- 
talline (juartz,  throughout  which  are  contained  minntt' blades  of  actinolite.  Tlie  mag- 
netite occurs  as  usual  in  bunchy  aggregations  of  sharply  outlined  crystals,  aud  also 
in  single  crystals  scattered  throughout  the  gronndmass.  In  slide  5475  there  is  a  gen- 
eral tendency  towards  a  concretionary  arrangement,  which  in  portions  of  the  section 
is  very  strongly  developed;  aud  after  having  seen  the  various  concretionary  develop- 
meuts  of  the  foregoing  rocks  one  has  no  hesitation  in  saying  that  these  are  of  the 
same  origin  with  all  the  rest. 

70.  Actinolitic  magnetite-schist,  from  a  low  horizon.  Specimen  12781  (slide 
5470);  from  70  N.,  1790  W.,  Sec.  15,  T.  47  N.,  R.  44  W.,  Michigan. 

A  very  heavy,  slaty,  dark  gray  rock,  analogous  to  that  last  described,  except 
that  certain  vaguely  defined  bands  have  a  pale  reddish  or  jaspery  appearance.  The 
content  of  magnetite  is  evidently  great,  as  large  sized  ijieces  are  easily  lifted  by 
the  magnet. 

The  section  is  again  one  of  the  ty^jical  actinolitic  magnetite-schists.  The  back- 
ground, as  usual,  is  a  minutely  crystalline  quartz.  In  this  are  contaiiied  fan-like  ag- 
gregates of  unusually  large  actinolite  blades,  and  crystals  of  magnetite,  partly  aggre- 
gated in  certain  bauds,  in  which  are  contained  a  large  proportion  of  red  and  brown 
iron  oxides. 

71.  Ferruginous  cherts,  ft'om  a  middle  horizon.  Specimens  12671  (slide  .5395), 
12672  (slide  5396),  12675  (slide  5398) ;  all  from  450  N.,  675  to  729  W.,  Sec.  16,  T.  47  N., 
K.  44  W.,  Michigan. 

The  rocks  are  brown  and  red  stained,  highly  ferruginous  cherty  schists,  made 
up  of  alternating  laminiB  of  black,  red,  and  brown  colors. 

The  thin  sections  are  composed  essentially  of  a  minutely  crystalline  to  amor- 
phous silica  with  the  red  and  brown  and  magnetic  oxides  of  iron.  These  oxides  are 
aggregated  more  especially  into  certain  lamime,   other  laminie  being  almost  jmre 


244  THE  PENOKEE  lEON-BEAEING  SEEIES. 

chert.  On  the  eilges  of  the  iron  oxide  huniiuB  the  individual  particles  of  hematite 
and  brown  oxide  are  seen  veiy  frequently  to  have  sharp  rhombic  outlines.  The 
magnetite  is  quite  subordinate  in  quantity  to  the  otlier  oxides,  being,  however,  more 
plentiful  in  some  bands  than  in  others.  It  occurs  in  sharply  outlined  crystals.  The 
hematite  particles  that  occur  in  these  bauds,  which  are  mainly  made  up  of  flinty  silica, 
are  often  arranged  in  a  radiate  manner. 

72.  Ferrugimnis  chert-schists,  from  a  high  horizon.  Specimens  12691  (slide 
5412),  126!)2  (slide  r.il3),  12G93  (slide  5414);  all  from  1380  N.,  1960  W.,  Sec.  15,  T.  47 
N.,  E.  44  W.,  Michigan. 

These  specimens  represent  alternating  lamina*  of  gray  and  red  stained  chert  and 
hematite  ii^on  ore.  Tlie  iron  oxide  seams  show  a  more  distinct  subordinate  kmina- 
tion  than  is  perceptible  in  the  cherty  portions.  Sp.  gr.  of  irony  layers  (12693),  3.25. 
The  cherty  layers  in  the  thin  section  are  seen  to  be  made  up  almost  entirely  of  a 
minutely  crystalline  to  nearly  amorphous  silica.  Tlierc  is  also  throughout  this  silica 
a  general  ^.endency  to  a  concretionary  structure,  which  is  brought  out  by  a  vague 
concentric  arrangement  of  the  more  or  less  completely  crystalline  particles.  The 
whole  appearaiuje  of  this  chert  is  very  strikingly  like  the  cherts  which  have  been 
above  described  and  ttgured  as  characteristic  of  the  limestone  member  of  thePenokee 
series.  In  these  cherts  occur  irregular  bunches  of  hematite  and  brown  iron  oxide,  the 
particles  of  which  often  show  most  distinctly  rhombic  outlines  of  the  carbonate  crys- 
tals, from  whose  alterations  they  are  taken  to  have  originated.  Some  of  these  rhombic 
crystals  are  of  unusually  large  size;  and  in  one  or  two  places  appear  to  still  retain 
portions  of  the  original  carbonate.  The  more  highly  ferruginous  laminaj  differ  from 
the  cherty  phases  mainly  in  ihe  large  proportion  of  the  oxides  of  iron,  which  now 
preponcerate  greatly  over  the  siliceous  matrix.  The  particles  of  iron  oxide  iire  here  * 
arranged  in  regular  lines,  which,  without  much  doubt,  mark  the  original  lamination  of 
the  rock.  The  rhombic  outlines  to  the  hematite  particles  arc  frequent,  and  some  of 
the  silica  shows  the  radiating  structure  characteristic  of  chalcedony. 

73.  Actinolitic  ferruginous  schist,  from  a  high  horizon  and  immediately  in 
contac*;  with  overlying  Keweenawau  greenstone.  Specimens  9381  (slide  3269),  9382 
(slide  3039).     From  670  K.,  1020  W.,  Sec.  16,  T.  47  N.,  E.  44  W.,  Michigan. 

The  specimens  present  an  alteruatiou  of  brown  cherty,  red  jaspery,  dark  green 
and  black  aphauitic  lamime,  the  whole  rock  having  a  distinct  slaty  or  parallel  struc- 
ture.    Sp.  gr.  of  the  black  laminic  (9382),  3.90. 

The  thin  sections  of  these  rocks  are  particularly  interesting  and  instructive,  in 
that  they  generally  show  in  a  single  section  all  of  the  characteristic  minerals  of  this 
class  of  rocks;  i.  e.,  brown  and  red  oxides  of  iron,  magnetite,  actinolite,  and  cherty  or 
flinty  silica.  These  diflcrent  minerals  occur  more  or  less  intermingled,  but  the  vary- 
ing appearance  of  the  lamime  as  seen  macroscopically  is  due  to  the  preponderance  of 
diflerent  minerals  in  the  diflerent  bands,     The  black  bands  are  particularly  rich 


Till':  ii;(>N-i!i:,\iMN(;  miimiuh;.  245 

ill  iiiiijiiictitc.  Tlic  led  luid  lnowii  InniiiKc  ;iit'  iic;iily  oiiatiuc  a^srciint  ions  (if 
particles  of  lieiiiutitc  aiul  liydro-scstiuioxide.  Tlic  greciiish  bands  are  iiarticularly 
ricli  in  actinolite.  Tlio  siliceous  snmiidmass  is  relatively  not  al)undant,  but  runs 
tliroujjliont  the  section.  The  bandiuj;-  of  this  rock  is  talctMi  to  be  d('i)cndent  ujion  an 
orifiinal  sedimentary  lamiiiation,  but  the  \va.\  in  wiiicii  tiio  bands  are  seen  now  to  hv 
swollen,  niinuiely  contorted,  or  abruptly  broken  otV  is  a  strong  indication  of  the  sec(»nd- 
ary  orif>in  of  a  i)ortion  of  the  minerals  whicli  arc  now  jnesent.  Besides  followiuy 
the  orifiinal  lamination,  the  various  groupings  of  tlic  minerals  lia\'e  often  traversed 
this  laniinatiou.  One  traversing  seam  of  actinolite  and  silica  is  particularly  note- 
worthy. This  is  a  structure  whicli  can  hardly  be  described,  but  is  of  importance  in 
its  bearing  upon  the  origin  of  tliese  singular  ferruginons  rocks. 

SECTION   II.— ORIGIN  OF  THE  ROCKS  OF  THE  IRON-BEARING  MEMBER.' 

Knowiug"  the  exact  facts  as  to  the  nature  and  method  of  occurrence  of 
the  iron-bearino-  rocks,  we  are  now  prepared  to  present  some  consistent 
account  of  their  origin.  In  the  detailed  tabulations  and  in  the  general 
account  of  their  macroscopic  and  microscopic  cliai-acters,  it  has  been 
necessary  to  anticipate,  to  some  extent  the  origin  of  certain  phases.  To 
the  degree  that  their  genesis  has  tluis  been  anticipated  it  is  not  in  the 
nature  of  theory,  but  fact,  because  the  stages  of  their  development  to  this 
extent  have  actually  been  observed.  Before  attempting  to  give  a  histor)'  of 
the  rocks,  it  will  perhaps  be  well  to  recapitulate  the  more  important  of  the 
observed  facts. 

(1)  The  Iron-bearing-  member  throughout  most  of  its  area  gives  abso- 
lutely no  evidence  of  a  fragmental  character.  This  characteristic  is  one  of 
the  greatest  importance.  The  quartz  rocks  of  the  iron-bearing  belt  have 
been  confused  with  the  underlying  qtiartzites.  By  most  observers  the}'  have 
been  taken  to  be  fragmental,  and  have  been  supposed  to  have  reached  their 
present  condition  b"\'  one  of  the  various  mysterious  processes  of  metamor- 
phism.  It  has,  however,  been  seen  that  the  fragmental  character  of  the 
undei'lying  quartzites  and  the  overlying  slates  is  manifest  when  their  thin  sec- 
tions are  examined  with  a  microscope.  Now,  this  Iron-bearing  member  lies 
between  these  two  fragmental  belts,  and  yet  nowhere  in  its  typical  rocks  is 

'Section  I  was  nearly  completed  when  Prof.  Irving's  sudden  death  occurred.  From  the  begin- 
ning of  this  section  the  junior  author  is  alone  responsible  for  the  form,  although  much  of  the 
substance  of  Sections  II  and  III  is  Prof.  Irving's  work. 


246  THE  PENOKEE  IRON-BEAIllN(J  SERIES. 

there  any  indication  of  a  fragmental  character.  Occasionally,  it  is  true,  in 
narrow  transition  bands  between  the  iron-bearing  and  the  fragmental  belts, 
there  is  mingled  with  the  nonfragmental  material  a  small  quantity  of  frag- 
mental quartz  and  feldspar.  Also  at  the  eastern  extremity  of  the  district 
clastic  and  nonclastic  sedimentation  have  alternated  to  some  extent.  How- 
ever, this  very  appearance  of  fragmental  material  l)ut  more  strongly  empha- 
sizes the  fact  of  the  nonfragmental  character  of  the  belt  as  a  whole,  by 
bi-ine'ing'  into  the  same  thin.sections  clastic  and  nonclastic  material.  The 
clastic  particles  are  recognized  at  a  glance  as  entirely  different  from  tlie 
mass  of  the  finely  crystalline  or  partly  amorphous  completely  interlock- 
ing material  which  composes  the  i-ocks  of  the  Iron-bearing  member.  (PI. 
XXXV,  Fig.  1.) 

(2)  Tlie  rocks  of  the  Ir(in-bearing  member  are  laminated.  Through 
laro-e  parts  of  its  area  the  lamination  is  as  perfect  and  minute  as  is  possible 
in  any  sedimentary  stratified  rock.  These  regularly  laminated  portions 
are  found  at  all  horizons,  and  nowhere  is  a  fair  degree  of  i-egularity  of 
stratification  absent.  The  most  perfectly  laminated  parts  of  the  belt  are 
of  the  first  type  of  rock — the  cherty  iron  carbonates.  As  the  lamination 
becomes  less  regular,  rocks  of  the  second  and  third  types — the  ferruginous 
cherts  and  the  actinolitic  slates — appear. 

(3)  There  have  been  observed  at  many  places  actual  stratigraphical 
transitions  of  the  regularly  bedded  carbonates  into  the  remaining  rocks  of 
the  belt.  This  gradation  sometimes  occurs  in  passing  from  east  to  west,  as, 
for  instance,  the  cherty  carbonates  and  the  ferruginous  cherts  grade  into  the 
actinolitic  slates,  or  the  gradation  may  be  a  transverse  one ;  that  is,  the 
cherty  iron  carbonates  at  a  higher  horizon  grade  into  the  ferruginous  cherts 
at  lower  horizons.  This  stratigraphical  gradation  of  the  three  types  of 
rock  into  each  other  is  alone  suflicient  to  make  very  probable  for  all  of 
them  a  common  origin.  The  gradation  is  repeated  in  thin  sections,  nearly 
all  stages  of  the  various  transitions  being  clearl}^  worked  out. 

The  original  rock — The  foregoing  facts  all  point  unmistakably  in  the 
same  direction;  that  is,  to  the  conclusion  that  these  rocks  at  one  time  Avere 
cherty  iron  carbonates.  There  may  be  differences  of  opinion  as  to  whether 
at  some  earlier  stage  they  did  not  have  another  form ;  there  may  be  differ- 


TIIK  IliON-liliAlUNd  MM.MIlKi;.  247 

encos  of  opinion  as  to  the  luninicr  in  wliicli,  iVoni  tlic  clicrtx'  cin-honatc  s, 
some  of  the  nuiltitndinous  plia.scs  of  rock  now  fonnd  haxc  liccn  foi'ni('(l  ; 
but  the  conclusion  can  not  he  escaped  that  tliese  rocks,  wliicli  still  make  u]) 
so  kirye  a  proportion  of  the  l)eU,  were  the  oi'i^inal  rocks  ot  the  nieinl)er. 
It  is  extremely  iniprol)ahle  that,  in  a  narrow  heU  of  rej^nlarly  stratilied 
rocks  about  800  feet  thick,  iuterstratitied  witli  otlicr  Ixdts  of  sedimentary 
rocks,  there  should  be  deposited  in  patches  here  and  tliei'e,  large  and  small, 
at  nearly  all  horizons,  cherty  iron  carbonate,  and  a  short  distance  east  or 
west  ferruginous  cherts  and  actinolitic  slates. 

The  c[uestion  arises  whether  the  cherty  iron  carbonates,  the  least 
altered  rocks  now  observable,  have  been  derived  from  an  earlier  and 
different  rock.  Thej^  are  apparently  unaltered,  and  that  they  Avere  origi- 
nally deposited  in  the  condition  now  found  is  probable  enough  from  analo- 
gies presented  by  later  geological  thnes.  The  cherty  ironstones  from  the 
Carboniferous,  which  occur  so  plentifull)^  in  our  own  country  in  Ohio  and 
Pennsylvania  and  so  extensively  in  other  countries,  are  like  tliese  thinly 
bedded  carbonates,  except  that  they  are  generally  more  argilla,ceous. 
Certain  Ohio  ores  are  so  remarkably  like  some  of  the  iron  carbonates  of  the 
Penokee-Gogebic  series  that  they  can  hardly  be  distinguished  in  hand 
specimen  or  thin  section  from  one  another.  This  essential  likeness  is  shown 
by  PI.  XXVII,  Figs.  1  and  2,  and  PL  xxix,  Fig.  4.  The  last  is  an  Ohio  cherty 
carbonate;  the  first  two  are  Penokee  carbonates.  The  only  difference 
betweeir  the  two  is  the  unimportant  one  as  to  the  nature  of  the  inclusions. 
The  clayey  character  of  the  Carboniferous  carbonates  is  no  more  than  an 
accident,  and  to  a  certain  extent  is  also  characteristic  of  the  older  carbon- 
ates of  the  Northwest.  It  simply  means  that  the  nonfragmental  sedimen- 
tation was  accompanied  by  mechanical  sedimentation  to  a  greater  extent 
in  the  Carboniferous  than  in  the  Iron-bearing  series  of  the  Northwest. 

In  Mississippi,  in  the  Claiborne  formation  of  the  Tertiary,'  there  are 
extensive  beds  of  cherty  iron  carbonate,  which,  according  to  analyses  pub- 
lished, are  almost  identical  in  composition  with  the  cherty  iron  carbonates 
of  the  Penokee  series.     A  third  analogy  presented  by  the  formations  of  more 

'  A  New  Discovery  of  Carbonate  Iron  Ore  at  Enterprise,  Mississippi,  by  Alfred  F.  Brainerd. 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  xvi,  1888,  pp.  146-149. 


248  THE  PENOKEE  lEON-BEAEING  SERIES. 

recent  times  is  in  the  ehert}^  limestones,  which  are  so  widely  found  at  all 
horizons.  It  may  be  a  question  as  to  these  limestones  whether  the  chert  was 
originally  in  nodules  and  layers  or  was  scattered  through  the  limestone  as 
disseminated  particles  of  silica,  but  from  later  investigations  it  appea,rs  certain 
that  in  some  cases  the  chert  was  originally  deposited  in  the  position  in  which 
it  is  now  found.  There  is  in  these  limestones  a  close  association  between  the 
cherts  and  calcium  and  magnesium  carbonates.  In  the  cherty  iron  carbon- 
ates of  the  Penokee-Gogebic  series  calcium  and  magnesium  are  present  as  in 
the  limestones,  and  we  need  onl}-  to  replace  a  portion  of  these  elements  by 
iron  to  have  rocks  -which  are  the  exact  analogue  of  the  cherty  limestones  of 
later  times.  As  has  been  seen  also,  such  chertj-  limestones  occur  in  deposits 
of  considerable  thickness  at  tlie  base  of  the  Penokee-Gogebic  series  itself 
From  analogy  it  is  then  extremely  probable  that  the  chei-t  and  iron  car- 
bonates were  simultaneously  deposited,  althotigh  it  may  be  a  possibility  (as 
has  been  maintained  with  reference  to  some  of  the  cherty  carbonates  of  later 
time)  that  the  chert  entered  very  early  in  the  history  of  the  rock  as  a 
pseudomorph,  replacing  carbonates. 

At  any  rate,  it  is  certain  that  a  large  portion  of  the  silica  now  present 
in  these  rocks  was  there  vei-y  early  in  their  history.  In  other  districts 
in  the  lake  Superior  country,  notably  at  Gunflint  lake,  in  the  Animikie 
series,  a  similar  cherty  carbonate  is  found  in  extensive  beds.  That  the 
chert  was  here  present  at  a  very  early  day  can  not  be  doubted.  The 
chert}'  bands,  where  there  is  no  folding,  are  evenly  interlaminated  with 
the  carbonates,  but  in  folded  areas  the  brittle  cherts  have  been  frac- 
tured in  every  direction,  so  that  the  rock,  instead  of  being  a  regularly 
laminated  one,  is  a  breccia,  which  contains  angular  fragments  of  chert  of 
greatly  varyiirg  sizes.  Such  a  brecciated'  rock  frequently  runs  into  the 
regularly  laminated  kinds  in  the  space  of  a  few  inches.  It  is  then  certain 
that  the  chert  of  these  beds  was  present  befor.e  the  folding  of  the  rocks. 
Further,  there  is  no  evidence  that  most  of  this  chert  was  not  deposited 
simultaneously  or  alternately  with  the  iron  carbonate.  The  i-elations  of  a 
portion  of  the  chert  and  the  iron  carbonate  are,  however,  such  as  to  show 
that  either  the  chert  has  entered  by  subsequent  solution,  or  that  the  silica 
originally  deposited  with  the  carbonate  was  subsequently,  to  a  greater  or 


TIIK  ll{ON-l!KAltlN(i  MKMlIKIf.  249 

less  oxtont,  r('iiiTanf>-e(l.  This  is  indicjited  l)\  the  l;icts  tliat  tlic  silicii  is 
f|iiit('  coiiipletcfly  crystallized,  and  that  tlic  silica  belts,  instead  of  heino- 
laminated  with  the  carbonate,  at  tin\es  break  aiToss  them  in  the  most  irreg- 
uhir  manner.  This  tissnring  must  have  been  subsequent  to  the  deposition 
of  the  carbonate,  and  may  imply  the  entrance  ot'  silica  from  an  extraneous 
source,  but  also  may  mean  no  more  than  that  silica  originall}'  present  has 
been  taken  into  solution  and  recrystallized.  In  either  case  there  is  no  proof « 
that  the  g-reater  part  of  the  silica  was  not  an  original  deposition. 

It  is  assumed  that  such  a  cherty  carbonate  is  water-deposited,  as  a 
direct  eruptive  origin  has  never  been  maintained  for  a  rock  of  this  char- 
acter. Such  an  origin  has,  however,  been  asserted  for  some  of  the  ferru- 
ginous cherts  and  jaspers  which  in  other  parts  of  the  lake  Superior  country 
are  found  associated  witli  the  carbonates.^ 

Taking  it  for  granted,  then,  that  this  cherty  carbonate  is  a  water- 
deposited  sediment,  the  questions  arise,  in  what  manner  the  iron  carbonate 
and  silica  were  originally  dissolved  and  how  they  were  precipitated.  We 
ma}',  without  varying  too  far  from  the  law  of  uniformity,  believe  that  in 
very  ancient  times  the  atmosphere  was  more  highly  charged  with  carbon 
dioxide  than  at  present.  We  may  also  believe  that  the  rocks  composing  the 
crust  of  the  earth  were  then  at  a  somewhat  higher  temperature.  An  increase 
in  heat  of  but  a  few  degrees  would  be  a  powerful  assistance  in  the  process 
of  solution  of  the  iron,  and  this  would  be  especially  true  if  the  atmosphere 
at  this  time  was  still  richly  charged  with  carbon  dioxide.  The  atmospheric 
waters  would  absorb  this  acid  and  carry  it  into  the  rocks  which  bordered  the 
ancient  sea,  would  decompose  them,  and  take  in  solution  ferrous  carbonate. 
Such  Avaters  escaping  into  the  shallow  ocean  at  hand  would  bear  the  material 
for  these  irony  deposits.  While  it  is  believed  that  these  conditions  may 
possibly  have  been  present,  their  assumption  is  not  necessary  to  account 
for  the  solution  of  the  iron;-  for  it  is  well  known  that  thick  beds  of  iron  ore 
have  formed  in  recent  times  by  virtue  of  the  solubility  of  iron  as  a  car- 
bonate under  ordinary  conditions.     It  is  also  not  impossible  that  its  solu- 

'Notes  on  the  Geology  of  the  Iron  and  Copper  Districts  of  lake  Superior,  by  M.  E.  Wadsworth. 
Bvdl.  Mus.  Comp.  Zool.  Harvard  Coll.,  whole  series,  vol.  vii,  Geological  series,  vol.  1,  No.  1, 
1880,  pp.  62-68. 


250  THE  PElSrOKEE  lEON-BEARlNG  SERIES. 

tion  was  greatly  assisted  by  means  of  terrestrial  life!  That  there  was  marine 
life  at  this  time,  as  will  be  seen,  we  have  strong  evidence.  Of  terrestrial 
life  we  have  no  such  proof,  but  it  is  highly  probable  that  life  existed 
on  the  land,  and  if  so,  tlie  organic  acids  would  be  of  assistance  in  decom- 
posing the  rocks  and  taking  iron  carbonates  into  solution.  The  iron  is  a 
carbonate  rather  than  a  hydroxide,  for  the  same  reasons  that  the  bedded 
*  carbonates  of  Carboniferous  times  are  so.  It  is  usually  taken  for  granted 
that  in  the  Carboniferous  deposits  the  presence  of  a  large  amount  of  organic 
matter  explains  the  presence  of  the  iron  as  a  carbonate.  Whether  the  iron 
was  originally  precipitated  as  a  carbonate,  or  was  decomposed  and  precipi- 
tated as  a  hydrated  sesquioxide,  just  as  limonite  now  forms  from  iron  car- 
bonate in  places  where  bog  ore  is  depositing,  is  uncertain.  If  the  latter  is 
taken  to  be  the  case  (and  it  is  perhaps  the  more  probable  supposition),  it  is 
necessary  to  believe  that  the  organic  matter  with  which  the  limonite  was 
associated  later  reduced  the  latter  to  the  protoxide,  and  by  its  decomposition 
furnished  the  carbon  dioxide  to  unite  with  this  pi'otoxide  and  thus  rej^roduce 
iron  carbonate.  Analyses  of  the  carbonates  of  the  Penokee  series  show 
conclusively  that  there  still  remains  in  these  rocks  quite  a  large  percentage 
of  organic  matter.  Also  in  the  thinly  bedded  argillaceous  slates  above 
them  the  percentages  of  hydrocarbons  are  at  times  quite  lai'ge.  Some  of 
the  black  slaty  carbonates  and  black  slates  of  the  other  iron-bearing 
series  in  the  Northwest  remarkably  resemble  the  black  carbonaceous  slates 
of  the  Carboniferous.  That  carbon  in  the  form  of  graphite  could  be  pro- 
duced in  other  ways  than  by  life  may  be  conceded ;  but  it  will  hardly  be 
urged  that  the  finely  disseminated  carbon  and  hydrocarbons  in  these  slates 
is  other  than  of  organic  origin. . 

Text-books  commonly  explain  chert  contained  in  limestones  as  of 
organic  origin  More  recentl)"  it  has  been  maintained  by  a  number  of 
writers  that  this  chert  is  a  chemical  sediment,  which  has  entered  the  carbon- 
ates as  a  pseudomorph  shortly  after  its  deposition.  The  evidence  that  an}"  of 
the  chert  is  not  of  organic  origin  is  of  a  negative  character;  but,  as  has  been 
said,  it  is  not  at  all  impossible  that  the  crust  of  the  earth  had  a  higlier  tem- 
perature at  the  time  of  the  formation  of  these  rocks  than  at  present.  If  so, 
we  need  not  necessarily  go  to  life  to  account  for  this  silica.     What  a  power- 


THE  IRONHEAllING  RIEMBEK.  '  251 

fill  assistance  an  increase  of  teinpei-jiturc  is  in  the  solnlioiidf  silica  is  slmwu 
by  the  geyserite  deposits  so  well  described  by  Hague.'  While  it  may  thus 
be  possible  that  a  pai't  of  the  silica  is  a  direct  cliciiiical  precipitate,  it  is  cer- 
tain that  life  is  sufficient  alone  to  collect  from  sea  waters  silica  in  solution 
and  form  extensive  deposits.  So  far  as  we  know,  this  was  first  shown  of 
the  Trinuningham  chalks,  the  chert  of  which  seems  without  question  to  be 
the  remains  of  life."  Later  it  has  been  shown  by  Dr.  G.  J.  Hinde  that  ex- 
tensive deposits  of  chert  in  Ireland,  England,  Wales  and  Spitzbergen  are 
largely,  and  possiblj-  wholly,  accumulations  of  sponge  spicules.  The 
deposits  of  Ireland  are  described,'  as  follows: 

They  consist  of  uodular  masses  of  irregular  form,  inclosed  in  beds  of  bard,  bluish 
limestones,  and  following  the  planes  of  bedding  much  in  the  .same  way  as  the  flints 
in  the  XTpper  Chalk;  but,  unlike  the  flints,  these  nodular  masses  are  not  sharply 
delimited  from  the  limestones  in  which  they  are  interbedded,  but  there  is  a  gradual 
passage  from  the  chert  to  the  limestone. 

More  frequently,  however,  the  cheit  exists  in  definite  beds  from  1  to  5  inches 
(.025-.12m.)  in  thickness,  which  intervene  at  irregular  intervals  between  beds  of 
limestone.  These  beds  sometimes  occur  also  as  well  marked  layers  in  the  central 
]3ortions  of  beds  of  limestone.  Both  the  nodular  aggregations  and  the  horizon- 
tally bedded  chert  usually  occur  in  the  same  series  of  rocks.  The  i)articular  mode  of 
deposition  probably  depends  on  the  extent  to  which  the  sponge  remains  (ot  which  it 
will  be  shown  the  chert  consists)  are  present  in  the  resijective  areas. 

It  will  be  seen  tliat  this  description  conforms  in  a  remarkable  manner 
to  the  occuiTence  of  chert  and  carbonate  in  the  Penokee  series.  At  Spitz- 
bergen, Axels  island,  Yorkshire,  and  North  Wales  there  are  alternations  of 
chert  and  limestone,  the  pure  cherty  layers  of  which  are  here,  however, 
often  quite  a  number  of  feet  in  thickness.  The  most  remarkable  occm-- 
rence  is  that  of  Spitzbergen  and  Axels  island.     The  beds  of  cherty  material 

'  Geological  History  of  the  Yellowstone  National  Park,  by  Aruokl  Hague.  Trans.  Am.  Inst.  Min. 
Eng.,  vol.  XVI,  1888,  pp.  783-803. 

-  On  the  Flint  Nodules  of  the  Trimmingham  Chalks,  by  Prof.  W.  J.  Sollas.  Auntils  Nat.  Hist. 
1880,  pp.  384  395,  437-461. 

'On  the  Organic  Origin  of  the  Chert  in  the  Carboniferous  Limestone  Series  of  Ireland,  and  its 
similarity  to  that  in  the  corresponding  strata  in  North  Wales  and  Yorkshire,  by  George  Jennings  Hinde. 
Geol.  Mag.,  London,  New  Series  (Decade  III),  vol.  rv,  pp.  435-446.  On  the  Chert  and  Siliceous  Schists 
of  the  Permo-Carboniferous  Strata  of  Spitzbergen,  and  on  the  characters  of  the  sponges  therefrom, 
■which  have  been  described  by  Dr.  E.  von  Dunikowski.     Dr.  Hinde.     Ibid.,  vol.  v,  pp.  241,  251. 


252  THE  PBNOKEE  lEON-BEAEIlsTI  SERIES. 

here  aggregate  870  feet  in  thickness.  That  the  cherty  layers  of  Wales  and 
England  are  almost  wholly,  if  not  wholly,  of  organic  origin  seems  to  be 
conclusively  shown.  Also,  it  is  certain  that  a  part  of  the  chert  of  Ireland, 
Spitzbergen,  and  Axels  island  is  of  organic  origin.  Dr.  Hinde  concludes 
that — 

It  is  true  that  the  number  of  specimens  of  chert  available  for  examination  are 
very  few,  and  they  might  be  regarded  as  insuflScient  of  themselves  to  warrant  the 
conclusion  that  this  gTeat  thickness  of  rock,  which  at  one  locality  on  Axels  island 
reaches  870  feet,  is  due  to  the  accumulation  of  the  skeletal  debris  of  siliceous 
sponges;  but  taking  into  consideration  the  fact  that  beds  of  similar  cherty  rock, 
which  in  Yorkshire  have  an  estimated  thickness  of  90  feet  and  in  North  Wales  of  350 
feet,  can  be  proved  to  be  due  to  sponge  remains,  there  is  nothing  extravagant  in  the 
supposition  that  this  much  greater  thickness  of  rock  has  had  a  similar  origin. 

These  cherts  in  Great  Britain  are  Carboniferous ;  those  of  Spitzbergen 
and  Axels  island  are  Permian.  It  is  of  interest  to  note  that  these  are  the 
terranes  in  which  the  most  extensive  beds  of  iron  carbonate  in  Paleozoic 
time  are  found.  Further,  the  sponge  remains  pass  into  a  "pure  translucent 
chert."  If,  as  Dr.  Hinde  maintains,  all  this  silica  is  derived  from  sponge 
spicules,  it  must  now  in  some  cases  be  extensivel}-  rearranged;  for  a  large 
part  of  the  chert  appears  from  the  descriptions  at  the  present  time  to  be 
cherty  or  chalcedonic  silica  in  forms  independent  of  organic  remains.  In 
the  Penokee  cherts  no  evidence  of  organic  origin  for  any  of  them  has  been 
found.  This  fact  does  not  seem,  however,  to  be  auj  proof  that  the  chert 
was  not  originally  deposited  in  the  form  of  sponge  spicules;  for  if  the 
sihca  in  deposits  so  late  as  the  Cretaceous  has  been  so  extensively  rear- 
ranged as  has  the  chert  in  the  Triramingham  chalks,  it  would  be  strange  if 
similar  deposits,  so  far  back  in  geological  time  as  the  Penokee  series,  had 
not  lost  the  evidence  of  organic  origin. 

Our  conclusion  is,  then,  that  we  have  no  satisfactory  proof  as  to 
whether  the  chert  of  the  Penokee  series  is  an  original  chemical  sediment  or 
the  remains  of  life,  but  the  latter  is  considered  more  probable.  The  quan- 
tity of  chert  which  is  contained,  supposing  the  whole  Iron-bearing  belt  to 
be  as  rich  in  silica  as  are  the  upper  horizons,  could  not  have  been  beyond 
300  or  400  feet.  It  appears  clear  that  original  formations  of  chert  have 
occurred  which  have  more  than  twice  this  thickness,  so  there  is  no  improb- 


THE  IKON-BEARING  MEMBER.  253 

ability  in  the  statement  ni;i(le,   tliat  this  material  was  deposited  simulta- 
neously with  the  iron  carbonate  with  which  it  is  so  closely  associated. 

The  ferruginous  slates.— The  microscopical  description  of  the  first  phase 
of  the  second  type  of  rock,  tlie  fen-u<;in(>us  slates  (pp.  203-206)  has  so 
fully  indicated  its  origin  that  but  little  more  need  be  said  here  than  to 
bring  together  the  actual  facts  of  observation.  Into  the  genesis  of  this 
phase  of  rock  little  or  no  theory  enters,  as  all  of  the  stages  of  its  growth 
have  been  seen.  The  only  conclusion  which  goes  beyond  the  observed 
facts  is  the  comparatively  safe  one  as  to  the  nature  of  those  exposures 
which  cannot  be  directly  traced  into  the  cherty  carbonates.  The  more 
important  of  the  facts  which  bear  upon  their  origin  are  as  follows:  The 
stratification  of  these  slates  is  precisely  like  that  of  the  original  carbonates; 
the  mineral  composition  of  the  two  rocks  is  the  same,  except  that  iron  oxide 
in  the  ferruginous  slates  takes  the  place  of  siderite ;  the  change  from  sider- 
ite  to  the  various  iron  oxides  of  the  slate  has  been  repeatedly  noted  in  all  its 
phases  in  thin  section  and  is  everywhere  found  in  the  slides  when  in  the 
field  the  change  from  the  cherty  carbonate  to  the  ferruginous  slate  could 
be  traced.  The  background  of  the  slates  is  at  times  more  coarsely  crystal- 
line than  in  the  cherty  carbonates ;  also,  the  rocks  are  cut  by  veinlets  of 
silica  to  a  greater  extent.  These  differences  imply  a  partial  rearrange- 
ment of  the  silica  which  they  originally  contained,  and  also  perhaps  the 
introduction  of  a  small  additional  amount  of  silica.  The  quantity  contained 
is  not  materially  greater  than  in  the  unaltered  cherty  carbonates,  and  it 
seems  a  sufficient  explanation  of  its  variation  in  character  to  suppose  that  it 
is  mostly  due  to  a  rearrangement;  that  is,  the  silica  has  been  taken  into 
solution  to  some  extent  and  subsequently  crystallized  (other  silica  perhaps 
at  the  same  time  being  added),  thus  becoming  more  perfectly  quartzose  and 
forming  the  veins  which  cut  across  the  laminse  of  the  rocks.  The  only 
chemical  change  implied  in  the  above  transformation  of  the  cherty  carbon- 
ates into  the  ferruginous  slates  is  the  decomposition  of  the  iron  carbonate 
and  the  peroxidation  of  the  iron  which  it  contained.  In  the  transition 
forms  which  occiir  between  these  slates  and  the  ferruginous  cherts  there 
have  been  doubtless  other  chemical  changes,  but  the}^  can  best  be  consid- 
ered in  connection  with  the  origin  of  this  rock. 


254  THE  PENOKEE  lEON-BEAEING  SERIES. 

The  ferruginous  cherts. — ;Iu  explaining  the  genesis  of  the  second  phase 
of  the  second  type  of  rocks  it  is  necessary  to  acconnt  for  its  concretion- 
ary and  brecciated  character;  for  the  production  of  brown  hydrated  hema- 
tite, red  hematite,  and  magnetite;  for  the  concentration  of  the  iron  oxide  in 
shots  and  bands;  for  the  large  quantity  of  silica  which  it  contains;  for 
the  somewhat  coarsely  crystalline  character  of  the  chert  as  compared  with 
that  of  the  original  carbonates;  and  for  the  presence  of  numerous  ramify- 
ing veins  of  finely  crystalline  quai'tz. 

In  the  microscopical  description  of  this  phase  of  rock  (pp.  205-209) 
the  concretionary  and  brecciated  character  has  already  been  accoxxnted  for. 
These  peculiar  areas  have  been  actixally  observed  in  all  stages  of  their 
formation,  so  that  their  history  is  not  theory,  but  definitely  observed  fact, 
and  will  not  be  repeated  here.  The  oxides  of  iron  have  been  produced 
from  the  iron  carbonate  just  as  are  the  oxides  of  the  first  phase  of  rock, 
the  only  difference  being  that  here  occasionally  magnetite  is  formed.  This 
mineral,  however,  is  readily  accounted  for  by  the  decomposition  of  iron 
carbonate  under  conditions  not  favorable  to  complete  oxidation.  It  is  not 
t(i  be  supposed  that  these  rocks  were  highly  heated,  but  it  may  be  remarked 
that  a  low  degree  of  heat  is  sufficient  to  change  iron  carbonate  into 
maa-netic  oxide  of  iron  with  the  liberation  of  carbon  monoxide  and 
dioxide.  It  is  not  believed  that  any  carbon  monoxide  has  been  liberated, 
for  the  oxygen  needed  to  change  the  protoxide  of  iron  into  the  proto- 
sesqxiioxide  was  doubtless  supplied  by  oxygen  in  percolating  water. 
The  manner  of  concentration  of  the  iron  oxides  in  bands  and  shots 
will  be  fully  discussed  in  considering  the  origin  of  the  iron  ores,  but 
it  is  here  necessary  to  mention  the  causes  which  have  produced  such  concen- 
tration. Percolating  waters  bearing  oxygen  in  solution  have  decomposed 
a  part  of  the  iron  carbonate,  the  carbon  dioxide  ])assing  into  the  water  and 
oxygen  passing  from  the  water  into  the  rock,  thus  simultaneoush'  forming 
sesquioxide  of  iron  and  solutions  capable  of  taking  up  other  iron  carbonate. 
Sflch  iron-bearing  waters  would  after  a  time  reach  some  crack  or  channel 
in  the  rock.  This  opening  in  many  cases  would  serve  as  a  passage  for 
other  waters  more  directly  from  the  surface  bearing  oxygen  in  solution, 
and  as  a  result  of  this  mingling  the  iron  in  solution  would  be  precipitated, 


THE  IRON  BEARING  MEMBER.  255 

and  thus  concentrations  of  ihv.  iron  oxide  in  bands  and  shots  would  occur. 
This  expUmatiou  is  not  a  wholly  suppositious  one,  hut  accords  with  the  facts 
of  observation  as  seen  in  lunncrons  exposures,  one  of  which,  the  cliff  bor- 
derin<r  the  outlet  of  Sunday  lake,  in  Sec.  13,  T.  47  N.,  R.  4G  W.,  Michigan, 
is  fully  described  (section  iv)  in  connection  witli  the  origin  of  the  ores. 
Tiie  concentration  of  silica  in  these  cherts  is  partly  explained  by  the 
abstraction  of  ii'on  carbonate  taken  into  solntit)U.  This  chert  is  i-arely  pure, 
and  frequently  contains  a  large  amount  of  iron  oxide;  and  this  is  what 
would  be  expected  from  the  character  of  the  jn-ocesses  above  indicated,  yet 
the  silica  is  much  more  abundant  in  the  ferruginous  cherts  than  in  the  unal- 
tered carbonates.  It  is  to  be  remembered  that  most  of  these  cherts. are  at 
low  horizons,  and  it  is  probable  that  they  have  to  a  considerable  extent 
been  silicified.  The  proof  of  this  silicification  lies  both  in  the  abundance 
of  the  silica  itself  and  in  the  manner  of  its  occurrence.  The  greater  part 
of  it  is  relatively  perfectly  crystalline  as  compared  with  the  chert  of  the 
carbonates.  The  cavities  which  the  rock  frequently  contain  are  lined  with 
quartz  crystals.  Numerous  veins  of  quartz  cut  through  the  rock  in  every 
direction.  The  concretions,  as  explained  (pp.  205-209);  are  areas  which 
were  originally  carbonate  of  iron,  but  are  now  largely  composed  of 
silica.  Many  of  them  have  been  severed  by  veinlets  of  silica.  All  these 
facts  imply  an  extensive  rearrangement  of  tlie  silica  originally  present  and 
the  introduction  of  additional  silica.  In  a  subsequent  consideration  of  the 
origin  of  the  ore  deposits  it  will  be  seen  that  it  is  probable  that  the  silica 
necessary  for  this  silicificatibn  was  derived  from  original  clierty  carbonates 
which  have  been  swept  away  by  erosion.  If  this  is  the  case,  a  portion  of 
the  silica  within  the  ferruginous  cherts  must  have  been  carried  some 
distance.  The  ferruginous  cherts  are  cut,  as  shown  by  mining  operations, 
by  numerous  dikes,  which  are  much  decomposed  and  are  certainly  more 
basic  than  they  were  originally.  This  alteration  of  the  dikes  may  also 
have  furnished  a  portion  of  the  silica  for  the  silicification  of  these  ferrugi- 
nous cherts.  The  nature  of  the  solutions  which  dissolve  silica  in  rocks  and 
deposit  it  in  other  places  is  becoming  better  known,  and  that  it  is  largely 
carried  is  eertain.-'     The  quantity  of  silica  required  to  fill  the  interspaces  of 

'For  foreign  localities,  see   Roth's    Allgenieiue   niul   Cbeiuisclie  Gcologie,  vol.  I,  1879.     For 
United  States  localitieSj  see  Bulletins  of  the  U,  S.  Geological  Survey,  No.  32,  List  and  Analyses  of  the 


256  THE  PENOKEB  lEOIf-BBAEIlSrG  SBEIBS. 

a  thick  bed  of  loose  sandstone  must  be  enormous;  yet  it  is  absolutely- 
certain  that  such  cementation  has  occurred  again  and  again.  Silica  in  sili- 
cates, in  the  amorphous  forms,  and  even  in  a  quartzose  condition,  is  soluble 
in  alkalies,  although  with  great  difficulty  in  the  latter  case.  Professor  Sollas, 
in  a  paper  already  alluded  to,^  has  drawn  a  sharp  distinction  between 
organic  and  mineral  silica,  the  first  being  easily  soluble,  while  the  latter 
is  relatively  insoluble.  Probably  most  of  the  silica,  even  in  the  cherty 
carbonates,  would  fall  in  his  class  of  mineral  silica;  but  a  part  of  it  is 
amorphous,  and  a  portion  of  it  certainly  is  quite  readily  soluble,  as  shown 
by  actual  laboratory  tests  with  caustic  alkalies.  In  accounting  for  this 
increase  in  the  quantity  of  silica,we  have  explained  its  relatively  coarsely 
crystalline  character  and  the  presence  of  the  veinlets ;  for  in  the  silicifica- 
tion  of  the  rock,  as  explained,  these  are  the  phenomena  which  we  would 
expect  to  find. 

The  genesis,  then,  of  these  feiTUginous  cherts  has  been  something  as 
follows:  The  rock  originally  occupying  the  place  now  taken  by  them  was  a 
cherty  iron  carbonate.  Percolating  solutions  have  decomposed  a  part  of 
the  siderite  and  at  the  same  time  have  taken  up  another  part.  This  iron 
carbonate  has  been  in  other  places  peroxidized  and  precipitated  in  bands  and 
shots.  Silica,  at  the  same  time  or  subsequently,  has  been  taken  into  solu- 
tion to  a  greater  or  less  extent,  and  in  other  places,  under  different  condi- 
tions, has  deen  deposited  as  chert.  Incoming  solutions  have  brought  with 
them  a  further  amount  of  silica,  which  has  tilled  the  spaces  left  by  the 
removal  of  the  carbonate.  It  is  probable  that  the  peroxidation  and  removal 
of  iron  carbonate  and  the  deposition  of  silica  have  to  some  extent  been 
simultaneous;  but  it  seems  to  be  the.case  that  they  have  in  general  been  in 
part  successive — that  is,  the  first  process  began  before  the  second  and  was 

Mineral  Springs  of  the  United  States,  Albert  C.  Peale;  No.  47,  Anal3'.ses  of  Waters  of  the  Yellowstone 
National  Park,  with  an  Acconnt  of  the  Methods  of  Analysis  employed,  Frank  Austin  Gooch  and 
James  Edward  V/^hitfield;  No. 8,  Ou  secondary  Eulargeraent  of  Mineral  Fragments  in  certain  Rocks, 
E.  D.  Irving  and  C.  R.  Van  Hise.  The  Pre-Cambrian  Rocks  of  the  Black  hills,  C.  R.  Van  Hise;  Bull. 
Geol.  Soc.  of  America,  vol.  1,  pp.  220-221.  Bulletin  No.  47  gives  over  forty  water  analyses,  in  all  of 
which  silica  is  found.  In  many  it  constitutes  25  or  more  per  ceut  of  the  total  soluble  material, 
while  in  one  case  it  runs  as  high  as  60  per  cent. 

=  Prof.  W.  J.  Sollas:  On  the  Flint  Nodules  of  the  Trimmingham  Chalks;  Annals  Nat.  Hist, 
1880,  pp.  384-395;  437-461. 


THE   IRON  BKAKING  MI^MHKK.  257 

completed  l)ef()re  the  latter  ended.  Tlie  arrangerneut  of  the  iron  oxide  and 
quartz  in  rough  layers  in  the  rock  and  in  concentric  belts  in  the  cavities 
g-o  to  show  tliis.  The  cavities  generally  have  as  an  exterior  casing  an  iron 
oxide.  This  was  formed  at  the  stage  in  which  the  iron  carbonate  was 
pnrtly  decomposed  and  partly  removed.  Later,  the  solutions  passing 
through  the  rocks  began  to  carry  silica,  and  finally  the  silicification 
was  tlie  chief  reaction;  as  a  result  of  which,  the  layers  of  quartz  crys- 
tals formed  within  the  cavities  and  in  veins  in  the  cracks,  oftentimes 
entirely  filling  them.  In  producing  the  many  peculiar  phases  and  mixtures 
of  chert  and  iron  oxides  the  conditions  must  have  varied  in  different  places. 
With  the  stages  of  growth  of  one  particular  form,  the  concretions,  we  are 
familiar.  In  them  the  series  of  operation  seems  to  have  been  exactly  as 
described  above. 

The  actinolitic  slates. — The  field  relations  of  the  actinolitic  slates  are 
such  as  to  show  that  they,  like  the  ferruginous  cherts,  are  almost  certainly 
derived  from  iron  carbonates.  There  is  not  here  the  intimate  association 
between  these  two  types  of  rock  that  there  is  between  the  cherty  carbonates 
and  the  ferruginous  cherts.  The  latter  are  for  many  miles  directly  overlain 
by  the  carbonates.  Within  their  mass  they  contain  large  exposures  of  this 
rock,  so  that  between  the  two,  both  in  exposure  and  in  thin  section,  we 
have  transitions  showing  all  the  intermediate  phases.  The  ferruginous 
cherts  and  iron  carbonates  which  occupy  the  central  portion  of  the  iron 
formation  grade  into  the  actinolitic  slates  to  the  east  and  west.  This 
transition  is  slow,  occupying  in  each  case  several  miles  of  distance ;  but 
above  the  actinolitic  slates  no  carbonate  is  found;  also  within  the  belt 
itself  there  are  no  bodies  of  this  material;  so  that  the  only  proof  of  the 
derivation  of  this  rock  from  the  cherts  and  carbonates  lies  in  the  facts  that 
the  two  classes  of  rock  occupy  the  same  horizon,  and  in  the  passage  from 
one  to  the  other  there  are  transition  phases. 

The  actinolitic  slates  are  not  so  different  from  the  ferruginous  cherts  in 
their  essential  characteristics  as  might  at  first  be  thought.  In  mineral  con- 
stitution the  two  types  of  rock  are  the  same,  except  that  in  the  actinolitic 
slates  the  additional  mineral  actinolite  is  present  and  magnetite  more 
plentiful.     In  the  peculiar  arrangement  of  the  iron  oxide  and  silica  the 

MON  XIX 17 


258  THE  PENOKEE  IRON  BEARING  SERIES. 

ferruginous  cherts  and  actinolite  slates  are  often  identical,  the  regular 
laminae  of  iron  ore  and  silica,  the  concretions,  and  the  breaking  of  these 
concretions  and  laminae  by  veinlets,  all  occurring  in  both  classes  of  rocks 
(PI.  XXIII,  Figs.  3  and  4;  PI.  xxiv,  Figs.  1'  and  2).  Also  the  two  types  of 
rock  are  alike,  in  that  they  both  contain  at  times  residual  iron  carbonate. 
It  is  true  that  the  amount  of  sidei'ite  is  not  so  g-reat  in  the  actinolitic  slates 
as  in  the  ferruginous  cherts.  A  further  likeness  between  the  two  rocks  lies 
in  the  occasional  pseudomorphous  aggregates  of  actinolite  and  magnetite 
after  iron  carbonate,  as  exhibited  by  PI.  xxviii,  Fig.  1.  The  only  essential 
difference,  then,  between  the  two  classes  of  rocks  is  the  introduction  of 
actinolite;  the  subordinate  differences  are  the  more  completely  crystalline 
character  of  the  quartz  and  the  abundance  of  magnetite.  To  account  for 
the  actinolite  is  comparatively  easy.  We  have  only  to  sujopose  that  the 
silica-bearing  waters  at  the  time  of  the  rearrangement  and  silicification  of 
these  rocks  united  with  a  portion  of  the  protoxides  of  calcium,  magnesium, 
and  iron,  thus  producing  actinolite.  As  has  been  seen  by  the  description, 
page  210,  the  order  of  crystallization  of  the  minerals  in  these  slates  is 
magnetite,  actinolite,  and  quartz.  The  condition  which  explains  the  large 
amount  of  magnetite  also  explains  the  production  of  the  actinolite.  In  the 
ferruginous  cherts  it  has  been  seen  that  the  oxidation  of  the  carbonate  was 
mainly  prior  to  the  rean'angement  and  introduction  of  silica;  so  in  the 
actinolitic  slates,  the  first  step  in  the  process  of  alteration  from  the  clierty 
carbonates  was  the  production  of  magnetite  by  the  decomposition  of 
siderite.  That  this  resultant  oxide  is  largely  magnetite  is  proof  of  an 
insufficient  amount  of  oxidizing  agent.  When  the  rearrangement  and  sub- 
sequent introduction  of  silica  occurred  a  portion  of  it  simply  united  with 
the  protoxide  bases  present  and  thus  formed  actinolite.  An  abundance  of 
protoxide  of  iron  was  here  perhaps  as  carbonate,  and  certainly  in  the  mag- 
netite. In  this  connection  the  relations  of  the  actinolite  and  magnetite  are 
of  great  significance.  The  actinolite  frequently  surrounds  the  magnetite 
areas,  less  often  penetrates  them,  and  is  usually,  although  not  always, 
associated  with  them.  In  the  final  stage  in  the  process  of  the  formation  of 
this  rock  the  most  of  the  silica  separated  as  quartz,  but  apparently  actino- 
lite continued  to  form  to  the  very  last,  as  it  is  almost  everywhere  included 


THE  IRON-BliAltING  MEMBER.  259 

within  tlic  quartz.  '1\>  expluiu  the  differences  between  these  rocks  and 
the  ferruginous  clierts  wo  need,  then,  only  to  suppose  one  different  con- 
dition— the  absence  of  a  sufKcient  amount  of  oxygen  at  the  time  of  their 
alteration  to  change  all  of  tlie  iron  to  the  peroxide.  As  has  been  sug-' 
gested  before,  it  is  possible  that  those  parts  of  the  iron  formation  in  which 
the  actiuolitic  slates  occur  were  more  highly  heated  than  the  other  parts. 
The  more  coarsely  crystalline  character  of  the  silica  so  prevalent  in  them 
may  also  be  explained  by  this  supposed  increase  in  temperature;  for,  as 
has  been  shown  by  laboratory  experiments,  a  temperature  considerably 
above  the  normal  is  favorable  to  the  production  of  coarsely  crystalline 
quartz.'  This  supposed  increase  of  temperature  is  rendered  plausible  by 
the  frequent  if  not  univiersal  association  of  large  quantities  of  basic  erup- 
tives  with  the  iron  formation  at  the  places  where  the  actinolitic  rocks  occur. 
Also  these  cutting  or  interlaminated  eruptives  may  have  excluded  to  some 
extent  surface  water  and  thus  caused  the  supply  of  oxygen  to  be  deficient. 

The  details  by  which  the  particular  phases  of  the  actinolitic  slates 
have  been  produced  cannot  be  satisfactorily  given,  for  in  so  few  of  them 
is  there  any  remaining  iron  carbonate.  However,  taken  in  connection 
with  their  position  and  intermediate  links,  there  can  be  little  question 
of  their  derivation  from  a  clierty  iron  carbonate.  If  there  is  any  doubt 
upon  this  point  it  is  set  at  rest  by  considering  the  occurrences  in  the 
Animikie  series  in  northeastern  Minnesota.  That  these  two  iron-bearing 
series  are  stratigraphically  equivalent  can  hardly  be  doubted.  It  has  been 
seen  that  in  the  Animikie  series  all  the  transition  phases  between  cherty 
iron  carbonates  and  the  most  crystalline  actinolitic  slates  are  as  perfectly 
illustrated  as  are  the  intermediate  phases  between  the  cherty  iron  carbon- 
ates and  the  ferruginous  cherts  in  the  Penokee  series. 

The  origin  of  these  actiuolitic  slates  is  analogous  to  that  of  the  tremo- 
litic  limestones,  so  widely  present  in  the  iron-bearing  series  of  the  North- 
west and  in  later  geologic  times.  Tremoliteis  calcium-magnesium-silicate; 
actinolite  is  calcium-magnesium-iron-silicate.  The  tremolitic  limestones 
are,  aside  from  the  tremolite,  mostly  composed  of  calcium-magnesium-car- 
bonate.    The    original    material  from    which  the    actinolitic    slates    were 

iRoscoe  and  Schorlemmer,  Treatise  on  Chemistry,  vol.  i,  p.  569. 


260  THE  PENOKEE  lEON-BEAEING  SERIES. 

derived  is  a  siliceous  iron  carbonate  bearing  calcium  and  magnesium.  In 
this  case  the  silica  had  but  to  unite  with  these  bases  to  produce  the  actino- 
lite;  like  solutions  in  the  limestones  had  only  to  unite  with  the  bases  there 
present  to  produce  tremolite. 

SECTION  III.— THE  ANIMIKIE  IRON-BEARING  SERIES. 

Before  leaving  this  part  of  the  subject  it  is  necessary  to  make  some 
allusion  to  like  formations  in  other  parts  of  the  lake  Superior  country. 
The  Animikie  and  Vermilion  lake  series  of  northeastern  Minnesota  and 
Ontario,  the  Marquette  and  Felch  mountain  series  of  Michigan,  and  the 
Menominee  series  of  Michigan  and  Wisconsin,  all  contain  large  develop- 
ments of  rocks  which  in  their  characters  are  almost  exact  reproductions 
of  the  iron-formation  rocks  in  the  Penokee  series.  In  each  of  the  districts 
mentioned  all  the  types  and  most  of  the  varieties  described  in  the  latter 
series  are  quite  widely  found.  Cherty  iron  carbonate  is  not  extensively 
known  in  some  of  them,  but  in  one  is  more  largely  developed  than  in  the 
Penokee  series  itself,  while  in  the  most  folded  and  altered  series  men- 
tioned this  rock  typically  occurs,  as  is  shown  by  PI.  xxiv,  Figs.  3  and  4. 
A  general  condensed  account  of  the  iron  formations  and  ore  bodies  of  these 
various  districts,  including  the  Iron-bearing  rocks  of  the  Penokee  series,  has 
been  published  by  us.^  While  a  continued  study  has  made  some  modifi- 
cations of  detail  necessary,  the  main  conclusion  of  the  papers  has  been 
rendered  more  certain;  i.  e.,  that  the  many  phases  of  peculiar  rocks  asso- 
ciated with  the  iron  ores  in  these  various  districts  have  been  derived  mainly 
from  a  clayey  or  cherty  iron  carbonate.  In  the  first  paper  refen-ed  to  an 
exception  was  taken  in  the  case  of  some  of  the  coarser  grained  actinolitic 
slates  which  are  found  in  the  Marquette  country,  "as  to  whose  relations  to 
the  other  ferruginous  materials  we  feel  now  unwilling  to  speak."  Later 
investigations  make  it  not  improbable  that  some  of  these  rocks  have  been 
derived  from  eruptives;  but  the  origin  of  this  class  of  rocks  has  not  yet 

1 R.  D.  Irving :  Origin  of  the  Ferruginous  Schists  and  Iron  Ores  of  the  Lake  Superior  Region ; 
Am.  Jour.  Sci.,  3tl  series,  vol.  xxxii,  1886,  iip.  255-272  C.  R.  Van  Hisc:  The  Iron  Ores  of  the  Mar- 
quette District  of  Michigan;  Am.  Jour.  Sci.,  3(1  series,  vol.  XLiii,  1892,  pp.  116-132.  C.  R.  Van  Hise: 
The  Iron  Ores  of  the  Lalie  Superior  Region;  Trans.  Wisconsin  Acad,  of  Sci.,  Arts,  and  Letters,  vol. 
viii,  1892,  pp.  219-228. 


THE  IKON  liEAKING  MEMBER.  201 

been  certainly  detenuined.  Nothing  will  be  here  added  to  what  was  said 
as  to  the  character  of  tlic  ferruginous  rocks  of  the  Marquette,  Menominee, 
and  Vermilion  lake  districts,  except  to  state  that  not  infrequently  mingled 
with  the  nonuiechanical  detritus  are  considerable  quantities  of  mechanical 
material.  Wliile  nuich  additional  work  has  been  done  upon  them,  it  has 
not  gone  far  enough  to  warrant  <i  detailed  account.  Howe\'er,  the  Animikie 
series,  in  its  iron  formation  and  its  simple  unfolded  condition,  is  so  remark- 
ably like  the  Iron-bearing  member  of  the  Penokee  series,  that  additional 
material  has  been  obtained  and  a  careful  study  made  of  it  in  order  to  com- 
pare its  character  and  origin  with  those  of  the  latter  series. 

The  following  general  description  of  that  part  the  Animikie  series 
which  contains  in  its  best  development  the  iron  formation  is  by  the  senior 
author.^ 

The  Animikie  rocks  are  exposed  in  four  distinct  areas,  one  of  whicli  is  separated 
from  the  others  by  an  overiap  of  the  great  gabbro  which  forms  the  base  of  the  Kewee- 
naw series.  The  others  are  separated  from  one  another,  so  far  as  known,  by  drift 
covering  only.  The  iirst  of  these  areas  is  that  which,  with  its  principal  development 
in  Canada,  along  the  shores  of  Thnnder  bay,  crosses  into  the  United  States  in  north- 
eastern Minnesota,  the  national  boundary  line  being  within  this  formation  from  the 
outlet  of  Guullint  lake  eastward  to  the  eastern  extremity  of  Pigeon  point.  Around 
Thunder  bay  the  rocks  of  this  series,  which  are  chieiiy  black  slates,  graywackes,  argil- 
laceous quartzite,  interstratifled  diabase,  and  gabbro  layers,  which  are  many  in  num- 
ber and  individually  often  have  a  considerable  thickness,  are  exposed  on.  a  large 
scale.  Immense  dikes  of  gabbro  and  diabase  also  penetrate  these  layers,  the  gabbro 
dikes,  which  are  at  times  several  hundred  feet  in  thickness,  being  noticeably  much 
closer  in  character  to  the  great  gabbro  at  the  base  of  the  Keweenaw  series  than  to 
those  gabbros  which  are  interleaved  with  the  Animikie  slates. 

In  the  vicinity  of  Thunder  bay  the  Animikie  rocks  are  often  nearly  horizontal, 
but  show  a  general  tendency  toward  a  southeastward  inclination.  As  the  formation 
crosses  into  United  States  territory  it  shows  more  marked  inclinations,  which  average 
probably  about  ten  degrees,  though  at  times  less  than  this,  and  sometimes  reach  as 
much  as  twenty  degrees.  The  national  boundary  line  is  situated  within  this  forma- 
tion from  the  mouth  of  Pigeon  river  to  Gunflint  lake;  but  on  the  north  side  of  the 
latter  lake,  and  again  to  the  north  of  the  next  lake  to  the  east,  called  North  lake,  the 
unconformable  abutment  of  the  Animikie  series  against  an  older  formation  of  granite 
and  schists  is  very  handsomely  shown.     Tlie  actual  contact  of  the  two  formations  is 

'R.  D.  Irviug:  Ou  the  Classification  of  the  Early  Cambrlau  and  Pre-Camhriau  Formations; 
7th  Annual  Report  U.  S.  Geol.  Survey,  1S88,  pp.  420-422. 


262  THE  PBNOKBB  IRON-BBAEmG  SBRIBS. 

not  seen,  but  tlie  exposures  approach  to  within  a  few  feet  of  each  other,  and  the  rela- 
tive  attitudes  of  the  two  formations  are  such  as  to  leave  no  question  whatever  with 
regard  to  the  unconformity.  Not  only  is  this  shown  by  the  vertical  position  of  the 
schists  as  contrasted  with  the  iiat  inclinations  of  the  slaty  series,  but  also  by  the  way 
in  which  the  latter  beds,  to  the  north  of  the  two  lakes  mentioned,  fit  into  the  sinuosi- 
ties of  outline  of  the  older  formations.  The  entire  contrast  as  to  lithological  charac- 
ters between  the  two  sets  of  rocks  furnishes  further  proof.  ...  So  far  as  it  is 
developed  along  the  national  boundary  line  the  lowest  layers  of  the  Animikie  series 
in  sight  are  those  on  Gunflint  lake  [see  PI.  xxxvii,  this  Monograph].  The  highest 
layers  are  thos^  in  the  vicinity  of  Grand  Portage  bay,  the  whole  succession  between 
these  points  being  some  thousands  of  feet  in  tliickness.  The  iron-bearing  horizon  at 
the  base  of  this  succession  is  lithologically  identical  with  that  of  tlie  Penokee  series 
of  northern  Wisconsin  and  Michigan,  while  the  black  slates,  graywackes,  etc.,  which 
succeed  the  iron-bearing  horizon,  are  in  turn  the  counterparts  of  those  which  form 
the  middle  and  xipper  jjortions  of  the  Penokee  series.  The  interstratified  gabbros  of 
the  Animikie  are  wanting,  however,  or  are  relatively  rare  in  the  Penokee  region. 

Tlie  iron  formation  of  the  above  described  area  of  the  Animikie  series 
contains  all  the  phases  found  in  the  Penokee  district,  with  the  exception 
that  no  extensive  ore  deposits  have  yet  been  developed.  There  is,  further, 
a  much  more  intimate  association  of  the  less  altered  and  most  crystalline 
forms  than  in  the  Penokee  series.  The  relations  of  the  original  cherty 
carbonate  with  the  variovis  phases  of  the  actinolitic  slates  are  as  intimate 
as  are  the  relations  of  the  iinaltered  rocks  with  the  ferruginous  cherts  in 
the  Penokee  series. 

The  clierty  iron  carbonates  (PI.  xxv). — The  cherty  iron  carbonates  are 
found  extensively  exposed  in  various  localities.  They  are  known  as  far 
east  as  lake  Superior,  on  the  ridge  which  is  just  back  of  port  Arthur. 
They  are  also  largely  found  on  the  Kaministiquia  river,  but  the  best  known 
and  most  characteristic  exposures  are  at  and  northeast  of  Grunflint  lake,  T. 
65  N.,  R.  2  and  3  W.,  Minnesota. ,  At  times  these  rocks  are  very  nearly  pure 
carbonate,  and  contain  from  35  per  cent  to  45  per  cent  of  metallic  iron 
(for  analyses  see  p.  192).  These  beds  of  comparatively  pure  carbonate 
are,  however,  not  usually  of  any  considerable  thickness.  They  alternate 
with  layers  which  contain  a  large  percentage  of  calcium  and  magnesium 
carbonates  and  with  belts  of  chert  of  greater  or  less  purity.  Sometimes 
the  transition  from  the  thick  bands  of  carbonate  to  those  of  chert  is  abrupt; 


THE  IKU^  iJEAKlNG  MEMBER.  263 

at  other  tiuu's  the  ciirbiniiite  layers  contain  thin  scams  of  chert  finely 
interhuninated  with  them.  Not  nnfre([nently  the  chert  is  in  nodular  forms, 
or  a  layer  of  chert  which  has  been  continuous  for  some  distance  has  a 
somewhat  abrupt  oval  termination  (PI.  xxv,  Fig.  4).  The  carbonate  areas 
may  contain  here  and  there  minute  particles  of  cherty  material,  just  as  the 
cherty  areas  contain  rhomboliedra  of  siderite.  In  fact,  there  is  every 
possible  gradation  between  bands  of  almost  pure  carbonate  and  others  of 
almost  pure  chert.  Upon  the  weathered  surface  these  rocks  show  in  a 
marked  degree  the  peculiar  weathering  exhibited  by  cherty  limestones. 
Because  of  the  resistant  character  of  the  chert  the  carbonates  ^-e  more 
rapidly  dissolved,  and  thus  leave  the  siliceous  bands  in  ridges  protruding 
from  the  face  of  the  rock,  varying  in  width  from  those  not  more  than  a 
hair's  breadth  to  those  several  inches  across. 

That  this  material  is  of  the  same  origin  as  the  rock  which  has  been 
regarded  as  an  original  one  in  the  Penokee  series  is  manifest.  The  rocks 
of  the  two  districts  are  so  remarkably  alike,  that  if  the  specimens  from 
them  were  mingled  many  of  them  could  not  be  separated  by  their  differ- 
ence of  appearance.  Upon  the  whole  some  phases  approach  more  nearly 
to  certain  of  the  Ohio  carbonates  and  those  near  old  Livingston  manor 
house,  Hudson  river,  New^  York,  than  do  most  of  the  Penokee  carbonates. 
That  the  silica  now  present  in  these  rocks  was  there  at  a  very  early  date  is 
shown  by  the  remarkable  brecciation  which  they  exhibit  where  folded. 
In  general  the  series  is  quite  uniformly  flat-lying  or  gently  tilted,  but  the 
exposures  upon  the  north  side  of  the  main  body  of  Grunflint  lake  exhibit  in 
places  somewhat  sharp  bowing.  The  layers,  where  cherty,  have  not  yielded 
to  this  bowing  uniformly,  but  have  'been  shattered  like  a  rigid,  brittle, 
inelastic  body,  and  thus  conform  themselves  by  a  succession  of  fractures 
to  the  general  bowing  taken  by  the  strata.  As  a  result  of  this  peculiar 
feature  the  chert  and  carbonate  are  in  places  as  finely  and  uniformly  lam- 
inated as  when  found  in  any  sedimentary  formations  (PI  xxv.  Fig.  3). 
From  this  regularity  they  suddenly  change  to  rough  breccias  along  zones 
of  fracture.  These  breccias  alone  might  be  taken  to  be  a  water-deposited 
fragmental  rock,  but,  as  examined  in  exposure,  their  positions  at  the  places 
of  abrupt   variation  in  the  bedding  of  the    rock  prove  that  they  have 


264  THE  PEIvTOKEE  lEON-BEAEING  SERIES. 

resulted  from  the  folding.  The  contained  fragments  are  exceedinglj^ 
angular,  as  would  be  expected,  and  give  no  indication  of  having  undergone 
any  considerable  change  since  the  folding,  except  that  their  interspaces 
are  cemented  with  chert  and  iron  carbonate  which  must  subsequently  have 
infiltrated. 

When  these  rocks  are  examined  in  thin  section  the  relations  of 
the  chert  and  carbonate  are  seen  to  be  exactly  like  those  in  the  Penokee 
series.  As  a  general  rule  there  is  a  regularity  of  interlamination  of  the 
two,  but  quite  frequently  also  the  cherty  layers  break  across  the  belts 
of  carb«iate  in  such  a  manner  as  to  imply  an  introduction  of  silica 
subsequent  to  the  carbonate  layers.  This  introduction  may  show  merely 
a  reari'angement  of  the  silica  which  was  oi'iginally  deposited  in  the  rocks, 
as  explained  with  reference  to  the  Penokee  series,  but  it  may  imply 
an  actual  introduction  of  silica  from  an  extraneous  source,  that  is,  a  sec- 
ondary silicification.  The  siderite  is  often  well  crystallized  in  perfect 
rhombohedra  or  oval  forms  (PI.  xxv.  Figs.  1  and  2).  These  rhombohedra 
often  average  larger  than  in  the  corresponding  rocks  of  the  Penokee  district, 
but,  as  in  them,  they  are  occasionally  argillaceous.  The  chert  also  varies 
greatly  in  the  coarseness  of  its  grains,  running  from  almost  completely 
amorphous  to  almost  completely  crystallized.  The  wholly  amorphous  con- 
dition is,  however,  unusual. 

The  ferruginous  slates. — The  ferruginous  slates  are  always  found  asso- 
ciated with  the  unaltered  cherty  carbonate.  They  are  not  known  to  be  so 
extensively  developed  as  in  the  Penokee  region.  In  microscopical  character 
they  differ  from  them  only  in  that  magnetite  is  relatively  more  abundant. 
Their  derivation  from  the  iron  carbonates  is  seen  in  all  its  phases.  A 
description  of  these  stages  of  gradation  would  be  but  a  repetition  of  what 
has  been  said  as  to  similar  rocks  in  the  Penokee  series  and  so  will  not  here 
be  given. 

The  ferruginous  cherts. — (Pis.  xxvi,  xxvii  and  xxviii.  Fig.  2.) — The  ferru- 
ginous cherts  are  abundant.  They  comprise  all  the  phases  described  in  the 
Penokee  series,  and  exhibit  as  finely  their  origin  and  also  the  character  of  the 
brecciation  which  sometimes  accompanies  these  concretionary  forms.  The 
chief  diff"erence  exhibited  by  the  ferruginous  cherts  of  the  Animikie  from 


THE  lUONBEAlUNG  MEMBER.  2(55 

the  Penokeo  district  is  relutivol)'  greater  abundance  of  magnetite.  To 
describe  the  ordinary  concretionary  and  brecciated  forms  would  be  a  repeti- 
tion of  what  has  ah-eady  been  given  as  applying  to  rocks  in  the  Penokee 
series,  so  that  here  all  that  will  be  said  is  in  reference  to  the  manner  of 
occurrence  of  the  magnetite.  It  has  Ijeen  noted  that  magnetite  is  occasion- 
ally pseudomorphous  after  siderite  in  the  Penokee  series.  This  is  often  the 
case  in  the  Animikie  series.  The  formation  of  magnetite  from  iron  car- 
bonate, as  exhibited  in  many  sections  in  all  its  phases,  is  finely  illustrated 
in  PI.  XXV,  Fig.  2,  which  shows  on  one  side  of  the  figure  oval  and  rhombo- 
hedral  areas  of  nearly  pure  siderite  and  in  the  middle  of  the  section  similar 
areas  which  have  completely  altered  to  magnetite.  In  the  intermediate 
parts  of  the  figure  all  the  transition  phases  can  be  made  out.  In  this  sec- 
tion the  magnetite  has  taken  its  forms  from  the  siderite  but  it  is  more  often 
the  case  that  its  crystal  forms  are  independent  of  the  original  areas  of 
carbonate.  In  sections  in  which  there  is  a  good  deal  of  both  siderite  and 
magnetite,  the  magnetite  is  ordinarily  found  scattered  through  the  section, 
disconnected  with,  adjacent  to,  or  included  in  the  exterior  parts  or  even 
quite  to  the  centers  of  the  siderite  areas.  The  relations  are  such  as  to  show 
that  the  magnetite  is  in  the  process  of  formation  from  the  siderite,  but  the 
crystals  of  the  oxide  included  in  it  generally  have  their  own  octahedral  form, 
and  their  outlines  consequently  bear  no  relation  to  those  of  the  carbonate 
from  which  they  are  derived  (PI.  xxix,  Fig  3).  Magnetite  and  hematite  in 
these  sections  are  most  closely  associated.  Generally  areas  are  made  up  of 
an  intimate  mingling  of  the  two.  In  very  thin  sections,  in  transmitted 
light,  the  blood  red  hematite  can  be  seen  as  spots  here  and  there  through  the 
magnetite  areas  (PI.  xxix,  Fig.  1);  or,  if  the  hematite  predominates,  the 
reverse  is  the  case.  In  some  cases,  however,  the  relation  between  these 
two  minerals  is  somewhat  peculiar.  The  magnetite  is  found  as  cores  in  tha 
hematite  areas.  In  this  case  it  is  either  possible  that  magnetite  was  origi- 
nally formed  from  the  iron  carbonate  and  that  it  h?is  subsequently  peroxi- 
dized  upon  its  exterior  and  thus  formed  hematite  ;  or  that  at  first  the  con- 
ditions were  favorable  for  the  production  of  magnetite  and  subsequently 
changed  so  as  to  produce  hematite. 


266  THE  PENOKBE  lEON-BEAEING  SERIES. 

The  adinolitic  slates. — (PL  xxv.  Figs.  3  and  4;  PI.  xxix,  Figs.  2  and  3.) — 
The  most  important  addition  to  our  knowledge  of  the  origin  of  iron-forma- 
tion rocks  given  by  the  Animikie  series  is  from  the  actinohtic  slates.  It  has 
been  seen  that  in  the  Penokee  series  the  transition  phases  were  somewhat 
sparingly  present.  This  is  not  the  case  in  the  Animikie.  The  man^^  phases 
of  imaltered  carbonate,  ferruginous  slate,  and  ferruginous  chert  contain  acfin- 
olite  and  magnetite.  Different  sections  exhibit  these  two  minerals  in  quantity 
from  minor  to  chief  constituents.  There  are  kinds  which  are  mainly  com- 
posed of  actinolite  and  siderite,  others  which  are  mainly  composed  of  actiu- 
olite  and  iron  oxide  including  magnetite,  and  still  others  which  are  com- 
posed of  actinolite,  iron  oxides,  and  chert ;  the  latter  including  the  most  con- 
cretionary and  brecciated  forms. 

The  evidence  that  the  actinolite  slates  have  been  formed  from  cherty 
iron  carbonates  is  like  that  which  proves  the  ferruginous  cherts  and  the  fer- 
ruginous slates  to  be  of  this  origin. 

- 
It  has  already  been  shown  that  the  magnetite  is  largely  a  secondary 

product  of  siderite,  being  very  frequently  pseudomorphous  after  it.     Also, 

when  not  thus  pseudomorphous,  its  relations  are  often  such  to  the  siderite 

as  to  show  that  it  is  secondary  to  it.     The  relations  of  magnetite  and  the 

other  oxides  of  iron  in  the  concretions  have  already    been  given.     The 

growth  of  these  concretions  has  been  traced  out  step  by  step  precisely  as 

in  the  Penokee  series ;  so  that  there  is  no  doubt  that  the  mag'uetite  in  them 

Is  secondary  to  the  siderite.     The  proportion  of  this  mineral  derived  from 

siderite  is  so  great  that  it  is  exceedingly  probable  that  it  is  of  secondary 

origin  even  in  those  cases  in  which  this  can  not  be  demonstrated. 

The  secondary  character  of  the  actinolite  can  not  always  be  clearly 

shown ;  but  the  cases  in  which  its  origin  can  be  made  out  are  so  numerous, 

that  the  conclusion  is  reached  that  all  of  it  is  of  the  same  nature.     That  the 

silica  in  the  actinolite  slates  has  been  extensively  rearranged  and  more 

coarsely  crystallized,  wkh  the  probable  addition  of  silica  to  that  which  the 

original  rock  contained,  can  not  be  doubted.      The  way  in  which  the  silica 

cuts  the  other  minerals  is  nicely  illustrated  by  PI.  xxix.  Figs.  1  and  2.    The 

actinolite  constantly  cuts  the  quartz.     The  association  of  the  actinolite  with 

the  magnetite  and  siderite,  that  is,  with  minerals  which  contain  protoxide  of 


TllK   lK().\.15l-:AltlN(;   MKMliKK.  2()7 

Iron,  is  charactenstic.  'Plic  actiuolite  began  to  form  at  tlie  time  of"  the 
entrance  and  rearraug-enicnt  of  tlic  silica,  the  magnetite  and  siderit(5  fur- 
nishing the  necessary  liases  and  the  silica  furnishing  the  acid  for  its  forma- 
tion. The  reactionary  natui'e  of  this  actiuolite  is  well  illustriited  by  V\. 
xxv,  Fig.  4,  where  the  actiuolite  constitutes  an  almost  solid  band  between 
a  cherty  nodule  composed  mostly  of  silica  and  the  finely  laminated  slate 
which  largely  contains  siderite  and  magnetite.  Also  actiuolite  and  magnet- 
ite occur  together  in  well  formed  concretions.  Frequently  in  the  concre- 
tions a  ring  of  magnetite  is  inclosed  upon  both  sides  by  a  border  of 
actiuolite.  Again,  in  other  places,  when  a  magnetite  ring  is  not  quite  com- 
plete, the  actiuolite  border  which  follows  the  magnetite  throughout  the  part 
of  the  concretion  where  it  is  present  completes  the  ring. 

It  has  been  suggested  of  the  actiuolitic  slates  in  the  Penokee  series 
that  the  conditions  which  explain  the  presence  of  the  magnetite  also  explain 
the  a^jpearance  of  the  actiuolite.  Yet  it  is  not  always  true  that  actiuolite  is 
accompanied  by  magnetite  ;  magnetite  may  also  appear  without  the  pres- 
ence of  actiuolite.  In  some  few  of,  the  sections  actiuolite  and  a  calcium- 
magnesium-bearing  siderite  are  the  only  important  constituents,  silica, 
however,  always  being  present ;  and  the  rock  then  becomes  an  actinolitic 
carbonate.  It  is  here  to  be  noted  that  this  rock  is  an  exact  analogue  of  the 
tremolitic  limestones ;  silica  in  the  case  of  the  limestones  has  united  with 
the  bases  present  to  form  treiuolite;  in  a  similar  way  the  actiuolite  has 
formed  in  the  ferriferous  carbonate  by  the  union  of  silica  with  the  bases. 

General. — It  need  hardly  be  said  that  the  rocks  of  this  formation  are 
mainly  nonfragmeutal.  It  is,  however,  the  case  that  fragmental  material  is 
mere  widely  mingled  with  the  iron  formation  of  the  Animikie  than  of  the 
Penokee  series.  A  good  many  thin  sections  contain  a  considerable  amount 
of  fragmental  quartz,  which  instantly  exhibits  itself  in  its  true  character, 
showing  its  rounded  outlines  and  frequent  enlargements.  This  occurrence 
then  furnishes  another  illustration  of  the  ease  of  separating  a  fragmental 
from  a  nonfragmeutal  quartz  when  the  rock  has  not  been  subjected  to 
dynamic  action  and  of  the  fundamental  difference  of  their  natures. 

The  relations  and  order  of  crystallization  of  the  minerals  contained  in 
the  actinolitic  slates  and  ferruginous  cherts  are  then  precisely  the  same  as  in 


268  THE  PENOKEE  lEON-BEARING  SEEIBS. 

the  Penokee  series.  A  chert  j  ferriferous  carbonate  was  the  original  rock. 
From  this  the  iron  oxides  have  been  in  the  main  the  first  to  form.  Before 
their  formation  was  complete,  when  actinolite  is  present,  it  has  begun  to 
develop,  and  is  included  in  the  oxides,  and  particularly  in  the  magnetite. 
Its  growth  is  most  pronounced  at  the  beginning  of  the  rearrangement 
and  introduction  of  silica,  as  .would  be  expected  from  its  requiring  this 
material  to  produce  it.  The  average  amount  of  silica  is  much  greater  than 
in  the  original  cherty  carbonates,  and  it  is  also  more  largely  in  the 
form  of  quartz.  The  final  stage  in  the  development  of  these  rocks  has 
been,  then,  a  rearrangement  of  silica  already  present  and  an  addition  of 
silica  to  a  greater  or  less  degree  at  the  time  when  most  of  the  quartz 
formed.  Clierty  carbonate  located  elsewhere  may  have  been  the  source 
whence  the  most  of  this  silica  was  derived,  as  will  be  more  apparent  after 
the  oriofin  of  the  ores  in  the  Penokee  series  is  considered.  Interbedded 
with  the  Animikie  rocks  are  also  heavy  beds  of  diabase  and  gabbro,  altera- 
tion of  which  may  also  have  furnished  silica  for  the  silicification.  The  like- 
ness of  the  Animikie  and  Penokee  iron  formations  is  further  exhibited  in  a 
striking  manner  by  the  fact  that,  in  the  most  typical  exposures,  the 
Animikie  series  upon  Grunflint  lake,  in  upper  horizons,  are  almost  pure 
cherty  carbonate ;  in  intermediate  horizons,  ferruginous  cherts  and  carbon- 
ates ;  and  in  the  lower  horizons,  actinolite  slates,  jaspers,  and  ferruginous 
and  concretionary  cherts,  which,  however,  often  contain  siderite. 

SECTION  IV,— THE  IRON  ORES.' 

Position  of  the  ores  in  the  Iron-bearing  member. — The  iron  ores  are  all 
located,  as  far  as  known  at  present,  in  that  part  of  the  iron-bearing  forma- 
tion between  Sec.  33,  T.  45  N.,  R.  1  "W.,  Wisconsin,  and  the  east  hue  of  T. 
47  N.,   R.  45  W.,   Michigan,  a  distance  of  about  30  miles.     The  greater 

'A  brief  account  of  the  contents  of  section  iv  was  published  m  advance  in  the  Am.  Jour.  Sci , 
3d  sorlfes,  vol.  XXXVII,  pp.  32-48,  by  the  junior  author.  Dr.  Irving  had  not  considered  this  x'^ii"*  of 
the  Mubject. 

This  section  indicates  the  development  of  the  mines  at  the  time  it  was  written.  A  recent  visit 
to  the  Penokee  range  enables  me  to  say  that  later  developments  accord  with  the  conclusions  herein 
contained,  but  the  mining  of  the  years  since  this  section  was  written  has  wholly  changed  the  maps  of 
the  mines. 


THE  IRON-BEARING  MKMBKR.  2(59 

rmmber  of  the  larger  deposits  are  found  in  tlu!  central  half  of  this  area. 
Also,  most  of  the  known  deposits  lie  at  the  base,  or  very  near  the  base  of 
the  Iron-l)earinp;-  member;  that  is,  they  rest  npon  or  close  to  the  coarse 
grained  fragmental  quartzite,  which  constitntes  tlie  uppermost  horizon  of 
the  Quartz-slate.  The  number  of  shii)ping  mines  is  about  twenty,  and  in 
them  all  the  deposits  lie  upon  this  fragmental  quartzite  except  the  follow- 
ing, which  are  given  in  order  from  west  to  east:  (1)  The  Tyler's  fork  mine, 
in  the  E.  ^  of  Sec.  33,  T.  45  N.,  R.  1  W.,  Wisconsin.  (2)  The  Iron  Belt 
mine,  NE.  i  Sec.  11,  T.  45  N ,  R.  1  E.,  Wisconsin.  (3)  The  Montreal 
mine,  a  short  distance  east  of  the  west  quarter  post  of  Sec.  33,  T.  46  N.,  R. 
2  E.,  Wisconsin.  This  mine  has  shipped  quite  a  large  quantity  of  ore  from 
a  deposit  which  was  at  first  an  open  pit  and  which  has  not  been  carried  to 
the  fragmental  quartzite.  This  deposit  at  the  present  time  is  not  the  more 
important  one  upon  the  property,  the  main  ore  body  resting  upon  the  frag- 
mental quartzite.  (4)  The  Mount  Hope  mine,  NW.  ^  Sec.  24,  T.  47  N.,  R. 
47  W.,  Michigan.  This  mine  has  a  large  deposit  of  ore,  situated  some  300 
feet  north  across  the  formation  from  the  underlying  quartzite.  This  body 
is  one  of  the  two  large  deposits  which  have  been  developed  above  the  base 
of  the  formation.  South  of  it,  and  a  short  distance  to  the.east,  is  another 
deposit  of  ore  which  rests  on  the  quartzite.  (5)  The  Bonney  mine,  NE.  J 
Sec.  24,  T.  47  N.,  R.  47  W.,  Michigan.  (6)  The  Colby  mine,  NE.  i  Sec. 
16,  T.  47  N.,  R.  46  W.,  Michigan.  This  is  the  second  of  the  two  large 
known  deposits  which  do  not  rest  upon  the  Quartz-slate  formation.  Directly 
south  of  this  body  is,  however,  a  still  larger  one  which  lies  upon  the  quartz- 
ite, and  the  deeper  workings  of  the  mines  show  that  these  two  deposits 
approach  very  close  to  each  other,  if  they  are  not  actually  connected.  (7) 
East  of  Sunday  lake,  in  T.  47  N.,  R.  45  W.,  several  mines— including  the 
Brotherton  and  Sunday  lake — which  are  a  considerable  distance  north  of  the 
fragmental  quartzites.  These  mines  have  peculiarities  which  distinguish 
them  from  other  working  deposits  to  the  west.  The  oi'e  bodies  lie  in  isolated 
and  irregular  patches,  which  are  generally  not  more  than  10  to  30  feet  in 
thickness.  These  lenses  ai-e  separated  by  barren  rock  which  is  at  times  as 
much  as  a  hundred  feet  thick.  With  the  exception  of  the  Mount  Hope  and 
Colby  none  of  the  above  mentioned  mines  are  of  large  size. 


270  THE  PENOKEE  IRON-BEAEING  SEEIES. 

It  is  not  meant  to  imply  that  the  more  numerous  class  of  deposits 
which  have  been  spoken  of  as  resting  upon  the  foot-wall  quartzite  liave 
the  clean  ore  always  in  contact  with  it.  Quite  often  there  is  a  layer  of 
what  the  miners  denominate  "paint  rock,"  or  a  layer  of  sand  rock  between 
the  quartzite  and  the  ore.  This  latter  material  is  sometimes  as  much  as  20 
feet  in  thickness,  although  it  is  usu:ally  not  more  than  a  few  inches,  or  at 
most  a  few  feet.  The  sand  rock  here  found  is  so  friable  as  to  readily  crumble 
between  the  fingers ;  in  fact,  is  no  more  than  coarse  sand.  The  paint  rock, 
a  commoner  material,  is  a  soft  red  substance,  rich  in  iron,  at  times  carry- 
ing more  than  50  per  cent.  It  appears  like  a  heavily  ferruginous  clay,  and 
varies  in  thickness  from  a  mere  film  up  to  5  feet.  Sometimes,  also,  there 
is  between  the  ore  and  the  quartzite  a  mass  of  greater  or  lesser  thickness  of 
the  ferruginous  chert  or  "mixed  ore"  of  the  miners.  Rarely  a  thin  layer 
of  nearly  pure  white  chert  is  found  between  the  ore  and  quartzite.  Not- 
withstanding all  these  exceptions  the  south  side  of  the  ore  never  penetrates 
the  quartzite  and  in  a  general  way  follows  it,  so  that  it  may  be  spoken  of 
as  resting  upon  it.  It  will  be  remembered  that  this  quartzite  has  an  aver- 
age dip  to  the  north  of  from  60°  to  70°,  and  it  thus  furnishes  an  approxi- 
mately regular  wall,  north  of  which  the  ore  lies,  and  is  consequently 
always  called  the  foot  wall  by  the  miners.  While  it  is  true  that  the  aver- 
age dip  of  this  quartzite  is  as  given,  it  has  subordinate  irregularities  as 
great  as  the  ordinary  eroded  surface  of  a  gently  undulatory  country.  In 
shafts  which  follow  it  the  dip  is  at  times  as  high  as  75°  or  80°,  while  in 
other  places  it  is  not  more  than  45°.  Also  in  horizontal  drifts  which  are 
following  the  quartzite  this  same  undulatory  character  is  seen.  For 
instance,  one  such  drift  in  the  Colby  mine  in  following  the  quartzite  bows 
sharply  at  one  place  and  runs  almost  due  south  for  some  feet  before  strik- 
ing the  quartzite,  although  it  has  just  before  been  alongside  of  it. 

The  so-called  "north  vein"  deposits  are  described  by  Mr.  J.  Parke 
Channing  as  always  having  regular  south  walls,  which  dip  with  the  forma- . 
tion  and  are  called  by  the  miners  "foot  walls."     Such  walls  may  be  well 
seen  at  the  open  pits  of  the  Montreal,  Mount  Hope,  and  Colby  mines. 

The  differences  between  the  foot  walls  (of  the  so-called  north  veins) 
and  the  southern  deposits  will  be  later  considered.     For  the  present  it  is  suffi- 


THE  lEONBEAlllNG  MEMBER.  271 

cient  to  say  that  wliether  the  ore  deposits  rest  upon  the  fragmental  quartz- 
ite  or  are  north  of  it,  they  have  as  their  southern  boundaries  an  approxi- 
mately reguhir  pUiue,  dipping-  to  the  north  at  an  average  an<»-le  of  from  G0° 
to  70°. 

Dikes  in  Iron-bearing  member. — The  eruptives  of  the  series  are  treated 
in  another  phxce.  They  are  known  to  be  very  numerous  in  the  Iron- 
bearing  member,  ahhough  their  abundance  liere  does  not  sliow  that 
they  are  not  as  plentiful  in  other  members  of  the  series;  for  mining 
developments'  have  cut  this  belt  in  every  direction  and  thus  found  the 
greenstones,  the  presence  of  which  would  not  have  been  suspected  from 
natural  exposures.  As  explained  in  their  description  (chapter  vii),  the 
greenstones  of  the  Iron-bearing  member  are  much  altered.  Many  of 
them  are  so  decomposed  as  to  be  soft  friable  masses  which  can  be 
picked  to  pieces  with  the  fingers  and  now  contain  none  of  the  original 
minerals  that  make  up  ordinary  basic  intrusives.  They  retain,  however, 
distinctly  their  diabasic  structure  and  can  occasionally  be  traced  into  com- 
paratively unaltered  phases  which  are  true  diorites.  These  much  altered 
greenstones  are  known  to  the  miners  either  as  soapstones  or  as  diorite 
dikes.  That  they  are  dikes  is  manifest  from  their  form  as  well  as  by  the 
way  in  which  they  cut  across  the  layers  of  the  Iron-bearing  member 
into  the  foot -wall  quartzite.  This  dike-like  character  is  finely  shown  by 
Pis.  XXX  and  xxxi. 

One  traveling  along  the  iron  range  is  at  once  struck  by  the  con- 
stancy of  the  association  of  the  iron  ores  and  these  so-called  soapstones. 
At  every  mine,  except  those  east  of  Sunday  lake,  a  greater  or  lesser 
amount  of  this  soapstone  is  found  among  the  debris  from  the  mines  and 
oftentimes  its  quantity  is  large.  This  association  was  found  to  be  so 
constant  that  Mr.  J.  Parke  Channing,  then  inspector  of  mines  for  Gogebic 
county,  Mich.,  was  employed  to  work  out  the  relations  of  the  ore  bodies 
and  dike  rocks.  What  follows  as  to  the  position  of  the  dikes  themselves 
and  as  to  the  position  of  the  ore  bodies  with  reference  to  them  is  wholly 
the  result  of  data  furnished  by  his  investigations. 

The  position  of  the  dikes  is  given  with  reference  to  the  iron  formation 
in  which  they  occur.     This  formation  has  a  northern  dip  and  a  general  east 


272  THE  PENOKEE  IRON  BEAEING  SERIES. 

and  west  strike.     As  used  in  reference  to  the  dikes,  aa  east  and  west  direc- 
tion means  parallel  with  the  iron  formation;  a  north  and  south  direction, 
transverse  to  it.     The  important  thing  for  the  present  purpose  is  not  the 
absolute  direction  in  which  the  dikes  run,  hut  their  relations  to  the  contain- 
ing formation.     The  dikes  vary  a  good  deal  in  their  dip  and  strike  in  dif- 
ferent mines,  and  the  same  dike  at  times  in  the  same  mine  also  varies  in  dip 
and  strike.     However,  certain  of  their  elements  are  quite  constant.     They 
always  dip  to  the  south,  and  generally  this  southern  dip  or  its  component 
transverse  to  the  formation  is  from  20°  to  30°.     The  northern  dip  of  the  • 
iron  formation  is  from  60°  to  70°.     It  follows  from  this  that  if  the  stratified 
rocks  were])laced  again  in  a  horizontal  position  the  dikes  would  he  vertical.     The 
true  dip  of  the  dikes  is,  however,  usually  not  exactly  transverse  to  the  for- 
mation, but  east  of  it,  so  that  a  component  along  the  dikes,  parallel  to  the 
strike  of  the  rocks,  has  usually  an  eastern  pitch.     This  pitch  may  be  as 
high  as  35°.     From  this   amount  it  varies  to  horizontality  or  even  to  a 
western  pitch  of  10°.     The  strike  of  the  rock*of  the  Iron-bearing  member 
in  wjiich  the  ore  bodies  occur  is,  throughout  most  of  the  distance,  north  of 
east.     For  a  short  way  in  the  eastern  part  of  T.  47  N.,  R.  46  W.,  Michigan, 
the  strike  of  the  rocks  is  nearly  east  and  west ;  but  in  passing  westward 
the  strike  begins  to  vary  to  west  of  south  and  in  the  western  part  of 
the  ore-bearing  area  is  from  25°  to  30°  west  of  south  (PI.  li).  If  these  rocks 
were  turned  back  to  a  horizontal  position  in  a  direction  at  right  angles  to 
their  strike,  the  strike  of  the  dikes  would  be  the  same  as  the  strike  of  the 
rocks  of  the  Iron-bearing  member  at  present,  when  there  is  no  eastern  or 
western  component  in  the  dip  of  the  dikes.     When  the  dip  component  of 
the  dikes  along  the  formation  is  east,  it  must  be  added  to  the  amount  that 
the  iron  formation  now  strikes  north  of  east  to  produce  the  strike  which  the 
dikes  would  have  if  the  iron  rocks  were  placed  in  a  horizontal  position. 
If,  on  the  contrary,  the  component  along  the  formation  is  to  the  west, 
it  must  be  subtracted  from  the  amount  that  the  strike  of  the  formation  is 
north  of  east.     The  position  of  these  dikes  with  reference  to  the  foot-wall 
quartzite  will  be  better  understood  by  an  examination  of  Pis.  xxx  and 
XXXI.     In  one  mine  in  which  the  pitch  of  the  dike  is  10°  to  the  west, 
the  Mount  Hope,  the  strike  of  the  formation  is  about  10°  north  of  east ; 


THE  IRON  HEAKUSTG  ME:\rP,EU.  278 

therefore,  the  strike  of  ihc  dike,  if  the  iron  rocks  wrw  turned  back  to  a 
horizontal  jtositiou,  would  Ir;  cast  and  west.  At  tht^  I'cucc  nunc  tlic  strike 
of  the  Ii-()n-l)ciirin,i;'  rocks  is  about  25  '  north  of  ciist,  while  tliu  dike  in  the 
mine  has  ;ui  eastern  component  ofaf)".  The  dik(i  would,  then,  if  the  Iron- 
bearinv-  rocks  were  turned  to  a,  horizontal  jjosition,  strike;  al)out  east  (10° 
north.  It  is  for  the  present  ;i.ssumed  that  the  dikes  cut  throu<^h  the  iron 
formation  before  their  uplifting-.  If  this  is  the  case,  it  would  follow  that, 
upon  the  average,  the  dij)  of  the  dikes  was  !)0°,  that  is,  they  were  per- 
pendicular to  the  strata  tbrough  which  they  came,  and  that  the  strike  of 
the  dikes  varied  from  east  and  west  to  east  60°  north.  Tlie  average  .strike 
of  the  larger  number  of  dikes  would  be  about  northeast  and  southwest. 

The  thickness  of  these  dikes  varies  greatly,  running  from  a  few 
inches  to  nearly  90  feet.  In  most  of  the  mines  in  which  the  deposits 
of  ore  are  of  any  considerable  size,  the  main  dikes  are  generally 
six  or  more  feet  in  thickness,  while  it  is  noticeable  that  generally  the 
hirger  mines  have  thick  dikes,  although  in  the  same  mines  there  are 
often  one  or  more  smaller  dikes.  The  main  Colby  dike  is  nearly  90 
feet  thick ;  the  main  Norrie  dike,  35  feet  thick.  The  only  ore  deposit 
west  of  Sunday  lake,  the  development  of  which  has  found  no  dyke,  is  the 
main  Ironton-Federal  body.  This  ore  body  pitches  to  the  east,  just  as  is 
later  seen  of  the  deposits  associated  with  dikes.  It  is  possible  tliat  when  the 
workings  of  these  mines  penetrate  deeper  they  will  come  iu  contact  with  a 
dike ;  one  is  known  to  come  to  the  surface  300  or  400  feet  west.  As  to 
the  mines  east  of  Sunday  lake,  it  has  already  been  noted  that  their  charac- 
ter is  quite  exceptional,  and  they  may  for  the  present  be  excluded  from  this 
discussion. 

From  the  eastern  pitch  of  the  dikes,  with  reference  to  the  rocks  of  the 
iron  formation,  it  is  evident  that  they  must,  if  they  continue  in  their 
observed  directions,  come  to  the  surface  to  the  west  of  the  workings  of  each 
of  the  mines  in  which  they  are  found.  xAs  the  present  workings  are,  in 
most  cases,  but  a  few  hundred  feet  deep,  it  follows  that,  when  these  pitches 
are  high,  the  dikes  would  reach  the  surface  but  a  short  distance  from  the 
ore  deposits.  It  thus  becomes  probable  that  there  are  as  many  dikes  in  tlie 
lower  horizons  of  the  Iron-bearing  member  as  are  seen  iu  all  of  the  differ- 
MON  XIX- — -18 


274  THE  PENOKEE  lEON-BEAEING  SERIES. 

ent  mines,  while  doubtless  there  are  many  more.  In  quite  a  number  of 
mines,  as,  for  instance,  the  Trimble  (PI.  xxxi.  Figs.  4  and  5),  NE.  J,  Sec.  33, 
T.  46  N.,  R.  2  E.,  Wisconsin,  there  are  at  least  three  parallel  dikes.  In  those 
cases  in  which  there  are  several  dikes  in  a  single  mine,  one  is  generally  known 
as  the  main  dike.  The  smaller  ones  are,  in  some  cases,  clearly  offshoots 
from  the  larger,  the  actual  connections  between  them  being  traced  ;  while 
in  many  other  cases  there  is  no  certain  connection  between  the  different 
dikes  in  the  same  mine. 

Position  of  ore  in  reference  to  the  dikes. — The  ore  has  been  spoken  of  as 
resting  upon  the  fragmental  quartzite  as  a  foot  wall  and  in  exceptional 
cases  (til o  so-called  "north  veins")  as  resting  upon  a  quartz  rock  which 
belongs  to  the  iron  formation,  but  which  nevertheless  forms  a  foot  wall  for  the  ■ 
ore  deposits  dipping  north  with  the  formation.  From  the  description  of  the 
position  of  the  dikes  and  the  quartzites,  it  is  evident  that  the  two  rocks  form 
V-shaped  troughs,  Avliich  have  at  their  apices  right  angles,  and  the  south 
arms  of  which  are  nearer  vertical  than  the  north  ai-ms,  the  first  being 
upon  an  average  20°  to  30*^  from  a  vertical,  while  the  second  is  20'^  to  30° 
from  a  horizontal  position.  The  relation  is  that  of  a  right-angled  trough 
tilted  toward  the  north  until  it  lacks  20°  to  30°  from  having  its  arms  in 
horizontal  and  vertical  positions.  In  one  or  two  mines,  for  a  short  distance 
these  troughs  do  not  incline  either  east  or  west,  but  at  most  of  them,  from 
what  has  gone  before,  it  is  evident  that  they  incline  to  the  east.  In  one 
case,  however,  there  is  a  westward  inclination.  Now,  the  ore  bodies  lie  in 
the  apices  of  these  roughli/  shaped  troughs.  Each  dejjosit  of  ore  in  following 
a  trough  will  evidently  be  at  different  depths  at  different  places  east  and 
west,  the  depth  depending  upon  the  nearness  of  the  dikes  to  the  svirface. 
All  ore  deposits  in  the  position  described  would  reach  the  surface  if  the 
underlying  dikes  pitch  to  the  east  or  to  the  west.  As  a  matter  of  fact,  many 
of  them  were  found  at  the  rock  surface,  but  others  were  found  after  cut- 
ting an  overlying  rock.  However,  at  present  (October,  1888),  the  mining 
developments  have  traced  to  the  surface  all  deposits  which  are  large  enough 
to  warrant  working,  with  two  exceptions,  and  these  exceptions  are 
newly  discovered  bodies,  which,  in  all  probability,  will  be  traced  to  the 
surface  in  the  future.     As  would  be  expected,  it  is  also  true  that  the  devel- 


THE  IKON-BEARING  MEMBER.  275 

opment  of  the  deposits  which  were  origiiiall}-  t'ouiid  at  tlie  surface  has 
carried  them,  in  every  case  in  wliich  they  are  of  any  magnitude,  below 
the  sm-tace  of  the  country  rock.  A  practical  result  of  tliis  is  that  many 
mines  Avhich  began  as  open  ])its,  continued  their  workings  as  iniderground 
mines  as  developments  went  on.  Hotli  of  tliese  facts,  the  tracing  of  the 
deposits  of  ore  discovered  at  depths  to  the  surface  and  those  discovered  at 
surface  beneath  rock,  are  inevitable  deductions  from  what  has  preceded  as 
to  the  })Osition  of  the  ore  with  reference  to  the  un(U'rlying  foot-wall  rock 
and  dikes.  It  also  follows  from  tlie  above  that  an  ore  deposit  upon  one 
property  will  pass,  sooner  or  later,  upon  an  adjoining  property  ;  so  that  sev- 
eral deposits  of  ore,  one  above  the  other,  each  bottomed  by  dikes,  will  be 
found  at  a  certain  place  among  a  row^  of  mines,  provided  that  the  deposits 
continue  to  a  sufficient  depth.^ 

Bock  above  the  ore. — The  rocks  wliicli  are  found  above  the  ore  deposits 
are  the  ferruginous  cherts,  the  rocks  which  have  l^een  spoken  of  as  the 
characteristic  ones  near  the  base  of  the  Iron-l:)earin<i:  mend)er  tlirouarhout 
the  area  in  which  the  ores  occur.  The  u])per  boundary  of  the  deposits 
differs  from  the  (;[uartzite  and  dike  boundaries  in  that  tlie  change  from 
ore  to  the  cherty  rock  is  a  gradual  transition  instead  of  an  abrupt  one. 
In  passing  upward  throiigh  an  ore  deposit,  as  its  Ijorder  is  reached,  the  ore 
becomes  mixed  with  chert  until  so  poor  in  iron  as  to  become  unsalable, 
although  perhaps  carrying  50  per  cent  or  more  of  metallic  iron.  In  pass- 
ing- still  farther  upwards,  the  amount  of  chert   becomes  greater,  until   a 

'  Already  (.July,  1890)  the  Asbltmd  lulue,  at  the  west  end  of  the  property,  has  gone  throui^h  the 
first  basement  dike  and  a  considerable  thiclvness  of  chert  and  ore  and  has  struck  another  ore  deposit 
presumably  bottomed  by  a  second  dike.  The  first  deposit  has  been  developed  (luito  to  the  east  end  of 
the  property,  so  that  the  managers  of  the  Norrie  felt  sure  of  finding  the  Asliland  deposit  by  jiuttiuo' 
a  shaft  down  through  the  basement  dike  of  the  deposit  before  known.  This  they  have  done  with  the 
anticipated  success.  The  Aurora  mine  has  at  present  fully  developed  the  ore  deposit  lying  on  its 
basement  dike;  so  that  practically  all  of  it  may  be  said  to  he  in  sight.  They  must  soon  attempt  to 
find  the  Norrie  deposit.  This  will  necessitate  a  shaft  of  very  considerable  deptli,  and  \\ill  wlien  tried 
be  an  interesting  experiment  as  throwing  light  upon  the  depth  to  which  large  deposits  may  be 
depended  upon  with  some  certainty  to  extend.  The  Colby  dike,  of  great  size  and  at  lirst  overliiin  l)v 
a  great  deposit  of  ore,  when  followed  to  the  east  lias  been  found  to  brcalv  up,  with  a  consequent 
breaking  up  and  dimiurition  of  value  of  the  ore  deposit. 

The  Mount  Hope  mine  has  been  said  to  have  a  westward  pitching  dike;  tliat  of  tlie  Aurora  is 
east;  that  of  the  Pabst  between  the  two  luis  little  j'itcli.  It  is  nearly  certain  that  this  is  the  same 
great  djkc  which  in  the  shape  of  a  great  half  saucer  holds  the  ore  d(^posits  of  these  t.hree  miucs, 


276  THE  PENOKEE  IRON-BEAEING  SERIES. 

fractured  chert  and  iron  ore,  known  to  the  miners  as  mixed  ore,  is  found. 
In  passing  up  still  further,  this  mixed  ore  grades  into  the  ordinary  ferrugi- 
nous chert  of  the  lower  horizon  of  the  iron  formation. 

To  summarize,  then,  the  boundaries  of  the  ore  deposits  are  to  the  south 
either  fi-agmental  quartzite  or  ferruginous  quartz  rocks  in  the  ore  forma- 
tion, generally  the  former;  under  the  ore,  the  dike  rocks;  and,  above  the 
ore,  the  typical  ferruginous  cherts  of  the  district. 

Practical  cleductiom  to  he  airplied  in  prosjpecting  and  mining. — The  first 
essential  in  wise  prospecting  in  the  Penokee  district  is  to  find  the  junction 
of  tlie  ore  formation  and  the  underlying  fragmental  quartzite.^  Having 
determined  tlie  position  of  the  foot-wall  quartzite  at  several  places,  the 
next  step  should  be  to  run  a  line  of  test  pits  east  and  west  just  north  of  the 
line  between  the  ore  formation  and  •  quartzite  across  the  pi'operty  to  be 
explored.  Other  conditions  being  equally  favorable,  the  west  end  of 
the  property  should  be  first  examined,  for  the  bodies  of  ore  almost 
always  pitch  to  the  east,  and  a  deposit  found  upon  this  part  of  the  property 
will  be  likely  to  remain  longer  on  the  land  to  be  explored.  If  the  above 
preparatory  work  does  not  develo})  an  ore  body,  but  a  dike  rock  is  struck, 
the  thickness  ()f  this  dike,  its  inclination  both  to  the  south  and  to  the  east 
or  west  should  be  carefully  determined.  If  its  dip  cannot  be  made  out, 
assume  that  it  is  to  the  east  of  south.  If,  then,  the  dike  is  actually  found 
to  have  or  is  taken  U)  have  an  eastern  pitch,  very  careful  exploration 
should  be  made  in  the  triangular  area  between  the  foot-wall  quartzite  and 
this  dike  to  a  distance  of  200  or  300  feet  east  of  the  junction  of  the  dike 
and  quartzite  and  an  increasing  distance  north  in  passing  to  the  east.  For 
if  an  ore  body  exists  within  a  reasonable  distance  from  the  surface  upon  the 
pi'operty  in  question,  tliis  is  the  place,  as  shown  by  the  actual  position  of 
previously  worked  deposits,  at  which  it  is  most  likely  to  be  found. 

The  known  existence  of  a  <like  just  west  of  a  property  to  be  explored 
would  be  a  second  got)d  reason  for  a  thorough  exploration  upon  the 
west  side  of  the  propert}',  leased  upon  the  chances  of  finding  an  ore 
deposit  which  rests  upon  this  dilce  and  the  quartzite,  which,  because  of  the 

1  Tlio  fliH'civucps  Ixitweeii  tlieso  two  cljissrs  of  loi'ks  liiive  Ijeeu  given  in  detail,  chapters  IV  and  V. 
Tht'y  arc  so  unlike  tluit  ;i  ])riicticiLl  man  who  has  cxiimined  the  chert  in  which  the  ore  occurs  iind  the 
foot-wall  quartzite  at  a  few  localities  can  readily  distinguish  between  them  at  sight. 


tin;  iiioN  r.KAUi  N(i  mk.miu'.k'.  277 

eastern  pitcli  of  the  Mir  in  (|iicsli(iii,  iii;i\-  niii  cast  tar  ciiouiili  1o  l»c  f'ouiul 
upon  the  a{ljoiniii<^-  propcrtw  A  well  di'liniMl  dike  tlius  locatcil  would 
warrant  pntthig  down  a  shaft  tlii-oii<^li  a  (•ousid('ral)l('  tliiidvncss  of  fcrrug'i- 
nous  I'lu'rt,  in  order  to  strike  an  ore  body  wliicli  mijilit  he  Ixdow;  l)ut  in 
general  a  property  ought  to  l)e  well  explored  by  test  pits  whicli  do  not 
go  below  the  surface  of  the  rock,  unless  there  is  a  great  thickness  of  drift 
above  it.  If  heavy  drift  overlies  the  rock  surface,  it  is  best  to  make  the 
most  of  a  shaft  once  down  to  rock  in  exploring  the  area  adjacent  to  it,  either 
by  drifts  in  the  rock,  or  l)y  means  of  the  diamond  drill.  As  a  matter  of 
course  such  explorations  are  nuich  more  expensive  than  in  the  cases  in 
which  the  country  rock  is  near  the  surface.  Under  no  circumstances  should 
money  be  expended  in  exploration  south  of  the  fragmental  quartzite,  or 
farther  north  than  400  or  500,  or  at  the  outside  600  feet  north  of  it;  and 
work  done  such  distances  north  of  the  quartzite  should  only  follow  failure  to 
iind  an  ore  deposit  resting  upon  the  fragmental  quartzite. 

The  large  number  f)f  the  working  shafts  upon  the  Penokee-Gogebic 
range  rest  upon  the  foot-wall  quartzite,  dipping  north  at  its  angle.  When 
a  shaft  thus  sunk  comes  in  contact  with  the  imderlying  dike  of  the  ore 
body,  it  is  useless  to  go  farther  for  the  purpose  of  working  the  deposits 
under  consideration,  unless  the  pitching  ore  body  is  to  lie  reached  at  lower 
levels  by  means  of  long  drifts.  It  is  of  course  the  part  of  wisdom  to  carry  a 
shaft  through  the  dike  a  short  distance  into  the  underlying  rock  in  order  to 
be  sure  that  it  has  struck  a  basal  dike.  It  might  be  the  case  that,  if  the  shaft 
Avere  carried  deep  enough,  it  would  strike  another  deposit  of  ore  resting 
upon  another  dike,  and  this  would  be  especially  probable  if  an  ore  deposit 
has  already  been  developed  within  a  short  distance  to  the  west."^  It  is  a 
good  maxim  among  jnining  men  in  exploring  work  to  follow  a  deposit 
already  found  rather  than  look  for  others  at  greater  depth.  After  the 
deposit  thus  found  has  been  developed  it  is  advisable  to  look  ahead  for 

I  Since  the  above  was  written,  shafts  put  down  throngli  the  livst  liaseiiieiit  (like  have,  in  some 
cases,  struck  the  ore  deposit  of  an  underlying  dike  after  passing-  through  a  cousiderahle  thickness  of 
ferruginous  cherfr.  Tlie  most  notable  instance  of  this  is  the  discovery  of  the  Ashland  ore  body  by  the 
Norrie  mine,  which  lies  to  the  east.  Here,  after  the  base  of  the  origiual  Norrie  deposit  was  reached, 
200  jr  300  feet  of  chert  was  passed  before  the  eastward-pitching  Ashland  body  was  found. 


278  THE  PENOKEE  lEON-BEAEmG  SERIES. 

future  supplies  of  ore.  From  the  shafts  which  stop  at  underlyiug  dikes  it  is 
possible  to  take  out  all  the  ore  to  the  west  and  the  ore  to  the  east  which  lies 
upon  a  higher  level.  Tliat  at  lower  levels  to  the  east  can  be  gotten  out  by 
carrying  the  shaft  to  a  greater  depth,  and  drifting  east  until  the  deposit  is 
again  found,  or  by  putting  in  other  shafts  farther  eastward.  If  the  eastern 
pitch  of  the  dike  (and  therefore  of  the  deposit)  is  high  enough,  the  first 
method  is  the  more  economical  for  a  certain  distance;  but  with  the  low 
eastern  pitch  which  generally  prevails,  the  second  one  has  in  practice  usu- 
ally been  followed.  Each  successive  shaft  to  the  east  must  pass  to  a  greater 
depth  through  barren  rock  before  it  strikes  the  ore  body  ;  and  if  the  devel- 
opments of  the  future  show  that  these  ore  bodies  are  of  great  depth,  a 
deposit  will  often  be  found  to  extend  upon  the  land  of  an  adjoining  mining 
company,  as  has  been  shown  to  be  the  case  with  some  deposits  at  the  pres- 
ent time.  From  the  shafts  thus  resting  upon  the  foot  wall,  drifts  are  ordi- 
narily run  along  it  east  and  west  at  each  of  the  levels,  and  from  these  main 
drifts  crosscuts  are  run  north.  The  drifts  running  west  are  continued  until 
the  underlying  dike  is  penetrated.  Upon  passing  through  this  dike,  if  it  is 
a  basal  one,  the  mixed  ore  or  ferruginous  chert  is  found.  The  drifts  running 
east  continue  in  ore,  alternating  often  with  the  lean  phases  and  horses  of 
rock,  until,  on  account  of  the  eastern  pitch  of  the  ore  deposits  folloAving  the 
dikes,  it  reaches  the  top  of  them,  when  it  runs  gradually  into  poor  ore,  mixed 
ore,  and  finally  ferruginous  chert.  In  general,  the  liorizontal  crosscuts  to  the 
north  are  continued  until  the  dikes  are  reached.  Beyond  the  dikes  is  found 
ferruginous  chert  or  the  regularlj^  banded  red  ferruginous  slates.  Clean 
ore  is  not  usually  continuou.?  in  all  these  directions  in  any  one  mine  for 
the  distance  above  indicated,  nor  are  the  rocks  found  at  these  outer  extrem- 
ities always  such  as  are  named  above,  but  the  boundaries  of  the  ore 
deposits  are  thus  defined,  at  least  in  a  general  way. 

It  is  a  question  whether  the  present  developments  have  not  gone  far 
enough  to  warrant  a  radical  change  in  the  manner  of  mining  iron  ore 
deposits  in  the  Penokee  district.  All  over  the  lake  Superior  country  such 
bodies  are  well  known  to  be  extremely  irregular.  The  deposits  of  the 
Penokee  range  give  promise,  however,  of  greater  regularity  than  has  been 
found  in  other  districts.     If  they  could  be  depended  on  to  continue  doWn- 


TnK  iK()\  r.KAitixc  aikmp.kr.  270 

ward  tor  sonic  distiinci'  witli  the  relations  which  thc\-  ccrt;iiiil\-  li;i\c  in  their 
lii<;'lier  hn'cls,  iind  tlic  dikes  arc  I'ound  to  lia\'e  tolcrnhh-  rei^nlarit \'  of  dips, 
inclined  shafts,  as  sugg-ested  by  Mr.  J.  Parke  (liainiino-,  could  be  sunk 
in  the  lower  slates  close  to  the  ([uartzite,  with  crosscuts  tin'ough  tiic  ([uartzite 
into  the  ore. 

What  has  been  said  in  reference  to  tlie  relations  of  the  oni  bodies  to 
the  dikes,  foot-wall  qnartzites,  adjacent  lean  material,  and  methods  of 
mining  can  be  better  understood  by  an  examination  of  I'ls.  xxx  and  xxxi. 

Nature  of  the.  rocks  of  the  Tron-heariiif/  mcnihrr  adjarrnt  to  tJic  ore  hodirs. — 
Before  considering  the  probable  origin  of  the  ores,  it  will  be  necessary  to 
recall  the  character  of  the  rocks  of  the  ore-lK\aring  formation  adjivcent  to 
the  ore  as  compared  with  the  rock  in  the  parts  in  which  ore  has  not  been 
found,  and  also  the  kinds  of  rocks  which  occupy  the  upper  horizons  of 
the  formation  through  the  area  in  which  paying  mines  occur. 

From  the  general  description  of  the  iron  formaticm,  p.  198,  and  from 
the  tabulations,  pp.  215-220,  it  will  be  seen  that  the  rocks  west  of  Tylers 
fork  are  actinolitie  slates.  The  magnetite  and  actinolite  are  important  in 
quantity  and  the  silica  is  always  completely  crystallized.  Also  the  rocks 
east  of  the  Sunday  lake  mines  and  west  of  the  Presqvie  Isle  river  are  of  the 
same  character,  except  that  the  quartz  is  not  always  whollj"  crystallized. 
The  Iron-bearing  member  east  of  the  Presque  Isle  is  treated  in  another 
place.  Also  these  actinolitie  slates  in  the  east  and  west  end  of  the 
Iron-bearing  member  constitute,  so  far  as  known,  the  whole  mass  of 
the  ore  formation.  The  part  of  the  iron  formation  which  bears  the  ore 
deposits  contains  throughout  most  of  its  extent  no  actinolite  and  magnetite. 
Its  eastern  and  western  extremities  do  contain  a  little  of  these  two  minerals 
through  the  few  miles  in  which  they  grade  into  Ijarren  actinolitie  slates 
farther  east  and  west.  Another  strong  contrast  between  the  part  of  the 
iron  formation  carrying  the  ore  bodies  and  the  barren  portions  is  found  in 
making  a  cross  section  of  the  member.  The  nature  of  the  rocks  in  which 
the  ore  deposits  actually  occur  has  already  been  mentioned.  Above  these 
ferruginous  cherts,  which  bear  bands  and  shots  of  ore  and  which  often 
grade  into  ore,  are  in  most  cases  regularl}'-  banded  red  ferruginous  slates. 
These  slates  have  been  already  described,  and  here  it  is  only  necessary  to 


^80  THE  PENOKEB  IRON-BEARING  SEIUES. 

remember  that  they  are  composed  of  chert  and  iron  peroxide;  that  they 
are  as  regularly  bedded  as  the  unaltered  carbonates,  and  that  they  readily 
cleave  along  the  bedding.  Above  these  slates  for  a  long  distance,  and 
particularly  well  exposed. in  T.  47  N.,  R.  46  W.,  Michigan,  constituting 
the  upper  horizon  of  the  ore  formation,  are  parti)'-  or  Avholly  unaltered 
chertjr  iron  carbonates.  While  the  above  section  is  known  to  occur  at 
several  of  the  more  important  mines,  it  can  not  certainly  be  said  to  be 
common  to  all  of  them.  Also  the  respective  thicknesses  of  the  ill-defined 
belts  are  ver}'  diiferent  at  different  mines.  A  general  statement  may  be 
made  that  at  most  of  the  mines  a  cross  section  of  the  iron  formation  shows 
the  proportion  of  unaltered  iron  carbonates  to  increase  in  passing  from 
lower  to  higher  liorizons  and  appears  to  be  greatest  in  quantity  at  the 
uppermost  horizon.  It  is  true,  however,  that  almost  solid  carbonate  occurs 
in  three  places  at  a  relatively  low  horizon,  although  none  of  them  are 
known  to  be  at  the  base  of  the  member,  while  one  is  certainly  underlain 
b)^  a  ferruginous  chert.  Also  at  several  localities  a  completely  altered  and 
brecciated  chert  is  found  at  very  high  horizons. 

The  cliaracter  of  the  ore. — The  iron  ore  of  the  Penokee-Gogebic  range 
is  a  soft  red  somewhat  hydrated  hematite.  By  chemical  analyses  it  is 
shown  to  be  more  or  less  manganiferous.  Much  of  it  is  so  friable  that  it 
can  be  broken  down  with  a  common  pick,  although  as  taken  from  the 
mines  a  large  portion  of  it  is  compact  enough  to  hold  together  in  tolerably 
large  lumps.  These  lumps  are  porous,  often  more  or  less  nodular,  and 
often  also  roughly  stratiform.  The  strata  conform  in  a  general  way  to  the 
strike  and  dip  of  the  formation.  Mingled  with  this  soft  hematite  in  a  few 
mines  is  a  small  quantity  of  aphanitic  hard  steel-blue  hematite,  which 
breaks  with  conchoidal  fracture  and  is  of  remarkable  purity.  In  general 
this  exceptionally  hard  material  is  found  in  contact  with  or  close  to  the 
diorite  dikes  of  the  mines.  The  following  analyses  and  facts  as  to  com- 
position are  mainly  taken  from  a  report  by  Mr.  John  Birkinbine:^ 

'John  Birkiubme:  The  ii-ou  ores  east  of  the  Mississippi  river;  Mineral  Eesourees  of  the  United  States,  1886,  pp.  67-73. 


I'HE  IRON  UKAUING  MEMDKK'. 


281 


AnahjucH  of  Oogehi 


f  trail  iiffH. 


[The  analyses  fi-oiu  the  Ciilby  arc  averages,    'I'hi' uiial.vHw  Ijcni  ihr  AshliiTicI  is  tim  average  nf  48  (cargoes.] 


Colby. 


lui-nu-rly 
Irou  Kiug. 


Kurth.    South.  I  Miirth. 


Iron I  Gl-00 

Manganese 2no 


Alumina 

Liiue 

Magnesia .. . 

Silica 

Phosphorus. 
Sulphur 


1'7.1 
■U 

■2;i 

4' 50 
•049 

•07 


59-;io 

4-00 

1-CK 

•10 

2^50 
•049 
•06 


(S0^8S 

I'M 

(') 

(*) 

(•) 
5-44 

■027 

(') 


South. 


55-74 

12-'2S 
(') 
(*) 
I*) 
3-47 
•034 
(*) 


Kurrli 


62-83 

(*) 
(*) 
(*") 
(*) 
5^18 
•0474 
(*) 


.\urora. 


62^93 

(') 
(*) 
(*) 
(*) 
3-65 
•0278 
(*) 


AHlilaiul 


64-50 

(') 
(*) 
(*) 
(*) 
365 
•047 
(*) 


'  Undetermined. 


The  south  deposits  carry  upon  an  average  more  manganese  than  the 
north  deposits.  The  content  of  manganese  in  the  south  Mount  Hope  is 
much  greater  than  the  average  for  the  district,  ahhough  some  quantity  of 
ore  has  been  taken  from  the  Colby  which  runs  above  30  per  cent  in 
metallic  manganese.  In  the  mine  itself  streaks  varj-ing  from  a  mere  film 
to  those  a  number  of  inches  in  width,  composed  of  almost  pure  pyrolusite, 
ma)^  be  seen  intersecting  the  main  mass  of  the  iron  ore.  "Alumina  is 
found  in  most  of  the  ores,  the  amount  varying  from  0-5  to  5  per  cent. 
The  sulphur  varies  from  0-03  to  013  per  cent.  Water  to  the  extent  of  6 
per  cent  exists  in  the  hard  ores,  and  to  a  greater  amount  in  the  softer  vari- 
eties." It  appears  that  a  portion  of  this  water  is  combined;  that  is,  that 
the  hematites  are  somewhat  hydrated. 

The  original  condition  of  the  rocks  of  the  ore  formation,  the  series  of 
alterations  by  which  they  have  become  changed  to  a  ferruginous  chert 
and  to  other  varieties  of  rock  in  the  formation,  have  been  fully  considered. 
In  several  localities  the  exact  facts  observed  as  to  these  alterations  have 
been  recorded.  We  then  have  before  us  the  character  of  the  iron  ores,  the 
shape  of  the  deposits,  their  relations  to  the  rocks  immediately  about  them, 
the  nature  of  the  rocks  of  the  iron  formation  above  the  ore  horizon,  and  the 
character  of  the  rocks  above  and  below  the  iron  formation.  An  attempt 
will  now  be  made  to  applj^  these  facts  to  the  changes  which  have  occurred 
to  the  particular  area  of  the  iron  formation  in  which  the  ore  occurs,  and 
to  suggest  an  explanation  of  the  character  and  location  of  the  ore  bodies. 


282  THE  PENOKEE  IROIf-BEAEma  SERIES. 

The  shape  of  the  deposits  and  their  i-elations  to  the  strata  of  the  iron 
formation,  are  such  as  to  exckide  the  idea  of  original  sedimentation  in 
place  ;  neither  can  they  be  considered  as  the  result  of  the  oxidation  of  iron 
carbonate  in  place  alone.  All  of  the  unaltered  sidei'ite  now  found  contains 
a  much  larger  quantity  of  silica  than  the  ores,  so  much  so  as  to  make  these 
carbonates  themselves  entirely  valueless.  Also  the  red  banded  slates  rather 
than  the  ores  described  (pp.  202-205)  give  ever)^  evidence  of  being  a 
material  whicli  has  resulted  from  the  oxidation  of  such  carbonate  in  place. 
Further,  the  large  amount  of  manganese  Avhich  the  ores  (especially  the 
south  deposits)  occasionally  contain  is  much  greater  than  the  amount  con- 
tained in  any  carbonate  from  which  analyses  have  been  made,  and  the 
average  content  of  manganese  in  the  ore  is  much  greater  than  the  average 
of  the  carbonates. 

While  it  is  thus  certain  that  the  ores  are  not  carbonates  of  iron  which 
have  altered  in  place  alone,  it  is  almost  as  certainly  true  that  the  siderite 
of  the  belt  has  been  the  source  whence  the  iron  oxides  for  these  ores  have 
been  derived.  This  statement  is  based  upon  the  facts  furnished  by  the 
detailed  description  of  the  rocks  of  the  iron  formation  as  a  whole  and  those 
developed  by  the  discussion  whicli  has  preceded.  That  manganese  is 
always  present  in  the  iron  carbonates,  frequently  in  some  quantity,- and  is 
yet  more  abundantl}"  found  in  the  ores,  is  an  additional  strong  indication 
that  the  iron  carbonate  has  been  the  source  whence  the  ore  deposits  derived 
their  iron  oxide. 

Since,  then,  the  iron  ores  can  not  be  explained  by  oxidation  of  carbon- 
ates alone  in  place,  and  since  the  carbonate  was  the  som-ce  whence  they 
were  derived,  they  are  necessarily  concentrations  of  ii'on  oxide,  combined 
perhaps  with  iron  oxide  furnished  by  oxidation  of  carbonate  in  place.  If 
this  explanation  is  adopted,  however,  it  is  necessary  to  explain  not  only  the 
presence  of  the  iron  oxide  in  its  peculiar  position,  but  the  nature  of  the 
whole  lower  part  of  the  iron-bearing  formation.  The  explanation  must 
account  for  the  great  increase  in  the  amount  of  silica  in  the  lower  horizon 
of  the  ore  formation  as  compared  with  the  original  cherty  carbonate ;  for 
its  almost  total  absence  in  the  ore ;  for  the  concentration  of  the  iron  oxide ; 
for  the  almost  complete  absence  of  carbonate  at  the  lower  horizons ;  for  the 


THK  IRON-BEARING  MEMHKR.  2B3 

red  baiulod  slates  and  (•••irlxniiilfs  in   the  middle  liori/.ons;   and  ior  the  rela- 
tively inucli  more  ;d)undant  unaltered  carbonate  in  tlie  ujiper  horizons. 

A  pdilicitldi'  occurrence  of  iron  ore. — Before  attemptin";'  to  i^-ivc^  a  <4'eneral 
explanation  of  these  facts  it  will  tirst  be  well  to  ay-ain  refer  to  an  occur- 
rence of  narrow  belts  of  iron  ore  upon  the  ])ank  of  Sunday  lake  outlet,  hi 
Sec.  13,  T.  47  N.,  K.  4(;  W.,  Michigan.  Here  the  actual  transformation  from 
lean  cherty  iron  carbonate  to  ore  is  seen  in  all  its  ])liases.  In  clefts  and 
joints  and  in  parting's  along  the  bedding  of  the  exposure  are  narrow  seams 
of  hematite.  In  passing  from  the  seams  the  hematite  becomes  mingled 
with  some  chert.  This  chert  increases  in  quantity  until  the  cherty  ground- 
mass  contains  many  rhombohedra  of  iron  oxide.  Proceeding  farther  away 
from  the  ore,  the  iron  oxide  gradually  changes  to  siderite,  the  transition 
from  one  to  the  other  being  quite  gradual.  The  siderite  is  in  perfect  rhom- 
bohedra, and  it  is  evident  in  thin  section  that  the  iron  oxide  adjacent  is 
pseudomorphous  after  it,  and  now  the  rock  is  a  sideritic  chert ;  it  is  of  a 
light  gray  color,  aphani tic  texture,  and  breaks  with  conchoidal  fracture. 
This  rock  is  manifestly  in  its  original  condition  at  this  place.  The  processes 
by  which  the  seams  of  iron  oxide  now  occupj^  the  space  once  taken  by  this 
sideritic  chert  are  plain.  The  iron  carbonate  has  decomposed  in  place  to 
iron  oxide,  the  rock  becoming  a  hematitic  chert.  Along  the  seams  waters 
bearing  iron  in  solution  have  passed.  These  waters  have  particle  by  parti- 
cle dissolved  out  the  chert  and  replaced  it  with  iron  oxide,  and  where  once 
was  lean  sideritic  rock  is  rich  ore.  A  portion  of  the  iron  oxide  is  due  to 
the  oxidation  in  place  of  the  iron  carbonate,  but  the  larger  portion  has  come 
from  a  greater  or  less  distance  there  to  be  deposited.  These  seams  of  iron 
oxide  at  this  place  are  but  a  few  inches  in  thickness,  but  it  is  possible  that 
the  process  of  alteration  which  has  here  taken  place  upon  a  small  scale  may 
upon  a  large  scale  explain  the  concentration  of  workable  ore  deposits. 

Chemistry  of  the  process  of  concentration.- — So  far  as  the  iron  oxide  has 
formed  from  iron  carbonate  in  place  the  process  is  simply  one  of  oxidation, 
and  it  is  only  necessary  to  suppose  that  percolating  waters  have  carried 
sufficient  oxygen  in  solution  to  accomplish  this  work.  In  the  removal  of 
iron  as  iron  carbonate  from  one  place  and  its  deposition  as  oxide  in  another 
place  the.  process  was  doubtless  as  follows :  Waters  having  in  solution  carbon 


^g4  THE  PmOKEB  IRON-BEARING  SERIES. 

dioxide  dissolved  the  iron  carbonate.  This  carbon  dioxide  may  have  been 
furnished  by  the  oxidation  of  other  iron  carbonate,  which  would  relieve  the 
percolating  water  of  oxygen  at  the  same  time  that  it  became  capable  of 
taking  iron  carbonate  into  solution.  Such  water  holding  iron  carbonate 
would  retain  it  until  some  more  or  less  open  cleft  or  passage  was  reached, 
by  means  of  which  the  carbon  dioxide  could  escape,  and  perhaps  oxygen 
reach  these  solutions  dissolved  in  other  waters,  and  thus  precipitate  the 
iron  oxide.  The  only  other  chemical  change  necessary  to  explain  this  con- 
centration of  ore  is  that  of  the  solution  of  the  silica,  which  in  part  probably 
occupied  the  place  now  taken  by  the  ore.  It  is  now  well  established 
that  great  quantities  of  silica  ai-e  carried  in  solution  and  deposited  in  rocks. 
The  large  amount  of  water  which  has  traversed  these  passages  has  been  able 
to  take  the  silica  into  solution  at  the  same  time  that  it  oxidized  the  iron 
carbonate  coming  in  by  lateral  secretion,  and  thus  simultaneously  precipi- 
tated iron  oxide  and  removed  silica.  There  is  every  reason  to  believe  that 
these  are  the  chemical  changes  which  have  formed  the  narrow  rich  seams 
of  ore  described.  It  is  most  probable  that  these  chemical  changes,  taking 
place  upon  a  large  scale,  have  produced  the  large  workable  deposits. 

Time  at  which  concentration  of  the  main  ore  bodies  occurred. — It  has  been 
stated  that  the  iron  belt  rocks  are  much  less  altered  as  a  whole  in  the 
higher  horizons.  In  passing  downwards,  it  will  be  remembered  that  an 
mcreasing  proportion  of  the  rock  varies  from  its  original  condition,  and  at 
the  lowest  horizons  there  is^  nowhere  known  any  unaltered  carbonate.  It 
follows  as  a  deduction  from  this  succession  that  the  series  of  changes  which 
have  so  completely  altered  the  lower  part  of  the  formation  have  taken 
place  subsequent  to  the  uplifting  of  the  series.  The  alterations  can  only 
be  explained  by  the  action  of  percolating  waters  bearing  oxygen,  and 
which  therefore  came  from  above.  If  the  layers  were  horizontal  when  the 
changes  occurred,  the  waters  passing  downward  would  have  altered  most 
that  part  of  the  formation  nearest  the  surface.  The  reverse  would  be  the 
case  if  the  alteration  was  subsequent  to  the  tilting,  for  the  apper  layers  of- 
the  iron  member  would  partially  escape  the  action  of  percolating  waters, 
as  will  be  readily  seen  by  glancing  at  any  of  the  sections  across  the  series 
in  that  part  of  the  range  under  discussion   (e.  g.,  Pis.  vi,  viii  andx;)  and 


THE  IKON  HEARING  MEMBEK.  285 

considering' ill (■  natiirt'  ol'  the  licit  di'  mck  iihoNc  the  ore  tnrmation.  It  is 
ii  iiH'iuhci-  (■(iiii|i()sr(l  ol  Miick  ami  j; ray  clay  slatt'.s,  grtiywackes,  and  gray- 
wacke-slatcs ;  all  oi'  which  contain  a.  lai-^c  amount  of  clay  and  arc  practi- 
cally impervious  to  water.  So  stron<;ly  rlayey  is  this  formation,  that  a  rock 
underlying;-  any  great  thickness  of  it  could  not  be  nnu-h  affected  by  waters 
from  above.  An  independent  proof  of  the  impenetrable  character  of  this 
rock  is  the  freshness  of  the  contained  greenstones,  which,  as  explained  in 
chapter  vii  are  almost  or  wholly  decomposed  in  the  lowei-  parts  f)f  the  iron 
formation.  However,  when  the  series  liad  Ijeen  uplifted  and  eroded,  this 
upper  impervious  member  would  have  been  removed,  and  the  Avaters  would 
come  directly  in  contact  with  the  lower  horizons  of  the  ore  formation, 
while  the  upper  horizons  of  the  lielt  would  still  be  somewhat  protected. 

'Before  going  further  it  is  necessary  to  consider  the  porosity  of  the 
rocks  which  underlie  the  ore  formation.  This  member  has  been  fully 
described  (chap.  iv).  It  consists  mainly  of  a  layer  of  feldspathic  quartz- 
slates.  These  are  interstratified  with  clay  slates,  consequently  the  rocks 
of  this  formation  are  almost  impenetrable  to  percolating  waters.  Above 
the  feldspathic  quartz-slates,  and  therefore  between  them  and  the  ore,  i^  a 
layer,  a  few  feet  in  thickness,  of  coarse  fragmental  quartzite — a  rock 
which  was  once  a  sandstone.  Before  the  induration  of  this  sandstone,  per- 
colating waters  may  have  penetrated  it,  but  they  would  have  been  stopped 
in  their  downward  passage  by  the  underlying  slates.  After  its  change  to 
a  quartzite  it  was  itself  a  barrier  to  the  passage  of  percolating  waters.  It 
is,  however,  not  so  perfect  a  check  as  the  underlying  slates,  because  of  the 
fracturing  to  which  it  has  been  subjected.  The  joints,  so  characteristic  of 
a  brittle  quartzite,  do  not  aifect  the  underlying  slates,  for  these  thiidy  lam- 
inated clay  rocks  are  flexible,  and  under  the  slight  bowing  which  they 
have  received  are  almost  or  quite  as  impervious  to  water  as  when  in  their 
horizontal  position. 

Process  of  concentration. — An  attempt  will  now  be  made  to  trace  the 
passage  of  percolating  waters  through  the  inclined  layers  of  the  iron 
formation.  PI.  xxxi,  Fig.  7,  is  a  section  showing  the  condition  of  this 
member  at  the  present  time  at  the  surface  and  illustrating  how  this  state 
of  aifairs  was  reached.     The  strata  of  the  formation  are  now  exposed  by 


286  THE  PENOKEE  lEOX-BEARING  SEEIES. 

theii-  dipping  at  a  high  angle  (66°)  to  the  north.  The  whole  Penokee  series 
of  which  the  iron  formation  is  a  part  is  exposed  in  the  same  fashion. 
Therefore  thousands  of  feet  of  the  iron  member  have  certainly  been  car- 
ried away  by  erosion.  The  figure  assumes  that  about  2,000  feet  have' 
been  eroded  from  this  member  since  it  was  upturned.  However,  it  would 
make  no  difference  with  the  argument  if  this  erosion  occurred  during  the 
time  of  the  uplifting.  The  upper  part  of  the  figure  represents  the  -surface 
of  the  iron  formation  and  a  part  of  the  underlying  and  overlying  rocks  at 
some  "past  time.  Near  the  bottom  of  the  figure  is  the  present  land  surface, 
showing  the  succession  of  rocks  from  north  to  south  which  are  now 
actually  found.  A  transition  from  unaltered  cherty  carbonates  to  com- 
pletely decomposed  carbonate  is  noted.  This  section  is  not  definitely 
known  to  occur  at  every  mine,  but  it  well  represents  the  usual  occur- 
rence in  that  part  of  the  formation  which  has  been  productive  in  iron  ore; 
that  is,  the  presence  of  more  unaltered  carbonate  at  high  horizons  than 
elsewhere  and  in  low  horizons  little  or  no  unaltered  carbonate.  At  the 
time  when  the  upper  supposed  land  surface  was  an  actual  one  the  present 
surface  would  be  but  little  exposed  to  the  action  of  percolating  water. 
Water  could  not  pass  through  the  slates  which  overlie  the  iron  formation, 
neither  could  it  get  in  through  the  underlying  feldspathic  quartz-slates. 
Therefore  most  of  the  water  which  at  that  time  was  able  to  reach  the 
present  land  surface  must  have  done  so  by  passing  down  along  and  through 
the  layers  of  the  iron  formation  itself.  The  dotted  broken  line  represents  a 
perpendicular  course  which  the  Avater  would  follow  were  its  passage  not 
deflected  by  the  lamina3  of  the  rocks,  but  this  water  would  have  a 
tendency  to  follow  the  bedding,  so  that,  entering  the  iron  formation 
at  its  uppermost  layer,  it  would  follow  the  somewliat  irregular  course 
marked  as  the  probable  line  of  percolation,  and  would  reach  the  foot- 
wall  quartzite  at  the  present  surface  of  the  country.  It  is  immaterial 
to  the  argument  whether  this  line  ought  to  vary  farfher  from  a  perpen- 
dicular than  marked  or  not,  for  in  any  case  nearly  the  whole  of  the 
present  surface  of  the  iron  formation  would  escape  the  percolating  waters, 
or,  if  not  this  surface,  some  other  yet  lower  down.  It  is,  however, 
probable  that  the  lower  horizons  of  the  formation  would  not  thus  escape 


THE  IRON-BEARING  MEMBER.  287 

until  ii  great  depth  was  reached,  for  the  waters  entering  the  formation 
would  steadily  work  their  w;iy  to  a  greater  and  greater  depth  along  the 
foot-wall  quartzite  until  sucli  depths  were  attained  as  to  prevent  its  farther 
penetration. 

Now,  suppose  erosion  to  gradually  sweep  away  the  rocks  which  are 
between  the  old  surface  of  the  country  mid  tlio  present  surface.  Begin- 
ning at  the  base,  the  rocks  of  the  formation  at  tlie  present  surface  would  be 
more  and  more  exposed  to  the  action  of  percolating  waters.  These  waters 
would  in  turn  affect  the  middle  and  finally  the  higher  layers  of  the  forma- 
tion until  its  whole  width  was  subject  to  the  agencies  of  alteration.  There 
is,  then,  a  gradual  increase  in  the  time  that  percolating  waters  have  acted 
upon  the  various  liorizons  of  the  formation  in  j)assing  from  south  to  north. 
The  difference  in  time  to  which  the  highest  and  lowest  layers  have  been 
subjected  to  such  action  is  at  least  the  length  of  time  that  it  has  taken 
erosion  to  remove  the  thickness  of  rock  'between  the  old  surface  of  the 
country  and  its  present  surface.  Therefore  the  slower  we  believe  the 
erosion  to  have  been  the  greater  the  difference  in  time. 

Next,  suppose  that  erosion  has  continued  until  the  surface  of  the  land 
is  at  some  intermediate  point.  In  tracing'  the  percolating  waters  in  theu' 
passage  through  the  formation  it  is  necessary  to  take  into  account  the  deflec- 
tion to  which  they  would  be  subject  caused  by  its  layers,  by  the  impene- 
trable character  of  the  underlying  slates,  and  by  intersecting  dikes.  The 
relative  positions  of  the  ore  bodies,  quartzites,  and  dikes  have  already  been 
given.  The  water  which  fell  upon  the  layers  of  the  iron  formation  nea.r  its 
base  would  readily  pass  through  the  rock ;  it  being  here  already  much 
altered  and  broken  by  the  long  action  of  water.  However,  in  the  most 
broken  cherts  there  is  evidence  in  the  somewhat  irregular  bands  of  ore  that 
the  path  of  percolating  waters  has  been  influenced  by  the  stratiform  char- 
acter of  the  formation.  Passing  through  these  ferruginous  cherts,  the  water 
would  quickly  reach  a  dike  or  the  fragmental  quartzite,  and  would  follow 
this  barrier,  deflected  to  the  north  if  upon  the  quartzite  and  to  the  south  if 
upon  a  dike,  until  it  reached  a  trough  made  by  the  dike  and  quartzite,  along 
which  it  would  travel  toward  the  east  as  it  penetrated  deeper.  Such  water 
would  be  likely  to  contain  oxygen  in  solution,  and  would  be  capable,  if  it 


288  TBE  PENOKEE  lEON-BEAEING  SERIES. 

contained  alkalies — which  might  readily  be  obtained  from  the  alteration  of 
the  basic  dikes — of  taking  up  a  small  amount  of  silica.  Other  water  falling 
upon  higher  layers  of  the  formation  would  make  its  way  slowly  and  with 
difficulty  through  these  less  altered  parts,  and  would  oxidize  iron  carbonate 
until  all  oxygen  had  been  -  extracted  from  it.  This  oxidation  of  a  part  of 
the  iron  carbonate  would  liberate  carbon  dioxide,  which  would  be  taken 
into  solution  and  added  to  the  carbon  dioxide  which  the  water  ah'eady  con- 
tained.^ Such  water  would  take  into  solution  unaltered  iron  carbonate.  It 
would  also^  in  its  upper  course,  take  iip  what  silica  it  was  able  to  carry.  As 
it  penetrated  farther  and  took  more  carbon  dioxide  into  solution,  and  con- 
sequently also,  more  iron  carbonate,  it  would  be  less  able  to  carry  silica, 
and  would  dejjosit  that  material  as  chert  in  the  lower  horizons.  This  water, 
thus  traveling  on  with  an  increasing  amount  of  iron  carbonate,  wotild 
finally  reach  a  dike  and  be  deflected  toward  the  foot-wall  quartzite.  This 
dike  it  would  follow  until  the  apex  of  the  trough  was  reached.  Here  it 
would  mingle  with  a  larger  amount  of  water  more  directly  from  the  surface, 
bearing  oxygen,  and  therefore  capable  of  oxidizing  the  iron  carbonate. 
The  iron  would  then  be  precipitated  in  the  apex  of  the  trough  as  more  or 
less  hydrated  sesquioxide  of  iron. 

Upon  the  other  hand,  the  silica  would  here  be  dissolved,  for  the  carbon 
dioxide  solution  containing  iron  carbonate  would  be  greatly  diluted  by  the 
large  amount  of  waiter  which  bore  the  precipitating  agent  for  the  iron,  and 
the  resultant  abundant  dilute  solution  of  carbon  dioxide,  bearing  perhaps 
alkalies  with  it,  would  be  capable  of  taking  up  silica,  which  was  either 
originally  present  or  had  been  subsequently  deposited  in  the  apex  of  the 
trough.  Such  solutions  may  have  furnished  the  silica  which  has  enlarged 
the  particles  of  quartz  in  the  foot  wall  and  thus  indurated  it."  The  result 
of  this  leaching  would  be  to  steadily  add  iron  oxide  to  and  to  remove  the 

'  In  this  tliscnssion  carbonated  water  is  taken  as  the  agent  of  solution.  It  is  likely  enough  that 
organic  acids  have  helped  to  take  the  iron  carbonate  in  solution  and  bear  it  to  the  points  of  precipi- 
tation. 

^The  chemistry  of  the  process  tlius  outlined  assumes  the  following:  That  the  oxygen  of  perco- 
lating waters  is  sufficient  to  oxidize  iron  carbonate  not  in  solution  and  set  carbon  dioxide  free;  that 
the  resultant  carbonated  waters  arc  sufficient  to  take  iron  carbonate  in  solution;  that  if  such  waters 
bearing  dissolved  carbonates  are  mingled  witli  other  waters  bearing  oxygen,  the  iron  carbonate 
or  aportioii  of  it  will  be  precipitated;  that  silica  may  be  carried  in  percolating  waters:  that  carbon 


THK  IRON-BEARING  MEIMBEK.  289 

silifii  from  tl\e  apices  of  the  trouglis  formed  ])y  the  ({uartzite  and  dikes,  and 
thus  to  foiiii  ore  bodies.  At  the  same  time  the  otliei"  parts  of  the  formation 
wouhl  be  steachly  impoveiislied  in  iron  content.  Much  of  that  which 
remained  disseminated  tln-ougli  the  formation  would  have  been  changed 
from  carbonate  to  oxide.  The  siUca  which  was  taken  into  solution  in  the 
upper  part  of  the  water's  course  would  be  preci})itated  in  its  lower  part. 

The  processes  thus  outlined  wt)uld  penetrate  to  deeper  j^arts  of  the 
formation  as  erosion  steadily  advanced  until  the  present  surface  of  the 
country  is  reached,  where,  as  shown  upon  the  diagram,  the  ore  bodies  thus 
formed  at  depth  are  now  found  at  svirface.  It  follows  that  a  large  amount 
of  the  iron  found  in  the  ore  bodies  was  originally  in  the  rock  which  has  been 
removed  by  erosion.  So  far  as  the  deposits  are  at  the  surface,  all  of  the 
iron  oxide  except  that  which  came  from  the  oxidation  of  iron  carbonate  in 
place  must  have  been  from  such  a  source,  while  it  is  probable  that  a  large 
part  of  the  deposits  located  at  considerable  depth  have  been  stored  from 
rock  which  has  been  broken  down  and  scattered  far  and  wide.  It  is  a 
further  consequence  that  it  is  of  no  moment  whether  the  amount  of  iron  in 
the  original  800  feet  of  thickness  of  the  fonnation  was  sufficient,  if  concen- 
trated at  its  base,  to  make  up  the  large  deposits.  These  are  not  so  much 
Concentrations  of  iron  oxide  which  were  originally  deposited  as  a  carbonate 
above  them,  as  from  the  layers  which  stretched  to  the  southward,  but  which 
were  subsequently  by  upturning  placed  over  the  ore  bodies.  Also,  the 
large  proportion  of  silica  now  found  near  the  surface,  and  particularly  in 
the  southern  half  of  the  belt,  is  probably  much  greater  than  was  here  origi- 

dioxido  is  sufficient  to  precipitate  silica  froui  such  solutions;  and  tliat  a  carbon  dioxide  solution 
strong  cnougli  to  precii)itato  silica  by  dilution  may  be  made  so  weak  in  carbon  dioxide  that  it  would 
be  capable  of  taking  silica  into  solution.  All  of  these  facts  and  principles  of  chemistry  are  so  well 
known  that  uo  discussion  or  reference  to  authorities  is  needed.  However,  the  following  statement  of 
Edscoe  and  Schorlemmer,  vol.  I,  p.  569,  is  of  interest  as  bearing  upon  the  solubility  of  silica  and  the 
temperature  at  which  it  crystallizes:  "In  all  three  conditions  silica  is  insoluble  in  water.  .  .  . 
Silica,  however,  is  easily  soluble  in  all  alkalies,  even  in  ammonia,  and  the  more  easily,  the  finer  its 
state  of  division.  The  amorphous  variety,  especially  if  it  contains  water,  also  dissolves  in  alkaline 
carbonates.  .  .  .  When  an  alkaline  solution  of  silica  is  heated  in  a  scaled  tube  the  glass  is 
attacked  and  an  acid  silicate  is  formed  from  which  silica  separates  out  on  cooling.  If  the  tempera- 
ture at  which  the  deposition  occurs  be  above  180°,  the  silica  separates  oiit  as  quartz ;  if  below  this 
point,  it  crystallizes  out  as  tridymite,  whilst  at  the  ordinary  temperature  of  the  air  it  separates  in 
the  form  of  a  hydrated  amorphous  mass." 
MON  XIX 19 


290  THE  PENOKEE  IRON  BEARING  SERIES. 

nally  j)resent.  As  erosion  steadily  carried  away  the  upturned  edge  of  the 
ore  formation,  percolating  waters  took  in  solution  a  part  of  the  silica  which 
it  contained,  penetrated  the  deeper  parts  of  the  ore  formation,  and  from  it 
silica  was  precipitated,  as  explained  above,  by  the  ore-forming  process.  The 
highly  cherty  rocks  associated  with  the  ores  may  then  represent  a  concen- 
tration from  many  hundreds  or  even  thousands  of  feet  of  rock  which  has 
been  swept  away,  just  as  the  ore  bodies  are  concentrations  from  the  iron 
carbonates  which  these  same  rocks  contained.  A  portion  of  this  silica  may 
have  come  from  the  alteration  of  dikes  contained  in  this  removed  material, 
but  doubtless  much  of  it  came  from  the  original  cherty  carbonate. 

Exceptional  localities. — At  three  places  iron  carbonate  is  found  near  the 
base  of  the  formation:  Sec.  6,  T.  45  N.,  R.  2  W.,  Wisconsin,  Sec.  13,  T.  47 
R.  N.,  46  W.,  Michigan,  and  Sec.  18,  T.  47  N.,  R.  45  W.,  Michigan.  At  the 
first  of  these  localities  the  rock  contains  a  considerable  quantity  of  clayey 
material.  At  the  second  and  third,  the  rocks  are  exceedingly  dense,  finely 
laminated,  and  bear  contortions,  as  shown  in  the  exposures,  without  breaking 
the  stratification.  In  all  of  these  jjlaces,  then,  the  original  cherty  iron  car- 
bonate had  exceptional  characteristics,  which  prevented  ready  penetration 
by  water.  The  presence  of  an  altered  carbonate  at  these  low  horizons  is, 
however,  interesting  on  two  accounts.  In  the  first  place,  it  greatly 
increases  the  probability  that  the  whole  base  of  the  formation,  which  now 
contains  so  little  carbonate,  was  once  a  cherty  iron  carbonate ;  secondly, 
one  of  these  exposures  shows  how  completely  the  presence  of  a  little  clay  in 
a  rock  may  prevent  the  action  of  percolating  waters,  even  when  the  remain- 
der of  the  belt  is  completely  altered  by  such  agencies. 

The  only  other  exception  to  the  facts  as  assumed  in  the  above  discus- 
sion are  the  occurrences  of  iron  ore  at  a  higher  horizon  than  the  foot-wall 
quartzite  and  the  ore  bodies  .east  of  Sunday  lake,  which  so  far  as  present 
developments  go  are  not  known  to  be  associated  with  dike  rocks.  It  has 
been  said  that  those  ore  bodies  in  the  main  part  of  the  range  which  are 
north  of  the  fragmental  quartzite  have  a  well  defined  cherty  quartz  foot- 
wall  of  a  regular  character.  In  these  cases  this  quartz  rock  has  served  as 
the  plane  which  checked  the  waters  on  their  downward  passage  before  they 
reached  the  fragmental  foot  wall  which  constitutes  the  basement  of  the 


TIIK   IKON  IMOAIUNG  MKM15EH.  291 

iron  formation.  Hero  the  relations  nf  the  ore  to  tlic  dikes  and  all  other 
of  its  chtu-acters  are  the  same  as  when  the  ore  is  found  upon  tlie  fragraental 
qiiartzite.  It  is  of  interest  here  to  note,  that  in  one  of  the  lar<^est  mines  of 
the  district,  the  Colby,  as  shown  by  PI.  xxxi,  Fig-.  3,  the  north  and  south 
deposits  have  as  their  basement  tiie  same  great  dike,  tlie  two  ore  bodies 
being  separated  at  the  top  merely  by  a  gigantic  horse  of  rock,  which  served 
as  the  impervious  layer  to  form  the  foot  wall  of  the  north  deposit.  The  ex- 
planation given  of  the  origin  of  the  ore  found  upon  the  fragmental  quartzites 
applies  perfectly  to  these  north  deposits  with  the  modifications  above  indi- 
cated. That  there  are  layers  of  the  iron  formation  which  are  not  readily 
pervious,  and  therefore  become  basements  along  which  the  down-flowing 
waters  passed,  is  not  at  all  strange.  It  would  be  stranger  if,  in  a  thickness 
of  water-deposited  sediments  of  800  feet,  there  were  no  layers  which,  at 
least  for  a  short  distance,  were  effectual  barriers  to  the  passage  of  percolat- 
ing waters.  The  chemistry  of  the  process  of  concentration  of  the  ore 
deposits  east  of  Sunday  lake  is,  in  all  probability,  like  that  of  the  typical 
•deposits  of  the  ranges  Their  concentration  is  apparently,  however,  more 
nearly  analogous  to  the  narrow  seams  of  ore  described  in  the  early  part  of 
this  section  than  to  the  typical  deposits.  The  formation  here  apparently 
being  cut  by  no  impervious  dikes,  the  waters  have  not  been  carried  over  to 
the  quartzite,  thus  forming  main  channels  of  percolation,  but  the  compara- 
tively small  ore  bodies  have  developed  here  and  there  as  favorable  condi- 
tions for  concentration  occurred.^  , 

The  above  explanation  of  the  origin  of  the  ore  deposits  accords  well 
with  the  facts  of  their  occurrence,  and  also  with  the  idea  that  the  iron 
formation  was  originally  an  impure  cherty  carbonate  of  iron.  It  explains 
perfectly  the  peculiar  position  of  the  ore  bodies  with  reference  to  the  dikes 
and  the  foot-wall  quartzite  ;  it  explains  tlieir  presence  in  a  similar  position 
in  the  few  instances  in  which  the  deposits  are  north  of  the  fragmental 
quartzite  ;  it  explains  the  flat  wedge-shaped  character  of  the  ore  deposit ;  it 
explains  the  nature  of  the  ore — a  soft  somewhat  hydrated  hematite,  bearing 

'  As  bearing  upon  tho  triithfulness  of  the  above  theory  as  a  whole,  it  is  an  interesting  fact  that 
the  iiractical  miners,  in  prospecting,  eagerly  follow  underground  water  channels,  hoping  that  they 
will  lead  to  ore  deposits. 


292  THE  PENOKEE  lEON  BEARING  SEEIES. 

more  or  less  of  manganese;  it  exjjlains  the  excess  of  manganese  which  the 
ore  carries  beyond  tlie  amount  found  in  the  unaltered  carbonates  and  its 
relatively  greater  abundance  in  the  south  deposits;  it  explains  the  presence 
of  large  quantities  of  unaltered  carbonates  in  the  upper  horizons  of  the  ii-on 
formation,  the  gradual  lessening  of  this  carbonate  in  passing  to  lower  hori- 
zons, and  its  absence  at  the.  base  of  the  formation ;  it  explains  the  large 
percentage  of  silica  contained  in  the  greater  part  of  the  lower  horizons  and 
the  low  percentage  at  the  apices  of  the  troughs. 

The  exceptions  of  carbonate  near  the  base  of  the  formation,  the 
occurrence  of  ore  deposits  at  horizons  above  the  foot-wall  quartzites,  and 
the  unusual  deposits  east  of  Sunday  lake,  are  all  due  to  exceptional  char- 
acters at  these  places.  These  exceptions,  with  the  ready  explanations,  are 
thus  rather  in  favor  of  than  against  the  general  idea  of  the  concentration 
of  the  ores. 

Probable  extent  in  depth  of  ore  bodies. — This  explanation  of  the  origin 
of  the  ores  may  throw  some  light  upon  the  depth  to  which  the  ore  bodies 
extend.  The  fact  that  all  of  them  have  been  traced  to  the  erosion  surface  • 
is  favorable,  rather  than  otherwise,  to  their  extending  to  a  considerable 
depth.  The  ore  bodies  at  the  depth  now  penetrated  must  have  formed 
almost  wholly  before  the  sweeping  away  of  the  rocks  of  the  iron  formation 
above  them.  They  could,  then,  have  received  but  little  of  the  iron  they 
contain  since  the  end  of  the  glacial  epoch,  for  erosion  was  then  terminated 
by  the  mantle  of  drift  dropped  over  the  disti'ict.  The  deposits  may  be  said, 
with  some  degree  of  probability,  to  continue  to  a  depth  at  which  the 
agencies  of  concentration  could  effectively  work.  In  estimating  the  depth 
it  must  be  remembered,  in  the  part  of  the  range  in  which  the  ore  deposits 
are  close  together  and  parallel  dikes  consequently  not  far  apart,  that  the 
deeper  dikes  will  be  screened  in  part  from  percolating  water  by  overlying 
dikes.  When  the  dikes  are  very  deep  an  overlying  dike  may  pene- 
trate the  upper  slate  to  the  north  before  reaching  the  surface  in  this 
direction,  and  the  lower  dike  can  receive  iron  ore,  if  it  holds  any  at 
all,  only  by  the  creeping  down  of  the  surface  waters  from  the  area 
between  the  outcrops  of  the  two  dikes.  Whether  the  depth  of  the  ore 
bodies    will    be    found   to    be    measured   in    hundreds    or   thousands    of 


TIIK  lUON  BEARING  MKMBEU.  293 

feet  the  data  at  present  arc  too  scant  to  indicate.  I  am  inclined  to 
believe,  however,  that  they  may  he  depended  npon  to  continue  for  a 
considerable  depth.  While  they  may  extend  m  miimpaired  i-ichness  and 
magnitude  to  a  deptli  as  great  as  can  be  penetrated  b)'  workings,  it  is  cer- 
tain that  they  do  not  coittiiuie  to  an  indefinite  distance.  There  is  also  a 
possibility  that  the  deposits  ma}'  liecome  somewhat  poorer  comparatively 
near  the  surface;  for  it  may  be  that  percolating  waters,  since  the  termina- 
tion of  the  glacial  epoch,  have  been  aide  to  remove  from  the  upper  parts  of 
the  deposits  a  small  percentage  of  silica.  Such  a  removal,  even  to  the 
extent  of  5  per  cent  or  less,  would  have  an  important  influence  upon  the 
value  of  the  deposits.' 

.  Emmons  on  ore  deposits. — In  this  connection  it  is  of  interest  to  com- 
pare the  conclusions  reached  with  those  of  Emmons^  as  to  the  origin  of 
the  silver-lead  deposits  of  Leadville,  Colorado.  He  finds  that  the  ore 
deposits  there  did  not  form  in  preexisting  cavities,  but  by  a  gradual 
replacement  of  the  rock  materials  by  substances  brought  in  solutions,  and 
also  that  these  solutions  did  not  come  up  from  below,  but  have  reached 
their  immediate  locus  by  passing  downward  through  the  rocks  above.  In 
his  discussion  upon  ore  deposits  in  general  he  maintains  that  a  like  origin 
is  much  more  common  than  has  been  believed.  It  will  be  seen  that  our 
own  conclusions  as  to  the  origin  of  the  iron-ore  deposits  of  the  Penokee- 
Gogebic  series,  arrived  at  independently  of  the  publication  of  Mr. 
Emmons's  monograph,  are  in  exact  harmony  with  his  general  conclusions. 
Iron  ores  in  other  parts  of  lake  Superior  country. — Before  closing  this 
chapter  some  allusion  must  be  made  to  the  nature  and  origin  of  the  iron 
ores  which  are  found  in  other  districts  of  the  lake  Superior  country.  Large 
deposits  of  ore  are  found  in  rocks  remarkably  like  those  of  the  Penokee- 
Gogebic  series  in  the  Vermilion  lake,  Marquette,  and  Menominee  disti-icts. 

Recent  investigations  of  these  districts  have  shown  that  the  pitching  ore 

• — ^__ 

'  Since  the  above  was  written  development  lias  extended  in  the  larger  mines  to  a  depth  of  sev- 
eral hundred  feet,  and  as  yet  there  is  no  appreciable  diminution  in  the  .size  or  richness  of  the  ore 
deposits. 

^Emmons,  Samuel  Franklin:  Monograph  U.  S.  Geol.  Survey,  1886,  pp.  375-379,  vol.  xii,  Geol- 
ogy and  Mining  Industry  of  Leadville.  Also  Structural  Relations  of  Ore  Deposits;  Trans.  Am.  Inst. 
Min.  Eug.,  vol.  xvi,  1888,  pp.  804-839. 


294  THE  PENOKBB  lEON-BEAEING  SERIES. 

bodies,  like  those  of  the  Gogebic  district,  are  secondary  concentrations 
produced  from  carbonates  by  downward  flowing  waters  and  resting  npon 
impervious  formations.^  .These  impervious  formations,  known  as  soap- 
stones,  are  frequently  altered  basic  eruptives,  but  in  certain  places  they 
are  clearly  sheared  clayey  phases  of  sedimentary  rocks.  It  is  an  interest- 
ing illustration  of  the  uniformity  of  nature's  processes  that  later  investi- 
gations have  shown  that  the  iron-ore  bodies  in  the  other  districts  of 
the  lake  Superior  country  have  an  origin  like  those  <if  the  Penokee- 
Gogebic  series. 

Summary  of  more  important  conclusions. — The  Iron-bearing  member  is 
separated  from  the  Quartz-slate  member  below  it  and  the  Upper  slate 
member  above  it  because  it  is  nonfragmental,  while  thej^  are  fragmental 
sediments. 

It  consists  of  three  main  types  of  rock:  (1)  cherty  iron  carbonate;  (2) 
ferruginous  slates  and  cherts,  and  (3)  actinolitic  and  magnetitic  slates. 

The  cherty  iron  carbonates  represent  the  original  condition  of  the 
whole  member,  the  other  types  having  reached  their  present  condition  by 
a  series  of  subsequent  alterations. 

In  the  change  of  cherty  iron  carbonate  to  ferruginous  slates  and  cherts 
the  siderite  has  been  oxidized,  and  to  some  extent  taken  into  solution 
and  redeposited  in  other  places.  The  silica  originally  present  has  been 
rearranged,  and  in  many  places  additional  silica  has  entered. 

In  the  altei-ation  of  the  cherty  iron  carbonate  to  actinolitic  slates  the 
oxidation  of  the  protoxide  of  iron  in  the  carbonate  has  not  been  complete, 
and  thus  magnetite  has  been  formed.  At  the  time  of  the  rearrangement 
and  introduction  of  silica  a  portion  of  it  has  united  with  the  bases  present 
and  has  formed  actinolite. 

The  iron  ores  ai-e  found  in  the  lower  horizons  of  the  Iron-bearing  mem- 
ber, most  of  the  deposits  resting  upon  the  upper  quartzite  of  the  quartz-slate 
formation.  Those  deposits  which  are  not  at  the  base  of  the  member  have 
also  somewhat  regular  foot  walls,  which  dip  to  the  north  like  the  quartzite 


I  Van  Hise,  C.  E. :  The  Iron  Ores  of  the  Marqnette  District  of  Michigan;  Am.  Jour.  Sei.,  3d 
series,  vol.  43,  1892,  pp.  116-132.  Also  The  Iron  Ores  of  the  Lake  Superior  Region;  Wis.  Acad.  Sci., 
Arts,  and  Letters,  vol.  8,  pp.  219-228. 


TIIK   IKON  15EAKIN(!   MEMBER.  295 

at  an  an<il('  of  (!()"  (ir  70  ',  ctinsistiui^'  of  tlic  fcrnif^iiioiis  clun't  of  tli(*  Iron 
iiicmlx'i-  itself.  '^Pliese  foot  walls  form  t.ho  soutlici-ii  hoiiiidary  of  t-lui  ore 
bodies. 

The  formations  are  cut  at  riglit  angles  b}-  a  series  of  dikes,  which  are 
in  such  a  position  that  the  same  dike  is  g-eiuM-ally  at  a  greater  depth  as  it  is 
followed  eastward,  i.  e.,  pitches  to  the  east. 

The  ore  bodies  rest  in  the  right-angled  troughs  formed  by  the  junction 
of  the  dike  rocks  and  foot  walls. 

The  iron  carbonate  is  the  source  of  the  ore  deposits. 

The  ores  have  not  been  chiefly  produced  ])y  the  oxidation  of  the 
carbonate  in  place,  but  b}'  concentration  from  the  lean  carbonates  of  the 
formation. 

The  concentration  has  taken  place  during  or  subsequent  to  the  uplift- 
ing of  the  series. 

Percolating  water  was  the  active  agent  of  concentration.  It  has 
taken  the  carbonate  into  solution,  and  in  its  passage  downward  has  been 
deflected  into  the  troughs  by  the  impervious  quartz-slate  formation  and  the 
dikes. 

In  the  apices  of  these  troughs  other  waters  more  directly  from  the 
surface,  bearing  oxygen,  have  precipitated  the  iron  as  an  oxide. 


CHAPTER   VI. 


By  C.  R.  Van  Hisb. 


THE  UPPER  SLATE  MEMBER. 

Section  I.     Details. 

Name  and  basis  of  separation.  Transition  from  Iron-bearing  to  Upper  slate  member.  Geographical 
distribution.  Topographical  features.  General  petrographical  character.  Petrograpbical 
characters  of  the  fonr  types  of  rock.     Tabulation  of  petrographical  observations. 

Section  II.     Origin  of  tlie  Upper  slate  rocks. 

(1)  Qnartzose  graywacke.  (2)  Mnscovitic  and  biotitic  graywacke.  (3)  Biotitic  graywacke. 
(4)  Muscovitic  biotite-slate.  (5)  Nearly  crystalline  muscovitic  biotite-scbist.  (6)  Crystalline 
muscovitic  biotite-schist.     Black  mica-slates.     Source  of  material.     Summary. 

SECTION  I  —DETAILS. 

Name  and  hasis  of  separation.— Al\  the  rocks  of  the  Penokee  series  above 
the  Iron-bearing  member  are  placed  together,  although  their  tliickuess  is 
for  a  distance  of  many  miles  several  times  as  great  as  the  three  lower  forma- 
tions of  the  series  combined.  Lithologically  the  great  mass  of  the  rocks 
are  slates.  Frequently  the  slate  becomes  somewhat  massive  and  occa- 
sionally becomes  a  quartzite ;  also  in  certain  localities  it  becomes  quite 
schistose ;  but  all  these  phases  may  be  said  to  be  exceptional.  It  is  not 
meant  to  imply  by  the  term  "slate"  that  any  of  the  rocks  have  a  slaty 
cleavage.  The  direction  of  the  easiest  parting  and  bedding  always  coiTe- 
spond,  and  none  of  the  rocks  possess  the  uniformity  of  composition  and  part- 
ing requisite  for  roofing  slates.  The  formation  is  given  the  name  "Upper 
slate  member,"  which  at  once  shows  its  stratigraphical  position  and  litholog- 
ical  character,  and  distinguishes  it  from  the  Quartz-slate  member,  which 
is  below  the  Iron-bearing  member. 

296 


THE  UPPEK  SLATE  MEMBER.  297 

This  Upper  slate  is  a  bell  of  IVaginentiil  rocks,  or  ro(tks  which  were 
originall)'  fraginental.  It  lias  the  same  basis  for  separation  tVoiii  the  sedi- 
ments of  the  Iron-bearing-  nu'iulxT  that  the  fraginental  Quartz-slate  below 
it  has.  In  general  the  fragmental  character  of  the  thin  sections  of  these  rocks 
is  recognizable  at  a  glance,  although,  as  will  be  seen  later,  certain  portions 
of  the  belt  have  by  subsequent  alterations  taken  a  crystalline  character.. 
These  rocks  both  in  hand  specimen  and  thin  section  are  separated  with  cer- 
tainty and  ease  from  those  of  the  underlying  Iron-bearing  member;  for  they 
ai'e  fundamentally  imlike  in  texture  and  mineral  content. 

Transition  from  Iron-heariny  to  the  Uj)pcr  slate  member. — The  change 
from  the  Quartz-slate  to  the  Iron-bearing  member  has  been  shown  to  be 
abrupt,  taking  place  usually  without  any  transition  heXi.  In  the  few  locali- 
ties in  which  the  transition  from  the  Iron-bearing  to  the  Upper  slate 
member  is  exposed  the  change  is  quite  gradual.  Beginning  at  the  Avest, 
the  first  known  exposure  of  this  sort  is  in  the  extreme  southern  part  of 
the  SW.  I  of  Sec.  11,  T.  44  N.,  R.  3  W.,  Wisconsin.  Here,  at  the  foot  of 
the  northern  slope  of  a  large  bluff  belonging  to  the  iron-bearing  formation, 
is  found  a  rock  (described  later)  which  in  its  characteristics  is  intermediate 
between  the  black  slates  found  in  the  upper  belt  at  this  locality  and  the 
biotitic  actinolite-slates  of  the  Iron-bearing  member.  Its  location  in  either 
formation  is  somewhat  arbitrary,  but  it  is  placed  in  the  tabulations  of  the- 
Upper  slate  membei-.  In  the  west  part  of  Sec.  6,  T.  46  N.,  R.  2  E.,  Wis- 
consin, and  near  by,  exposures  in  the  Iron-bearing  and  in  the  Upper  slate 
members  are  found  quite  close  to  each  other.  That  in  the  latter  is  a  transi- 
tion rock,  as  it  contains  iron  oxide  as  a  chief  constituent,  a  large  part  of  which 
is  magnetite,  although  it  is  nearer  the  slates  than  the  iron-belt  rocks.  At 
the  Black  river,  in  the  southern  part  of  Sec.  12,  T.  47  N.,  R.  46  W.,  Michi- 
gan, there  is  a  continuous  exposiu'e  of  200  feet  of  rock  at  the  horizon  in  which 
the  change  from  the  Iron-bearing  to  the  Upper  slate  member  occurs.  Here 
is  a  transition,  and  the  location  of  the  line  between  the  two  belts  is  more  or 
less  arbitrary.  Just  south  of  this  line,  although  the  specimens  contain  fi-ag- 
mental  material  in  considerable  quantity,  the  nonfragmental  material  is  pre- 
ponderant; and  at  the  southern  end  of  the  exposure,  150  feet  south  of  this 
line,  the  fragmental  material  is  still  tolerably  abundant.     North  of  the  line 


298  THE  PENOKBB  lEON-BBAUmG  SERIES. 

separating  the  formatious  the  exposure  is  strongly  clayey,  and  contains  a 
preponderating  amount  of  fragmental  material.  The  exposure  does  not 
continue  far  enough  northward  for  the  rock  to  become  completely  frag- 
mental. It  thus  appears  that  the  passage  from  the  nonfragmental  iron- 
bearing  rocks  to  the  fragmental  rocks  of  the  upper  slates  is  not  complete  in 
a  surface  distance  of  200  feet,  which,  at  a  dip  of  57°,  which  here  prevails, 
corresponds  to  a  thickness  of  rock  of  about  170  feet.  This  is  the  only 
locality  in  which  the  actual  change  from  one  belt  to  the  other  is  so  well 
exposed.  It  is  of  course  possible  that  in  other  localities  the  change  is  more 
abrupt,  although,  as  will  be  seen,  the  nature  of  the  rocks  at  the  base  of  the 
upper  slate  belts  are  such  as  to  render  it  probable  that  the  change  is  often 
a  transition. 

Geographical  distrihution. — The  westernmost  exposure  found  in  the  upper 
slates  is  near  the  center  of  Sec.  18,  T.  44  N.,  R.  3  W.,  Wisconsin,  while  the 
easternmost  exposure  is  at  Black  river,  Michigan,  Sec.  12,  T.  47  N.,  R.  46  W. 
Probably  the  rocks  of  the  member  extend  somewhat  beyond  these  points 
before  being  cut  off  l^y  the  Keweenaw  rocks.  At  the  western  extremity  of 
the  belt  the  rocks  on  the  map  are  carried  to  near  the  north  and  south 
quarter  line  of  Sec.  14,  T.  44  N.,  R.  4  W.,  Wisconsin.  Whether  they  ex- 
tend beyond  this  point  -  is  not  known.  At  the  east  end  of  the  belt  the 
Keweenaw  rocks  upon  the  map  are  represented  as  cutting  off  the  upper- 
belt  rocks  about  1^  miles  east  of  Black  river,  near  the  east  line  of  Sec.  7, 
T.  47  N.,  R.  45  W.,  Michigan.  While  it  is  not  certain  just  where  the 
Upper  slate  member  first  emerges  from  the  Keweenawan  eruptives  at  the 
west,  it  is  certain  that  the  belt  rapidly  widens  until  it  attains  a  great 
breadth  and  thickness.  At  a  short  distance  east  of  Penokee  gap  the  sur- 
face width  of  the  belt  is  about  If  miles,  or  nearly  9,240  feet.  The  dip  is 
here  about  70°.  This  surface  width  therefore  represents  a  thickness  of 
8,630  feet.  From  this  place  the  width  of  the  member  gradually  and  uni- 
formly increases,  until  at  the  east  line  of  Range  2  W.,  Wisconsin,  the  dis- 
tance across  the  belt  north  and  south  is  about  2  J  miles,  which  represents 
a  width  perpendicular  to  the  strike  of  the  rocks  of  about  2^  miles.  Through 
R.  1  W.,  Wisconsin,  this  width  is  about  the  average.  At  Tylers  fork  the 
width  reaches  its  maximum,  and  2  miles  east  of  Tylers  fork  falls  some- 


TUE  UPPEli  SLATE  MEMUEK.  2i»9 

what  below  its  averag-e  through  the  tovvushi]).  Tlu;  surface  width  at  Tylers 
fork  is  almost  2i  miles  (more  aocurately  isiihout  12,800  feet.)  Tlic  dip  at 
this  locality  is  from  70^^  to  80°  (upon  an  avcnigi*  it  may  be  taken  as  Tf)''), 
Mill  I  the  surface  width  then  represents  an  actual  thickness  of  rock  of  12,3()0 
i'vi'\.  Through  the  township  Iv.  1  W.,  Wisconsin,  the  average  thickness  of 
the  belt  is  not  far  from  11,000  feet,  and  nowhere  does  it  fall  below  10,000 
feet.  Though  R.  1  E.,  Wisconsin,  the  belt  gradually  and  quite  unifoi-mly 
narrows,  and  at  the  east  line  of  this  range  has  a  surface  width  of  about 
1§  miles,  or  7,260  feet.  As  the  dip  is  here  upon  an  average  close  to  80°, 
this  width  represents  a  thickness  of  about  7,150  feet.  This  width  and 
thickness  is  maintained  with  considerable  uniformity  through  R.  2  E., 
Wisconsin,  to  the  Montreal  river.  From  the  Montreal  river,  the  boundary 
between  Michigan  and  Wisconsin,  there  is  a  continuous  narrowing  of  the 
member  through  Ranges  47  W.  and  46  W.,  Michigan,  until  at  the  Black 
river  the  surface  width  is  but  1,650  feet,  and  as  the  dip  is  here  but  65°, 
this  represents  a  thickness  of  only  1,495  feet.  Here,  as  before  stated, 
are  the  last  exposures  of  this  formation,  and  a  short  distance  east  the 
whole  member  is  cut  out  by  the  Keweenawan  greenstones. 

As  the  rocks  of  the  Upper  slate.  Iron-bearing,  and  Qtiartz-slate  mem- 
bers are  entirely  conformable,  there  is  exposed  throughout  the  whole 
extent  of  the  rocks  of  the  upper  member  the  full  thickness  of  the  lower 
members  of  the  series.  However,  a  very  considerable  portion  of  the  area 
which  should  be  occupied  bj"  the  slates  of  the  upper  formation  is  taken  by 
the  Keweenaw  series.  It  is  not  certain,  even  at  Tyler's  fork,  where  the 
maximum  thickness  of  the  formation  is  exposed,  that  its  total  thickness  is 
really  shown.  If  at  Tylers  fork  we  have  the  full  thickness  of  this  mem- 
ber, the  surface  which  the  upper  formation  rocks  cover  in  the  area  in 
which  they  occur  at  all  is  about  one-half  of  the  area  which  they  would 
cover  had  they  not  been  removed  by  erosion  before  the  beginning  of 
Keweenaw  time;  for  at  Tylers  fork  the  member  gradually  and  quite  uni- 
formly nari'ows  both  east  and  west  until  it  disappears.  The  sui'face  then 
covered  by  this  member  may  be  roughly  compared  to  a  triangle,  the  area 
of  which  is  one-half  that  of  a  rectangle  of  the  same  base  and  altitude  which 
these  i:ocks  ^vould  extend  over  were  they  not  cut  off  by  the  Keweenaw 


300  THE  PENOKEE  IRON-BE  AKIN  G  SERIES. 

series.  When  it  is  considered  that  in  all  probability  this  upper  member- 
of  the  series  once  extended  east  and  west  as  far  as  the  lower  members,  the 
proportion  of  this  formation  that  is  removed  must  be  concluded  to  be  much 
greater  than  one-half 

Throughoiit  the  area  which  the  member  covers,  the  exposures,  with  the 
exception  of  a  few  localities,  are  not  particularly  numerous,  and  yet  a  suf- 
ficient number  haA'e  been  found  and  mapped  to  give  a  clear  idea  of  the 
nature  of  the  rocks  of  the  entire  area.  Nowhere  are  there  full  sections. 
At  Penokee  gap  the  exposures  are  most  numerous,  but  even  here  little  more 
than  one  quarter  of  its  total  thickness  is  uncovered.  At  Tylers  fork  the 
exposures  are  also  numerous,  but  here  not  more  than  one-fifth  or  one-sixth 
of  the  member  is  shown.  Tlie  conception  of  the  member  as  a  whole  is 
formed  b}^  uniting  and  correlating  the  results  obtained  from  many  detached 
exposiu'es. 

The  rocks  of  the  upper  formation  have  been  said  to  strike  across  the 
country  in  perfect  conformity  to  those  of  the  Iron-bearing  member  (Pis. 
V  to  xii).  In  the  vicinity  of  Penokee  gap  and  English  lake  the  rocks  of 
the  upper  belt  strike  nearly  east  and  west,  and  here  the  strikes  of  the  iron- 
bearing  belt  are  approximately  in  the  same  direction.  In  the  west  part  of 
R.  2  W.,  Wisconsin,  the  iron-bearing  formation  begins  to  take  a  northern 
trend  and  the  strata  of  the  upper  belt  conform  to  it.  Through  Ranges  1  W. 
and  1  E.,  Wisconsin,  the  iron-bearing  hoi'izon  has  a  strike  from  E.  25°  N. 
at  the  east  side  of  R.  1  W.  to  about  E.  30°  N.  at  the  east  side  of  R.  1  E., 
and  to  these  strikes  the  slates  above  almost  exactly  correspond.  In  R.  2 
E.,  Wisconsin,  and  R.  47  W.,  Michigan,  in  passing  eastward  there  is  a  grad- 
ual bowing  towards  a  more  easterly  course  in  the  rocks  of  the  iron  formation, 
and  the  upper-belt  rocks  also  show  a  corresponding  bowing;  and  finally, 
when  Black  river  is  reached,  on  the  east  side  of  T.  47  N.,  R.  46  W.,  Mich- 
igan, the  Iron-bearing  and  Upper  slate  members  both  strike  nearly  due 
east  and  west.  This  exact  con'espoudence  of  strike  between  the  two  mem- 
bers is  accompanied  by  as  noticeable  a  correspondence  in  their  dips,  as 
will  readily  be  seen  by  an  examination  of  the  plates  referred  to.  The 
foregoing  facts  could  hardly  be  explained  upon  any  other  hypothesis  than 
that  of  a  continuous  conformable  series,  which  has,  as  a  whole,  undergone 


THE  UPPER  SLATE  MEMliKK.  301 

the  same  process  of  tiltini>-,  and  wliicli  lias  ri'sult(Hl,  in  coinuictioii  with 
erosion,  in  ""ivinji^  the  series  its  present  surface  distribution. 

Topofiraphical  features. — Tlie  topof^raphy  of  the  Penokee  district  (Pis. 
VII,  IX,  and  xi)  shows  in  a  general  way  the  boundaries  of  the;  Upper  slate 
membex*.  The  ridge  known  as  the  Penokee  range  has  been  described  in 
detail,  pp.  145-146.  It  is  always  found  within  the  Quartz-slate  or  Iron- 
bearing  members,  or  in  the  granite  just  to  the  south.  North  of  the  ii'on 
formation  the  ground  slopes  rapidly  in  most  cases  to  a  nearly  level  plain, 
which  ordinarily  has  a  gentle  slope  northward,  as  is  shown  by  the  direction 
of  the  flow  of  the  streams  (PL  ii).  A  considerable  portion  of  the  ground 
is  of  a  swampy  nature,  and  the  part  not  a  swamp  is  largely  low,  rich, 
heavily .  timbered  and  thick  with  underbrush.  The  northern  boTindary  of 
the  Upper  slate  is  for  much  of  the  distance  defined  by  the  trap  range, 
which  constitutes  the  base  of  the  Keweenaw  series.  In  traveling  from  the 
iron  range  north  across  the  slate  belt  to  determine  its  northern  limit,  the 
approach  to  this  line  is  indicated  by  a  rise  in  the  ground.  Ascending  the 
swell  for  a  short  distance,  one  is  pretty  sure  to  find  the  eruptives  of  the 
Keweenaw  series,  and  oftentimes  in  the  form  of  a  series  of  bold  bluffs, 
which  are  a  prominent  topographic  feature  of  the  region.  In  general,  then, 
the  Upper  slate  member  lies  in  a  valley,  bounded  by  the  Penokee  range  on 
the  south  and  the  trap  range  on  the  north.  At  many  points  on  both  of 
these  ranges  the  valley  may  be  overlooked.  The  tree  tops  are  seen  stretch- 
ing as  an  almost  level  mass  of  forest  for  miles  both  east  and  west.  This 
forest  is  now  rapidly  disappearing  under  the  necessities  of  the  neighboring 
mines. 

Within  the  belt  the  exposures  are  usually  low  and  small.  This  results 
from  the  soft  character  of  the  slate.  It  is  to  be  presumed  that  the  judg- 
ment made  up  from  the  known  exposures,  conveys  a  somewhat  erroneous 
impression  with  reference  to  the  character  of  the  member  as  a  whole ;  that 
is,  the  rocks  which  have  a  certain  amount  of  the  more  resistant  massive 
graywacke  or  quartzite  are  more  apt  to  be  exposed  than  the  softer  slates. 
In  the  vicinity  and  a  short  distance  west  of  Bad  river  the  exposures  are 
quite  numerous ;  also  a  little  way  west  of  this  locality,  east  of  English  lake, 
.  there  are  again  frequent  exposures.     This  unusual  immber  is  here  probably 


302  THE  PENOKEE   lEON  BEARING  SERIES. 

due  to  the  fact  that  the  rock  in  large  measure  becomes  a  somewhat  resistant 
mica-schist,  which  is  interstratified  with  quartzites  and  massive  graywackes. 
East  of  this  locahty  the  exposures  are  not  numerous  until  Tylers  fork  is 
reached.  Here,  while  they  are  large,  they  are  in  a  good  measure  due  to  the 
cutting  action  of  the  river.  No  bluffs  and  hills  of  slate  are  found,  as  west 
of  Bad  river.  Numerous  large  exposures  are  in  the  south  part  of  Sec.  12, 
T.  45  N.,  R.  1  E.,  Wisconsin,  just  south  of  a  small  lake.  The  slates  and 
graywackes  here  are  the  north  slope  of  a  steep  bluff  which  rises  rapidly 
from  the  water  of  the  lake.  The  only  remaining  large  exposures  in  the  belt 
are  those  along  the  railroads  in  the  west  part  of  Sec.  28,  T.  46  N.,  R.  2  E., 
Wisconsin.  The  slates  and  graywackes  are  here  a  prominent  landmark, 
rising  in  a  bluff  of  some  size  in  the  valley  between  the  iron  and  trap 
ranges. 

General  petrograpMcal  character. — It  has  already  been  said  that  the 
rocks  of  the  member  under  consideration  are  distinctly  clastic,  or  at  least 
are  rocks  which  can  be  shown  to  have  been  once  fragmental.  While  they 
are  alike  in  this  fundamental  point  there  is  great  variety  in  minor  charac- 
teristics. The  varieties  may  be  grouped  under  the  heads:  (1)  Mica-schists 
and  mica-slates;  (2)  Grayivackes  and  graywache-slates;  (3)  Clay-slates  or 
phyllites;  (4)  Quartzites  and  conglomerates.  Each  of  these  main  types  has 
the  various  phases  shown  by  the  following  tabulation: 

'  Muscovitic. 


Mica-schist  and  mica-slate. 


Micaceous i.  Biotitic. 

1,  Muscovitic  and  biotitic. 

\  Cliloritic  and  biotitic. 

Micaceous  and  chloritic < 

>  Cnlontic  and  sericitic  or  muscovitic. 

•i  Biotitic. 
Micaceous < 

(  Biotitic  and  muscovitic. 


Graywacke  and  graywacke-slate. 


Micaceous  and  chloritic Cliloritic  and  biotitic. 

(Cliloritic. 
Magnetitic  and  chloritic. 
Ferruginous  and  chloritic. 
\  Chloritic. 
J  Chloritic  and  magnetitic. 

V  Clayey. 


Quartzite  and  conglomerate 


'  Feldspathic, 


TllK  Ul'PKK  SLATK  M  KM  HER.  303 

In  the  subsequent  (letiiiled  (lesc'ri[)tioiis  of  tlii^  rocks  of  this  ineiiiber 
they  'AYG  grouped  into  several  ureas  geographically.  '^Phe  (listriljution  cor- 
responds in  a  loose  way  to  the  classification  of  the  rocks  ahove  given.  vVt 
the  west  end  of  the  member  the  mica-schists  and  mica-slates  are  the  only 
rocks  found.  At  Bad  river  and  vicinity  these  rocks  greatly  predominate. 
In  i)assing  eastwai'd  micacecnis  graywackes  and  graywacke-slates,  chloritic 
graywackes  and  graywacke-slates,  and  clay-slates  are  each  in  turn  the  pre- 
ponderating rocks.  In  the  transition  from  one  type  of  rock  to  the  next 
one  there  are  cross  sections  in  which  two  or  more  of  these  kinds  may 
be  found. 

Before  passing  to  the  petrographical  characterization  of  the  types  of 
rocks  of  this  group,  it  will  perhaps  be  well  to  give  the  kinds  of  rocks  foimd 
in  the  geographical  divisions  as  given  in  the  tabulations.  All  the  rocks  of 
the  English  lake,  Penokee  gap,  and  Mellan  junction  sections,  and  also  the 
exposure  in  Sec.  18,  T.  44  N.,  R.  3  W.,  Wisconsin,  have  as  their  chief  con- 
stituents quartz,  feldspar,  and  mica.  The  rocks  in  Sec.  1 8  and  those  in  the 
English  lake  section  are  all  mica-schists  and  mica-slates.  At  Penokee  gap 
are  some  graywackes  and  thin  belts  of  quartzite,  but  both  are  micaceous. 
At  the  Tylers  fork  section  the  mass  of  the  rocks  are  graywackes  and 
graywacke-slates.  However,  in  exposure  all  show  lamination  with  suffi- 
cient plainness  to  enable  one  to  readily  determine  strike  and  dip.  The 
difference,  then,  between  graywackes  and  graywacke-slates  is  of  minor 
importance,  and  is  used  only  for  convenience  to  show  relative  degrees  of 
massiveness.  The  rocks  at  the  section  between  Tylers  fork  and  Potato 
river  and  at  the  section  in  the  Adcinity  of  Potato  river  differ  in  no  respect 
from  those  at  the  Tylers  fork  section,  with  the  exception  that  biotite  is  less 
and  chlorite  more  plentiful.  Also,  at  one  exposure  in  the  extreme  northern 
part  of  the  member  are  found  some  conglomerates  and  quartzites,  which 
will  be  further  referred  to.  The  rocks  in  the  section  located  in  the  north 
part  of  T.  45  N.,  R.  1  E.,  and  along  the  west  range  line  of  R.  2  E., 
Wisconsin,  are  mostly  chloritic  graywackes,  although  there  are  also 
found  magnetitic  clay-slates.  The  rocks,  then,  differ  from  those  found 
at  Tylers   foKk    and   Potato   river    in   that   biotite  has   almost   entirely 


304  THE  PBNOKEE  lEON  BEARING  SERIES. 

disappeared  and  chlorite  has  become  the  predominant  mineral  aside 
from  the  quartz  and  feldspar.  At  the  extreme  •  east  end  of  this  area 
appear  the  first  exposures  found  of  the  clay-slates.  These  clay-slates 
or  phyllites  differ  chiefly  from  the  finer  grained  graywacke  slates,  into 
which  they  grade  apparently  oidy  in  that  a  portion  of  their  mineral  con- 
stituents are  so  fine  grained  as  to  make  exact  determinations  impossible  in 
many  cases.  The  rocks  in  the  section  between  the  east  range  line  of 
R.  1  E.  and  West  branch  of  the  Montreal,  the  section  between  the  west 
branch  of  the  Montreal  and  Montreal  river,  and  "the  section  at  the  Montreal 
river  and  vicinity  extend  over  a  distance  east  and  west  of  about  6  miles. 
The  westernmost  exposure  placed  in  these  areas  is  about  3  miles  west 
of  the  west  branch  of  the  Montreal,  while  the  easternmost  exposure  is  just 
east  of  the  Montreal  river.  In  this  locality  the  exposm-es  are  more  numer- 
ous than  in  any  of  the  previous  ones  except  those  at  Bad  river  and  Tylers 
fork.  Also,  some  of  the  exposures  are  of  large  size.  The  rocks  here 
included  are  wholly  graywackes  and  graywacke-slates,  which  are  always 
chloritic.  Only  in  one  or  two  of  them  is  any  biotite  found,  and 
then  only  in  subordinate  quantity.  Some  of  them  are  ferruginous  in 
appearance.  The  slates  differ  but  little  from  those  of  the  Potato  river  and 
Tylers  fork  sections.  The  areas  west  of  Black  river  and  at  Black  river 
include,  with  a  single  exception,  all  the  exposures  found  in  Michigan. 
They  are  widely  scattered,  few  in  number,  and  are,  with  one  exception, 
small.  While  graywackes  and  graywacke-slates  are  found  among  them, 
they  are  very  fine  grained  and  approach  closely  to  clay-slates,  of  which 
the  greater  number  of  the  exposures  are  composed.  Mica  is  found  only  in 
a  single  exposure  and  in  subordinate  quantity. 

Petrographical  characters  of  the  four  types  of  rock. — In  giving  a  charac- 
terization of  the  rocks  of  the  Upper-slate  member  the  following  order  will 
be  followed:  (1)  The  quartzites  and  conglomerates;  (2)  The  clay-slates  or 
phyllites;  (3)  The  graywackes  and  graywacke-slates;  (4)  The  mica-slates 
and  mica-schists.  This  order  is  that  of  alteration,  the  first  mentioned  being 
nearest  its  original  condition  of  deposition.  By  following  this  order  the 
sections  will  in  the  main  come  from  exposures  in  a  direction  from  east  to 
west. 


THE   III'PKU  SLATK  MEMBEK,    .  305 

Tlie  only  expusui-c  of  tiaartzite  and  confjlomerate  of  any  inii<ruitu<le  is  in 
Sec.  il,  T.  45  N.,  R.  1  W.,  Wisconsin.  At  this  place  the  rocks  of  the 
Copper  series  He  innuediately  to  the  northward,  there  being'  between  the 
quartzites  and  greenstones  an  interval  of  bnt  a  few  feet.  The  rocks  of  tliis 
exi)osnre,  described  in  the  tabulations,  are  feldspathic.  They  are  notable  in 
shovvinji'  nicely  the  alteration  of  fragniental  feldspar  to  both  nmscovite  and 
biotite  with  a  simultaneous  development  of  quartz,  which  is  thus  inclosed  in 
the  feldspar  and  nungled  with  the  other  secondary  products.  (PI.  xxxii,  Fig. 
1.)  The  rather  unusual  alteration  of  feldspar  to  actinolite  is  also  here  illus- 
trated. This  actinolite  occurs  in  the  enlargements  of  the  quartz  grains,  but 
not  in  their  cores,  thus  showing  that  it  has  formed  subsequently  to  the 
de})Ositiou  of  the  fragments  of  the  rock. 

The  clay-slates  or  phijllites  are  not  numerous,  being  found  in  but  a  few 
localities  in  the  eastern  ^Jart  of  the  Upper-slate  member.     It  is  probable 
that  these  rocks  make  up  a  greater  part  of  the  upper  belt  than  would  be 
inferred  from  the  few  exposures,  for  they  are  so  soft  that  they  would  rarely 
outcrop  even  if  widespread.     The  known  exposurea  of  clay-slates  are  in 
Sees.  11,  12,  and  17,  T.  47  N.,  R.  46  W.,  Michigan;  Sec.  13,  T.  47  N.,  R. 
47  W.,  Michigan;  and  near  the  line  between  Sec.  1,  T.  45  N.,  R.  1  E.,  and 
Sec.  6,  T.  45  N.,  R.  2  E.,  Wisconsin.     Among  the  clay-slates  are  jjlaced 
only  rocks,  which  are  excessively  fine  grained,  the  particles  that  comjjose 
them  being  so  minute  as  t(>  make  the  determination  of  their  mineral  com- 
position in  part  uncertain.     A  portion  of  each  thin  section  is  sufficiently 
coarse  grained  to  certainly  show  that   it   contains  small  fragmental  yax- 
ticles  of  quartz  and  feldspar.     It  can  also  be  seen  that  it  contains  much 
chlorite,  kaolin  or  sericite,  finely  crystalline  quartz,  and  iron  oxides.     As 
the  minerals  thus  known  to  be  present  are  the  same  that  ordinarily  con- 
stitute clays,  and  as  they  are  those  whicli  compose  the  graywackes  and 
graywacke-slates  with  which  the  clay-slates  are  closely  associated,  there  is 
little  doubt  but  that  the  greater  number  of  them  are  thus  chiefly  consti- 
tuted.    One  of  the  slates  contains  a  small  quantity  of  biotite.     The  iron 
oxide  in  the  slates  along  the  line  of  Sec.  1,  T.  45  N.,  R.  1  E.,  and  Sec.  6,  T. 
45  N.,  R.  2  E.,  Wisconsin,  is  so  abundant  as  to  be  one  of  the  chief  constit- 
uents of  the  slate.     This  iron  oxide  is  very  largely  magnetite  in  minute 

MON  XIX 20 


306 


THE  PENOKEK  IRON-BEAEING  SERIES. 


crystals.  One  of  the  slates  has  a  peculiar  mottled  appearance  when 
viewed  under  the  microscope,  resembling  that  presented  by  raindrops  on 
shale,  but  the  spots  are  indefinitely  smaller.  They  seem  to  be  due  to  the 
relative  proportions  of  the  mhierals  which  compose  them.  This  an-ange- 
ment  may  be  caused  by  the  decomposition  of  feldspar,  each  one  of  the 
hghter  spots  perhaps  representing  a  rounded  fragment  of  that  mineral. 

In  order  to  reenforce  the  microscopical  determination  of  the  minerals  in 
these  obscure  slates,  analyses  of  two  of  the  raagnetitic  ones  were  made  in 
the  U.  S.  Geological  Survey  laboratory  by  Mr.  L.  G.  Eakins,  with  the  result 
of  confirming  fully  the  observations  made.  The  first  specimen  is  from  NW. 
J  Sec.  6,  T.  45  N.,  R.  2  E.,  Wisconsin;  the  second  from  NE.  J  Sec.  1,  T.  45 
N.,  R.  1  E.,  Wisconsin.  The  amount  of  magnetite  would  appear  in  both 
cases  to  be  as  much  as  15  per  cent. 

*■  Analyses  of  slates. 


SiOj 

AI2O3 

Fe^Os 

TeO 

MnO 

CaO 

MgO 

KjO 

NojO 

U-fi 

H2O 

P.O5 

Total 


53-44 

19-62 

11-118 

5-35 

trace 

-42 

1-58 

1-73 

2-61 

trace. 

4-07 

trace. 


100-20 


n. 


52-58 

20-76 

12-17 

408 

-21 

•30 

1-33 

4-87 

-37 

trace. 

3-43 


100-10 


The  graywackes  and  graywacke-slates  cover  much  the  largest  territory 
of  any  class  in  the  Upper  slate  member.  The  graywackes^  have  always  as 
chief  constituents    fragmeutal  quartz  and  feldspar.     The  strength  of  the 


'  The  term  gray  wacke  is  here  used  in  a  lithological  sense,  in  accordance  witli  the  deiiuitiou  of  the 
term  given  by  Gcikie,  Text  Book  of  Geology,  2d  ed.,  p.  162:  "A  compact  aggregate  of  rounded  or  sub- 
angular  grains  of  quartz,  feldspar,  slate,  or  other  minerals;  or  rocks,  cemented  by  a  paste,  whicli  is 
usually  siliceous,  but  may  be  argillaceous,  fcldspathic,  calcareous,  or  anthracitic.  Gray,  as  its  name 
denotes,  is  its  prevailing  color;  but  it  passes  into  browu,  brownish  purple,  and  sometimes,  -where 
anthracite  predominates,  into  black.  The  rock  is  distinguished  from  ordinary  sandstone  by  its  darker 
hue,  its  hardness,  the  variety  of  its  component  graius,  and  above  all  by  the  compact  cement  in  -which 
the  grains  are  embedded," 


THE  UPPER  SLATE  MEMHKlt.  307 

roi'k  i.s  usually  <>-ivcui  hy  "siliruous  [lasU;,"  althouj^li  laucli  of  thu  cuiuoiit  is 
trequeutly  hi  the  t'orui  of  (|uartz  added  to  the  rounded  fraynieutal  grains. 
There  is  frequently  "  arg-iUaceous  aud  calcareous"  and  occasionally  car- 
bonaceous material  present  as  accessories.  Tliere  is  also  found  in  all  of 
these  graywackes  abundant  chlorite  or  mica,  or  both.  The  mica  is  chiefly 
biotite,  although  muscovite  or  sericite,  or  both,  are  plentiful.  In  impor- 
tance, these  minerals  are  secondary  only  to  quartz  and  feldspar.  The  class 
of  rocks  in  this  district — graywackes  and  graywacke-slates — has  been 
divided  into  three  phases — micaceous,  micaceous  and  chloritic,  and  chlo- 
ritic.  These  divisions  have  a  geographical  significance  as  well  as  a  litho- 
logical  one.  All  of  the  graywackes  and  graywacke-slates  in  Michigan  and 
Wisconsin  as  far  west  as  the-  Potato  river  are  chloritic ;  the  Potato  river 
rocks  are  mostly  micaceous  and  chloritic,  these  two  minerals  being  in  about 
equal  quantity ;  about  half  of  the  rocks  in  the  vicinity  of  Tylers  fork  are 
both  chloritic  and  micaceous ;  the  remaining  half  micaceous  only;  west  of 
Tylers  fork  and  vicinity  tlie  graywackes  and  graywacke-slates  are  mica- 
ceous oidy.  We  have  thus  a  series  of  graywackes  which  at  the  east  end 
are  wholly  chloritic ;  in  passing  to  the  west  mica  appears  with  the  chlorite, 
becomes  more  and  more  plentiful,  and  ultimately  entirely  replaces  the  chlo- 
rite. How  gradual  and  complete  this  transition  is  will  be  readily  seen  by 
turning  over  the  tabulations  of  these  rocks  on  subsequent  pages.  The 
graywackes  and  graywacke-slates  are  so  closely  associated,  both  strati- 
graphically  and  lithologically,  with  the  mica-slates  and  mica-schists,  that  a 
more  detailed  characterization  of  them  will  be  deferred  until  the  general 
character  and  distribution  of  the  latter  are  given. 

The  mica  slates  and  mica-schists^  (Pis.  xxxiii  and  xxxiv)  of  the  district 
have  always  as  a  chief  constituent  quartz,  and  usually  tliis  mineral  is  the 
most  abundant  of  all.  Feldspar,  as  in  the  graywackes,  is  almost  always 
present,  but  its  quantity  is  much  less,  and  in  the  typical  slates  and  schists 
is  almost  wholly  absent.  Biotite,  musco-si's,  sericite,  and  rarely  chlorite, 
or  two  or  more  of  these  minerals,  are  always  plentiful,  the  individuals 
generally  being  in  well  defined  folia.     The  accessories  present  are  the  same 

'  Although  in  one  place  chlorite  Is,  aside  from  the  feldspar,  the  chief  c.oustituent,  they  will  all 
be  called  niicft-schists  aud  mica-alates,  to  distinguish  tliein  from  the  graywackes  and  graywacke-slates. 


308  ^  THE  PElN'OKEE  lEON-BEAEING  SEEIES. 

as  in  the  graywackes — ferrite,  pyrite,  some  cai'bouate,  and  rarely  carbona- 
ceous matter.  As  biotite  is  usually  the  prevalent  mica,  the  slates  and 
schists  have  as  a  whole  a  darker  color  than  the  graywackes,  ranging  from 
very  dark  gray  to  black.  The  fundamental  differences,  however,  which 
separate  them  from  the  graywackes  are  their  crystalline  appearance  and 
absence  of  feldspar.  Very  many  of  them  when  viewed  under  the  micro- 
scope, unless  closely  examined,  taken  by  themselves,  show  no  trace  of 
a  clastic  origin ;  although  the  greater  number,  upon  closer  examination, 
are'  seen  to  have  something  of  a  fragmental  character.  The  distinction 
made  between  mica-schists  and  mica-slates  is  based  upon  structure  and 
coarseness  of  grain.  Most  of  the  coarser  grained  upper  member  mica  rocks 
have  a  well  developed  schistose  structure  parallel  to  the  bedding  of  the 
rock — a  structure  which  could  fairly  be  called  foliated  in  some  places.  They 
never  become  very  thinly  foliated,  or  contorted  with  a  brilliant  sheen  upon 
the  foliation  surface,  as  do  some  of  the  most  crystalline  mica-schists.  A 
few  exposures  have  a  dark  and  light  banded  appearance,  suggestive  of  a 
fine  grained  gneiss,  which  in  fact  they  are  when  the  feldspar  is  a  chief  con- 
stituent. The  rocks  called  mica-slates  are  generally  finer  grained  than  the 
mica-schists ;  they  cleave  with  a .  very  smooth  slate-like  parting  parallel  to 
the  bedding.  Many  of  them  have  a  black  color,  due  to  abundant  dark 
biotite,  particles  of  ferrite,  and  in  some  places  to  carbonaceous  material. 
Like  the  graywackes  and  graywacke-slates,  the  mica-slates  and  mica-schists 
have  been  divided  into  three  divisions :  those  in  which  chlorite  is  the  chief 
constituent  aside  from  the  quartz  and  feldspar ;  those  in  which  mica  takes 
this  place ;  and  those  in  which  both  chlorite  and  mica  are  abundant.  The 
first  division  is  represented  by  but  a  single  exposure.  The  chloritic  and 
micaceous  schists  and  slates  are  quite  j)lentiful,  but  by  far  tlie  greater  num- 
ber of  the  slates  and  schists  are  micaceous  only.  As  with  the  graywackes 
and  graywacke-slates,  tlieir  classification  corresponds  to  geographical  dis- 
tribution, and  the  distribution  is  of  the  same  sort  as  that  of  the  graywackes 
and  graywacke-slates.  The  one  exposure  of  sericitic  chlorite-schist  is  but 
a  short  distance  west  of  the  Montreal  river,  in  Sec.  14,  .T.  46  N.,  R.  2  E., 
Wisconsin.  In  passing  to  the  west,  the  mica-slates  at  the  Potato  river  first 
appear.     At  this  locality  only  one  exposure  is  known  which  falls  under 


TIIH   riM'EK  SLATE  iMK.Ml'.KH.  309 

this  lieiul.  At  Tylers  fork  iiinl  \iciiiity  tlicrc  nrc  quite  a  miniber  of  expo- 
sures of  cliloritic  and  liiotitic  slates  jind  hut  a  single  one  of  pure  cliloritie, 
shites.  West  of  Tylers  fork  the  schists  jiud  slates  are  almost  wlioll}-  luiea- 
ceous,  although  chlorito  is  yet  occasionally  found.  There  are,  then,  precisely 
the  same  stratigraphical  relations  with  reference  to  chlorite  and  mica  in  the 
slates  and  schists  that  there  are  in  the  graywackes  and  graywacke-slates. 
The  two  classes  of  rocks  are  also  interlaininated  with  each  other.  In  Sec. 
14,  T.  46  N.,  R.  2  E.,  a  chlorite-schist  is  but  a  short  distance  from  an  expo- 
sin-e  of  typical  graywacke.  At  Potato  river  again  the  one  biotite-slate 
there  found  is  interstratified  with  graywackes  and  graywacke-slates.  At 
Tylers  fork  the  graywackes  and  graywacke-slates  and  the  mica-schists  and 
mica-slates  are  interstratified  in  the  most  intimate  manner,  both  rocks  occur- 
ring at  times  in  the  same  exposure.  Beginning  at  the  top.  of  the  Upper 
slate  member,  the  order  of  succession  of  the  different  phases  of  rock  as  here 
found,  as  taken  from  the  tabulations,  are  biotite-slate,  chloritic  graywacke, 
chloritic  biotite-slate,  biotitic  graywacke,  biotitic  graywacke-slate,  chloritic 
graywacke-slate,  chloritic  graywacke.  At  Bad  river,  the  order,  beginning 
at  the  top,  is  as  follows:  mica-schists,  micaceous  graywacke,  biotite-schists, 
micaceovis  graywacke,  mica-slates.  West  of  Bad  river,  in  the  vicinity  of 
English  lake,  mica-schists  and  mica-slates  only  are  found. 

TABULATION  OF  PETRO GRAPHICAL  OBSERVATIONS. 

Exposure  in  Sec.  18,  T.  44  If.,  E.  3  W.,  Wisconsin. 

1.  Biotite-slate.  Specimen  167  Wr.,  from  1,000  N.,  1,250  W.,  Sec.  18,  T.  44  N., 
E.  3  W.,  Wisconsin.! 

The  rock  contains,  in  an  aplianitic  background,  very  numerous  small  crystal  sur- 
faces which  feebly  reflect  the  light,  is  black,  and  cleaves  somewhat  irregularly  parallel 
to  the  bedding. 

The  thin  section  is  composed  of  a  finely  crystalline  background  and  individuals 
and  clusters  of  individuals  of  feldspar,  in  about  equal  proportion.  The  background 
consists  of  finely  crystalline  quartz,  small  brown  folia  of  biotite,  very  numerous  black 
particles  of  ferrite  or  biotite,  or  both,  and  probably  also  some  carbonaceous  material. 

'  The  numbers  of  specimens  .and  slides  are  usually  those  of  the  collection  of  the  lake  Superior 
division.     Specimens  with  Wr.  after  the  numbers  are  from  the  collection  of  the  late  Mr.  Charles  E. 
Wright.     Specimens  with  Wis.  after  the  numbers  are  from  the  collection  of  the  Wisconsin  Geological  - 
Survey.     Locations  arc  given  from  the  southeast  corner  of  the  section  in  steps  of  2,000  per  mile. 


310  THE  PENOKEE  IRON-BEAlimG  SERIES 

The  feldspar  areas  are  rounded  and  are  decomposed  to  a  greater  or  less  extent,  the 
secondary  products  being  biotite  and  quartz.  This  alteration  is  more  extensive  upon 
the  outside  than  in  the  interiors  of  the  particles.    These  feldspars  also  include  ferrite. 

From  the  section  at  east  end  of  English  lake. 

2.  Biotite-slates  from  a  middle  horizon.  Specimen  126  Wr.,  from  1,300  N.,  200 
W.,  and  specimen  128  Wr.,  from  1,000  IS.,  GOO  W.,  Sec.  9,  T.  44  N.,  E.  3  W.,  Wisconsin. 

These  rocks  are  dark  gray  to  black,  flue  grained,  quite  massive,  mottled,  the 
mottling  being  due  to  numerous  small  cleavage  surfaces. 

In  the  thin  section  the  cleavage  areas  are  seen  to  be  well  rounded,  altered  grains 
of  feldsijar.  They  are  set  in  the  grouudmass,  which  consists  of  intimately  mingled 
small  grains  of  quartz  and  small,  brown  jparticles  of  biotite,  with  a  considerable 
quantity  of  ferrite.  The  partial  decomposition  of  the  clastic  feldspars  has  resulted 
in  the  formation  of  very  numerous  small  folia  of  biotite  and  a  few  larger  ones  of 
muscovite,  the  transition  to  these  minerals  being  beautifully  shown.  In  places  also 
the  feldspar  is  replaced  by  saturating  quartz.  In  each  of  the  feldspar  areas  the 
secondary  biotite  and  muscovite  are  found  most  plentifully  at  or  near  the  exterior  ol 
the  grain,  although  in  almost  every  case  the  alteration  has  proceeded  in  a  greater  or 
less  degree  quite  to  the  center.  In  the  matrix  it  is  impossible  to  determine  which 
part,  if  any,  of  the  quartz  is  fragmental.  The  biotite  of  the  matrix  is  precisely 
like  that  found  in  the  feldspar;  it  is  all  deep  brown,  very  strongly  dichroic,  and 
therefore  probably  bears  a  large  percentage  of  iron.  Tliis  biotite  has  doubtless  been 
furnished  its  iron  by  the  abundant  oxide  present.  The  peculiar  spotted  appearance 
of  the  section  when  held  up  to  the  light,  taken  in  connection  with  its  appearance 
under  the  microscope,  gives  a  clear  idea  of  the  manner  in  which  the  rock  reached 
its  present  condition.     (PI.  xxxiii.  Figs.  2, 3,  and  4.) 

3.  Muscovitic  biotite-schist  from  an  iipper  middle  horizon.  Specimen  73,  Wis. 
(slkle  24),  from  south  shore  of  English  lake,  Sec.  9,  T.  44  IST.,  E.  3  W.,  Wisconsin. 

The  thin  section  is  a  rather  fine  grained,  apparently  completely  crystalline, 
typical  mica-schist.  The  groundmass  consists  chiefly  of  quartz,  mingled  with  which 
is  feldspar,  both  orthoclase  and  plagioclase.  Biotite  in  rather  small  fine  folia  of 
uniform  size  is  very  plentiful ;  muscovite  is  much  less  abundant.  That  all  the  mica 
is  a  secondary  alteration  of  feldspar  can  not  be  proven,  but  a  portion  of  it  is  certainly 
of  this  nature.  Many  grains  of  feldspar  are  partly  surrounded  and  cut  by  folia  of 
mica,  while  many  of  the  larger  particles  of  feldspar  contain  numerous  flakes  of  mica, 
which  in  magnitude  and  appearance  are  precisely  like  the  great  mass  of  the  mica  in 
the  section.  Quite  numerous  black  grains  and  crystals  of  a  mineral  which  is  taken 
to  be  pyrite  are  included  alike  in  the  quartz,  feldspar,  and  mica. 

4.  Miiscovitic  biotite- schists  from  an  upper  horizon.  Specimen  78  Wis.  (slide 
26),  specimen  79  Wis.  (sUde  27^,  from  SW.  I  of  Sec.  4,  T.  44  N.,  E.  3  W.,  Wisconsin. 


T 1 1 1:  UPPER  SLATE  M EM BEH.  3 1 1 

Tlipse  sections  arc  like  .'{,  excopl  tliut  Miey  are  somewhat  (uiarser  giained  iiiul 
contain  more  ninsc((vite  and  apijroacli  nearer  (especially  Klide  27)  to  a  typical  mica- 
schist. 

5.  Riotite-acliist,  IVoiii  an  upper  horizon.  Specimen  IRJ  Wr.,  335  N.,  1,050  W., 
Sec.  4,  T.  44  N.,  K.  :i  W.,  Wisconsin.  ;; 

The  rock  is  dark  gray,  of  a  rather  fine,  nniform  grain,  finely  laminated,  yet  so 
compact  as  to  break  quite  readily  aca-oss  the  plane  of  lamination.  The  rather  large 
black  abundant  flakes  of  mica  give  the  specimen  tlie  appearance  of  a  typical  mica- 
schist. 

The  thin  section  shows  this  I'ock  to  be  a  mica-schist.  It  has  an  interlocking 
quartzose  background,  mingled  with  which  in  subordinate  (piantity  are  both  ortho- 
clase  and  plagioclase.  Contained  in  this  background  is  nuich  biotite  in  tolerably 
wide,  long,  well  defined  blades,  which  cut  through  both  ((uartz  and  feldspar.  No 
grains  of  quartz  are  found  which  are  plainly  enlarged.  This  rock  is  the  most  com- 
pletely-crystalline of  any  mica-schist  in  the  formation.  It  contains  the  merest  trace 
of  old  feldspar  grains,  no  quartz  which  can  be  shown  to  be  fragmental,  and  a  consid- 
erable quantity  of  fresh  feldspar.  In  short,  if  this  schist  has  formed,  as  from  its  posi- 
tion we  are  obliged  to  bglieve,  from  a  fragmental  rock,  no  trace  of  the  alteration  is 
discoverable.  The  fresh  feldspars  in  all  probability,  in  this,  as  in  other  cases  where 
they  are  present,  are  of  secondary  origin,  not  original  fi-agmental  particles.  (PL 
XXXIV,  Fig.  2.) 

6.  Biotite-schist,  from  an  uppermost  horizon.  Specimen  154  Wr.,  500  N.,  1,000 
W.,  Sec.  4,  T.  44  N.,  R.  3  W.,  Wisconsin. 

This  rock  differs  only  from  153  Wr.  in  that  it  is  finer  grained. 

In  section  the  chief  constituents  are  quartz,  biotite,  and  feldspar,  the  first  being 
preponderant,  and  the  last  comprising  both  orthoclase  and  plagioclase.  The  section 
contains  also  quite  a  quantity  of  black  ox^aque  material.  Many  of  the  grains  of 
quartz  have  undergone  secondary  enlargement.  That  all  the  biotite  is  secondary  is 
probable,  although  only  a  portion  of  it  can  be  shown  to  be  of  this  nature. 

From  the  section  at  Bad  river  and  vicinity. 

7.  Garnetiferous  biotite- slates,  from  west  side  of  fault  at  base  of  formation. 
Specimen  9552  (slide  3187),  0  K,  1,800  W.;  Specimen  9554  (slide  3189),  0  N.,  1,750 
W.,  Sec.  11,  T.  44  jST.,  R.  3  W.,  Wisconsin. 

These  rocks  are  of  a  reddish  or  greenish  black  color,  aphanitic,  finely  laminated, 
and  readily  cleave  along  the  plane  of  lamination.  The  cleavage  surfaces  are  lustrous 
and  covered  by  many  small  protuberances,  which  are  taken  to  be  due  to  contained 
garnets. 

The  sections  are  mostly  composed  of  small  flakes  of  biotite  with  particles  of  a 
black  substance  set  in  a  fine  quartzose  groundmass.     The  quartz  is  in  fine  and  closely 


312  THE  PBNOKEE  IRON  BEAlilNG  SERIES. 

fitting  grains.  Whether  any  of  it  is  fragmental  is  difficult  to  say.  A  few  grains  are 
found  which  are  larger  than  the  remainder  of  the  mineral,  and  these  are  clear  and 
appear  to  be  clastic.  Biotite  in  very  small  flakes,  with  the  greatest  diameters  ordi- 
narily in  a  common  direction,  compose  fully  one-half  of  the  section.  Mingled  with 
this  biotite,  and  giving  the  rock  its  color,  are  large  quantities  of  the  black  or  very 
brown  material  before  mentioned.  This  is  not  magnetite,  but  appears  to  be  a  very 
dark  colored  ferrite,  or  such  ferrite  mingled  with  pyrite  or  carbonaceous  material,  or 
both.  In  each  section  there  chftnces  to  be  but  one  or  two  garnets.  Included  in  each 
of  the  garnets"  are  all  of  the  remaining  constituents.  In  the  main  the  garnets  are 
not  crystal  outlined,  but  in  places  they  are,  and  here  biotite  blades  often  abut  sharply 
against  the  garnets  and  abruptly  terminate.  These  rocks  are  plainly  intermediate 
between  the  Iron-bearing  and  Upper-slate  members. 

8.  Biotite-slate,  from  east  side  of  fault  and  near  base  of  formation.  Specimei\ 
9568  (slides  4497  and  4498),  1700  N.,  1000  W.,  Sec.  14,  T.  44  N.,  R.  3  W.,  Wisconsin. 

This  rock  is  black,  very  fine  grained,  massive,  breaks  with  hackly  subconchoidal 
fracture,  contains  many  grains  of  colorless  and  flesh-colored  feldspar,  which  upon  the 
broken  surface  of  the  rock  show  well  developed  cleavage  planes. 

The  thin  sections  are  composed  of  comparatively  coarse  fragmental  particles  of 
feldspar  set  in  a  much  more  abundant  matrix,  consisting  largely  of  biotite,  quartz,  and 
feldspar.  The  larger  fragments  are  mostly  more  than  1  mm.  in  diameter.  They  vary 
in  magnitude  from  this  to  those  so  fine  as  to  be  lost  in  the  matrix,  their  numbers 
increasing  as  their  magnitudes  diminish.  These  sections  differ  from  any  of  the  other 
biotite-schists  and  biotite-slates  in  the  remarkable  freshness  of  the  larger  feldspars. 
They  are  in  their  interiors  generally  clouded  but  slightly,  and  the  plagioclase  gives 
sharp  twinning  bands.  The  gTeater  part  of  the  feldspar  is,  however,  orthoclase.  The 
smaller  feldspars  have  all  largely  and  some  of  them  wholly  altered  to  biotite  and 
quartz,  and  even  the  larger  feldspars  are  often  affected  more  or  less  deeply  upon  their 
exteriors  by  this  biotitic  decomposition,  and  sometimes  complex  areas  of  biotite  are 
found  quite  a  distance  in  the  feldspars.  Every  stage  of  this  change  is  seen  from  per- 
fectly fresh  unaffected  feldspar  to  that  in  which  but  a  trace  of  feldspathic  material 
remains  in  a  fine  aggregate  of  biotite  and  quartz.  The  amount  of  fragmental  quartz 
is  very  small,  not  more  than  from  a  tenth  to  a  twentieth  as  plentiful  as  the  feldspar. 
The  biotite  is  in  small  dark  brown  folia  intimately  -mingled  with  finely  crystalline 
quartz,  and  all  the  particles  of  these  two  minerals  have  Avithout  doubt  formed  by 
the  decomposition  of  the  feldspar.  The  absence  in  many  cases  of  large  fragmental 
grains  of  quartz  in  the  more  altered  mica-schists  and  mica-slates  is  explained  by  this 
section.  Here  are  abundant  fresh  large  fragmental  feldspars  and  but  few  small  frag- 
mental quartzes.  It  is  plain  that  this  rock  was  once  mainly  a  feldspathic  sediment,  the 
metasomatic  alterations  of  which  have  formed  a  dark  colored  mica-slate.  Doubtless 
the  greater  mass  of  the  fragmental  material  was  finer  grained  than  the  unaltered 


TlllO  i;iM'i:i.'  SLATK  jMKMIiKi;.  313 

largo  grains  of  feldspar.  It,  would  simmu  from  (liis  socrtioti  that  tliP  fin(>  grained  niica- 
scliists  in  wliicli  tliii  feldspar  is  deeouiposed  to  a  inncrli  greater  extent  than  in  tiiis 
rock,  and  which  contain  no  hirge  fragments  of  quartz,  must  originally  have  been  I'eld- 
spathic  sediments. 

it.  Bhiick  biotitie  slate,  from  east  side  of  fault  and  near  base  of  formation.  Speci- 
men itr)(i!)  (slide  .'5378),  from  17UU  N.,  lOUO  W.,  Sec.  14,  T.  44  N.,  1!.  ,".  W.,  Wisconsin. 

A  black  fine  grained  coTupact  rock,  which  breaks  with  a  conchoidal  fracture. 

Very  small  clastic  particles  of  (piartz  and  ferdsi)a.r  eomi)ose  one-half  of  the  area 
of  the  section.  The  quartz  grains  are  ordinarily  distin(;tly  enlarged,  while  quite  a 
good  many  of  the  grains  of  feldspar  are  tolerably  fresli.  The  greater  nuiid)er  of 
tbeui  are  altered  to  some  extent  to  biotite,  while  freciueutly  they  are  almost  completely 
thus  altered.  All  stages  of  this  process  are  seen,  and  while  the  particles  of  feldspar 
and  secondary  folia  of  biotite  are  very  small,  the  transformation  is  distinctly  made 
out.  The  interstitial  material  composing  tlie  other  lialf  of  the  section  coiisists  of 
exceedingly  fine  crystallized  qnartz,  of  minute  folia  of  biotite,  and  of  an  abundiyit 
black  opaque  material  which  is  doubtless  ferriferous  and  possibly  also  carbonaceous. 
All  of  the  biotite  in  the  section  is  believed  to  be  of  secondary  origin. 

10.  Black  biotitie  slates,  from  west  side  of  fault  and  at  the  lower  middle  horizon. 
Specimens  9550  (slide  3322)  and  1480  Wis,,  (slide  207),  fi'om  500  N.,  1400  W.,  Sec  .11, 
T.  44  N^.,  E.  3  W.,  Wisconsin. 

Fine  grained  and  ftnely  laminar  rocks,  which  cleave  readily  along  the  plaues  of 
lamination.  Contained  in  the  fine  material  are  numerous  roundish  black  lustrous 
cleavage  areas,  which  are  taken  to  be  large  fragmental  particles  of  feldspar.  A  lens 
shows  very  uumerous  minute  crystals  of  pyrite  or  pyrrhotite,  probably  the  latter, 
for  the  rock  gives,  finely  pulverized,  a  maguetitic  powder,  which  may,  however,  be 
due  to  magnetite. 

The  thin  sections  show  the  rooks  to  consist  of  two  parts,  a  finely  crystalline 
mati'ix  and  coarse  well  rounded  fragmental  feldspars.  These  feldspars  have  always 
altered  to  a  greater  or  less  degree,  the  alterations  resulting  in  the  formation  of  the 
biotite,  many  small  folia  being  always  found  in  a  single  individual  of  feldspar. 
Every  gradation  of  the  change  is  seen  from  grains  of  feldspar  which  contain  but 
few  folia  of  biotite  to  those  in  whicli  the  remaining  feldspar  is  just  sufficient  in  quan- 
tity to  enable  one  to  perceive  that  the  detached  areas  are  parts  of  one  individual. 
Doubtless  also  the  alteration  to  biotite  and  quartz  has  completely  taken  place  in 
many  cases,  when  the  outlines  of  the  original  clastic  areas  would  be  entirely  lost. 
Accompanying  the  biotite  secondary  to  the  feldspar  is  a  large  amount  of  black 
opaque  material  in  minute  particles.  This  is  in  all  probability  also  secondary,  as  its 
amount  increases  as  the  quantity  of  biotite  increases.  The  black  roundish  spots 
spoken  of  under  the  maitroscopic  descriptions  are  evidently  these  altered  fragmental 
feldspars.     The  matrix  of  the  rock  is  composed  of  intimately  mingled  quartz,  feld- 


314  THE  PENOKEE  IKON-BE AKING  SERIES. 

spar,  biotite,  and  black  opaque  material,,  which  is  mostly  somewhat  altered  pyrite  or 
pyrrhotite,  but  which  may  also  contain  carbonaceous  material.  A  portion  of  the 
matrix  is  certainly  frag'mental,  as  is  shown  by  secondary  enlargements  of  the  quartz 
grains.  The  biotite  is  all  believed  to  be  due  to  the  alteration  of  feldspar ;  much  of  it 
is  certainly  of  this  nature. 

11.  Black  chiastoliti(;  biotitic  slates,  from  west  side  of  fault  at  lower  middle  hori- 
zons. Specimen  9576  (slide  3381),  650  N.,  1350  W.;  specimen  9574  (slide  3380),  670 
N.,  1350  W.;  specimen  9572  (slide  3379),  680  N.,  1350  W.,  Sec.  11,  T.  44  N.,  E.  3  W., 
Wisconsin. 

The  rocks  are  black,  and  exceedingly  fine  grained  to  aphanitic.  Specimen  9572 
breaks  with  a  conchoidal  fracture  and  9574  and  9576  are  finely  laminar.  In  9576 
are  large  cleavage  areas  like  those  described  in  10.  lu  the  specimens  numerous  crys- 
tals of  pyrite  are  contained. 

The  thin  section  3381  is  almost  precisely  like  tlie  thin  section  3322  in  10,  above 
described,  the  only  difference  being  that  the  decomposition  of  the  large  fragmental 
feldspars  has  proceeded  somewhat  further.  Thin  section  3380  differs  from  3381  in 
that  the  large  feldspars  have  been  less  numerous  apparently  and  the  alteration  of 
those  present  has  not  gone  so  far.  The  appearance  of  the  rocks  as  seen  in  hand 
specimens  corresponds  with  their  apiiearance  under  the  microscope,  the  extent  of 
the  alteration  of  the  feldspars  as  determined  by  the  thin  sections  corresponding  exactly 
with  the  condition  of  the  large  feldspars  as  seen  macroscopically.  In  specimen  9550, 
in  which  the  areas  of  feldspar  are  generally  well  developed  and  show  distinct  cleavage 
surfaces,  the  biotitic  alterations  characteristic  of  these  rocks  have  taken  place  to  but 
a  comparatively  small  extent.  In  specimen  9574  the  obscurely  outlined  feldspars  and 
irregular  cleavage  surface  are  accompanied  by  extensive  biotitic  alteration.  The 
section  of  9572  is  much  like  the  finer  parts  of  9550  (in  10)  and  9576.  Scattered  sparsely 
through  the  finer  material  of  9572  and  9574  are  quite  large  crystals  and  intersecting 
clusters  of  crystals  of  chiastolite.  These  chiastolites  often  include  such  a  quantity  of 
the  other  minerals  of  the  section  as  to  be  almost  indistinguishable  in  ordinary  light. 
The  cloiids  of  inclusions  are  arranged  in  parallel  lines,  which  correspond  to  the  some- 
what obscure  slaty  cleavage  of  the  rock.     Pyrite  is  an  abundant  accessory. 

12.  Biotite-slate,  from  west  side  of  fault  at  a  lower  middle  horizon.  Specimen 
1440  Wis.,  (slide  261),  700  N.,  0  W.,  Sec.  10,  T.  44  N.,  E.  3  W.,  Wisconsin. 

The  thin  section  is  very  fine  grained,  and  consists  of  intimately  mingled  quartz, 
feldspar,  biotite,  and  minute  particles  of  dark  opaque  material  a  portion  of  whicli  is 
doubtless  pyrite.  The  biotite  composes  about  one-third  of  the  rock.  A  portion  of 
this  mineral  is  certainly  secondary  to  feldspar,  and  all  of  it  is  probably  of  this  origin. 

13.  Biotitic  and  muscovitic  slates,  from  west  side  of  fault,  at  a  lower  middle  hori- 
zon. Specimens  129  Wr.  and  130  Wr.,  850  JST.,  200  W.,  Sec.  10,  T.  44  N.,  E.  3  W., 
Wisconsin. 


THE  UlTEU  SLATE  iMEMl'.EH.  315 

8i)00iiiipu  11'!)  Wr.  is  likciinTfi  in  U,  silxtvo  described.  Specimen  I'M)  Wr,  differs 
from  ll'it  iiibeiiiy-  of  a  dark  j^ray  color,  and  in  tlmt  itsliows  hut  few  clea\'iifre  surfaces. 

Tlie  tlim  section  of  12!)  Wr.  resembles  very  closely  ;5.')L'li  in  10  and  .t.tSl  in  1 1 ;  tlie 
oidy  point  111'  (Urierciuu'  beinj;-  tiiat  pyrite  and  black  material  in  ininnte  specks  ai'e 
particularly  abundant.  In  K5(t  Wr.  the  alteration  of  larj-e  fragmental  feldspar  to 
mnscovite  and  biotite  is  nicely  shown.  The  Hakes  of  mus(!ovite,  which  are  of  a 
greater  magnitude  than  tlio.se  of  biotite,  are  clustered  about  and  i>onctra-te  the  parti- 
cles of  feldspar.  The  section  is  nimsually  free  from  black  material.  In  other  resi)ects 
it  does  not  differ  from  the  other  mica-slates  of  the  vicinity.     (PI.  xxxiv,  Fig.  3.) 

14.  Black  biotite-slates,  from  west  side  of  fault,  at  a  middle  horizon.  Specimens 
9549  (slide  3321),  1050  N.,  UKtO  W.;  954S  (slide  33130),  1070  N.,  1000  W.;,9547  (slide 
3094),  1090  N.,  IGOO  W.,  Sec.  11,  T.  44  N.,  R.  3  W.,  Wisconsin. 

The  rocks  are  black,  exceedingly  fine  gi'ained,  finely  laminar,  and  cleave  readily 
along  the  plane  of  lamination.  As  in  9550  in  10,  and  957(!  in  11,  which  they  are  almost 
precisely  like,  nnmerous  rounded  cleavage  areas  are  contained  in  the  Hue  material. 
Pyrite  in  small  crystals  is  an  accessory. 

The  thin  sections  are  almost  i^recisely  like  3322  in  10,  the  only  difference  of 
importance  being  that  in  them  the  biotitic  alteration  of  the  fragmental  feldspars  has 
taken  jilace  more  largely  than  in  3322.  The  correspondence  between  the  appearance 
of  hand  specimens  and  tWn  sections  described  as  applying  to  10  and  11,  due  to  feld- 
spar decomposition,  is  here  equally  well  seen. 

15.  Muscovitic  graywacke,  from  west  side  of  fault,  at  a  middle  horizon.  Speci- 
men 9540  (slide  3093),  1100  N.,  1000  W.,  Sec.  11,  T.  44  K.,  R.  3  W.,  Wisconsin. 

The  rock  is  a  light  gray,  medium  grained  one,  with  uniform  texture,  which 
cleaves  irregularly  along  a  schistose  plane. 

The  thin  section  is  composed  of  coarse  grains  of  quartz,  having  generally  oval  or 
roundish  forms  set  in  a  fine  matrix,  which  also  appears  to  be  chiefly  quartzose, 
although  with  it  are  feldspar,  chlorite,  and  mnscovite.  While  there  is  a  wide  grada- 
tion in  the  size  of  the  quartz  grains^  there  is  an  approximate  separation  of  the  coarse 
and  fine  quartz.  At  first  sight  the  section  apjiears  to  have  but  a  small  amount  of 
coarse  fragmental  feldspar.  Ilowever,  upon  closer  examination  it  is  seen  that  the 
remnants  of  many  such  feldspars  are  present.  In  some  cases  these  grains  are  honey- 
combed by  saturating  quartz,  many  detached  areas  often  being  in  a  single  individual. 
Again  a  feldspar  area  contains  many  small  individuals  of  quartz  mingled  with  chlorite 
and  mnscovite,  and  occasionally  a  feldspar  particle  is  found  which  is  tolerably  fresh. 
Evidently  this  rock  was  originally  mostly  composed  of  rounded  fragmental  ijarticles 
of  quartz  and  feldspar.  The  fragmental  quartz  grains  are  enlarged,  and  thus  are  now 
sharx)ly  angular.  The  x>i^rticles  of  feldspar  are  largely  replaced  or  decomposed,  as 
above  described,  and  consequently  the  rock  has  now  become  at  first  sight  an  almost 
completely  cirystalline  one.     Had  the  i^rocess  continued  until  all  .the  feldspars  had 


316  THE  PENOKEE  IRON  BEARING  SERIES. 

been  thus  decomposed,  the  only  trace  of  any  original  fragmental  material  would  be 
the  general  oval  or  roundish  character  of  the  larger  particles  of  quartz. 

IG.  Biotitlc  and  muscovitic  graywacke,  from  west  side  of  the  fault,  at  a  middle 
horizon.  Specimen  9544  (slide  3092),  1105  N.,  1600  W.,  Sec.  11,  T.  44  N.,  R.  3  W., 
Wisconsin. 

A  gray,  coarse  grained,  massive  rock,  having  a  conchoidal  fracture. 

Large  fragmental  grains  of  quartz  and  feldspar,  with  the  alteration  and  replac- 
ing products  of  the  latter,  make  up  the  mass  of  the  thin  section.  The  areas  of  quartz 
are  enlarged,  and  consequently  minutely  angular,  although  still  retaining  their  general 
roundish  form.  The  feldspar  is  nuich  fresher  than  in  15,  many  individuals  showing 
no  alteration  influence  except  a  little  kaolinization.  Other  individuals,  however, 
include  many  grains  of  quartz  or  large  reticulating  quartz  individuals  with  numerous 
flakes  of  muscovite.  Here  these  minerals  are  plainly  alteration  products  of  the  feld- 
spar. In  many  cases  this  alteration  has  proceeded  so  far  as  to  leave  irregular  cores, 
which  are  entirely  surrounded  with  the  secondary  quartz,  biotite,  and  muscovite. 
Again  in  other  cases  the  original  rounded  outlines  of  the  feldspar  are  distinct,  the 
alterations  having  occurred  in  spots  through  the  grains.  The  finer  portions  of  the 
rock  are  composed  of  quartz,  biotite,  and  muscovite,  often  without  any  feldspar. 
These  portions  are  also  probably  alteration  products  of  feldspar,  the  individuals  per- 
haps being  small,  or  at  any  rate  are  alteration  products  of.  feldspar  mingled  with 
detrital  quartz.  Through  the  finer  grained  portions  of  the  rock  are  numerous  small 
particles  of  black  opaque  material.     (PI.  xxxii.  Fig.  2;  PI.  xxxiii.  Pig.  1.) 

17.  Biotitic  graywacke,  from  the  west  side  of  fault  at  a  middle  horizon.  Speci- 
men 9545  (slide  3319),  from  1100  K,  1600  W.,  Sec.  11,  T.  44  N.,  R.  3  W.,  Wisconsin. 

The  rock  is  gray,  coarse  grained,  massive,  and  has  a  conchoidal  fracture.  It 
contains  numerous  particles  of  limpid  quartz  and  less  plentiful  ones  of  white  or  pale 
pink  feldspar,  which  are  so  large  as  to  enable  one  to  easily  see  their  rounded  forms. 
The  rock  in  appearance  approaches  that  of  a  gray  quartzite. 

fn  the  thin  section  very  large,  rounded,  often  complex  areas  of  quartz  are  very 
numerous.  These  grains  as  to  general  form  are  well  rounded,  but  their  outlines  are 
minutely  irregiilar,  and  these  irregularities  are  doubtless  due  to  enlargements.-  The 
feldspar  also  is  found  in  rounded  areas.  It  is  often  more  or  less  decomposed,  the 
method  of  alteration  and  the  products  produced  being  precisely  as  in  16.  Contained 
in  the  section  are  large  areas  composed  almost  entirely  of  finely  crystalline  quartz, 
brown  folia  of  biotite  mingled  with  small  remnants  of  felds])ar,  each  of  which  with- 
out doubt  represents  a  single  fragmental  feldspar. 

18.  Biotitic  schists,  from  west  side  of  fault  at  middle  horizon.  Specimens  9543 
(slide  3318),  and  1494  Wis.  (slide  273),  from  1110  N.,  1600  W.,  Sec.  11,  T.  44  N.,  R.  3 
W.,  Wisconsin. 

A  dark  gray  mottled,  fine  grained,  and  quite  massive  rock. 


THE   UJM'KK  SLATK  MKMl'.Elt.  317 

At  first  sij^lit  the  sections  appear  like  ordinary  liiotite-schists.  They  contain 
grouiidmasses  of  quartz,*  through  whi(;li  arc  [deiitifully  and  unil'oinily  scattered 
brown  fohui  of  biotite  and  less  numerous  tlalces  of  inuscovitc.  On  closer  examination 
numy  of  the  larger  grains  of  this  (luartz  are  ])lainly  fragmental,  and  they  have  often 
been  enlarged.  Tiic  biotite  is  to  a  large  extent  secondary  to  feldspar,  many  Hakes  of 
this  mineral  being  found  mingled  with  a  small  qiumtity  of  feldspalhic  material,  wliich 
throughout  a  considerable  area  acts  as  a  unit.  •  In  a  few  places  fragments  of  ortho 
clase  and  plagioclase  are  found,  in  which  the  mica-schist  decomposition  is  but  begun, 
but  in  the  greater  part  of  the  sections  but  little  of  the  original  feldspar  remains.  We 
have  now  the  explanation  of  the  vaguely  outlined  spots  seen  in  the  hand  specimens. 
They  represent  original  fragmental  feldspars,  which  once  composed  a  very  large  pro- 
portion of  the  rock;  but  now  in  most  cases  they  have  so  completely  altered  to  biotite, 
muscovite,  and  quartz  that  only  here  and  there  in  the  thin  section  is  an  area  well  out- 
lined. This  rock  was  then  originally  like  2,  but  now  apin'oaches  very  closely  to  a 
typical  mica-schist.  In  2  there  is  everywhere  a  plain  distinctiou  between  the  feld- 
spathic  and  nonfeldspathic  areas.  The  larger  portion  of  the  sections  resembles  the 
fine  grained  matrix  of  2.  Oxide  of  iron  in  small  jiarticles  is  an  abundant  inclusion 
in  the  biotite,  feldspar,  and  much  of  the  ([uartz.  The  quartz  containing  such  quanti- 
ties of  this  oxide  of  iron  is  taken  to  be  secondary,  the  larger  plainly  fragmental 
grains  being  free  from  this  feixite. 

19.  Muscovitic  and  biotitic  graywacke,  from  west  side  of  fault  at  an  upper  mid- 
dle horizon.  Specimen  9540  (slide  3316),  from  1600  N.,  0  W.,  Sec.  10,  T.  44  N.,  K.  3 
W.,  Wisconsin. 

A  light  gray,  line  grained  rock,  which  is  quite  massive,  but  which  breaks  most 
readily  along  its  obscure  schistose  i>lane. 

The  section  is  chiefly  composed  of  particles  of  quartz  and  feldspar  of  greatly 
varying  sizes  and  quite  deeply  interlocking.  At  first  sight  the  only  indication  of  a 
fragmental  nature  is  the  general  rounded  contours  of  the  larger  particles  of  quartz 
and  feldspar,  these  grains  being  minutely  and  sharply  angular  upon  their  borders. 
Upon  close  examination  many  of  the  grains  of  quartz  show  indications  of  enlargement. 
The  lines  between  the  sui>posed  nuclei  and  their  enlargements  are  in  no  case  so 
distinct  as  in  the  ferruginous  fragmental  quartzites,  but  consist  of  small  detatched 
particles  of  chlorite,  kaolin,  air  bubbles,  and  sometimes  iron  oxide.  In  a  few  cases 
the  cores  of  the  quartz  grains  are  clearly  made  out.  Oftentimes  there  are  considera- 
ble breaks  in  the  lines,  and  the  whole  line  in  most  grains  might  be  taken  to  be  ordi- 
nary inclusions.  However,  the  constancy  of  the  oval  or  rounded  forms  of  the  inclu- 
sions and  their  positions  near  the  exteriors  of  the  large  grains  lead  to  the  conclusion 
that  they  mark  the  outlines  of  true  fragmental  cores  which  have  become  eidarged 
until  they  interlock.  The  irregularity  of  the  exterior  of  the  grains  of  feldspar  is  due 
to  another  cause,  their  partial  alteration  to  chlorite,  muscovite,  and  biotite,  with  con- 


318  THE  PENOKEE  lEON-BEAEmG  SERIES. 

sequent  separation  of  silica.  These  minerals  are  not  plentifully  present,  but  are  so 
situated  witli  respect  to  the  feldspars  as  to  leave  little  doubt  of  the  connection 
between  them.     A  portion  of  the  finer  grained  quartz  is  certainly  secondary. 

20.  Biotite-schist,  from  west  side  of  fault  at  an  upper  luiddle  horizon.  Speci- 
mens 9539  (slide  3315),  1442  Wis.  (slide  2G2),  frorn^  1650  N.,  0  W.,  Sec.  10,  T.  44  N., 
E.  3  W.,  Wisconsin. 

The  rocks  are  like  2. 

The  thin  sections  are  like  those  of  2. 

21.  Muscovitic  biotite-schists,  from  west  side  of  fault  at  a  middle  horizon.  Speci- 
mens 9537  (slide  3314),  50  N.,  1950  W.;  2001  Wis.  (slide  281),  30  N.,  1950  W.;  2002 
Wis.  (slide  282),  130  N.,  1950  W.;  9536  (slide  4223),  150  N.,  1950  W.,  Sec.  2,  T.  44 
N.,  E.  3  W.,  Wisconsin. 

The  rocks  are  gray  or  green,  fine  grained,  quite  massive,  yet  showing  a  more  or 
less  obscure  schistose  structure. 

The  thin  section  4223  resembles  very  closely  those  of  3  aiid  4.  It  differs  from 
them  in  that  the  alterations  have  not  gone  <inite  so  far,  and  consequently  the  original 
fragmental  nature  of  the  rock  is  clearly  made  out.  Many  of  the  more  angular  graiu^ 
of  quartz  are  distinctly  seen  to  be  widely  enlarged.  In  places  saturating  quartz  has 
penetrated  through  and'tlirough  large  grains  of  feldspar.  In  this  manner  large  sec- 
ondary individuals  of  quartz  have  formed,  no  part  of  which  is  fragmental.  The  mica 
is  abundant  and  much  of  it  is  secondary  to  feldspar,  Avhile  there  is  little  doubt  that  all 
or  nearly  all  of  the  biotite,  muscovite,  and  enlarging  quartz  are  secondary  products, 
having  resulted  from  decomposition  of  the  feldspar.  The  section  3314  differs  from 
4223  in  that  the  alterations  of  feldspar  and  enlargements  of  quartz  are  not  so  easily 
traced,  and  in  that  a  considerable  quantity  of  chlorite  has  resulted  from  the  decom- 
position of  the  feldspar.  The  thin  sections  281  and  282  contain  numerous  well  rounded 
somewhat  coarse  particles  of  quartz  and  feldspar,  so  that  they  are  much  nearer  their 
original  condition  than  4223  and  3314  from  tlie  same  locality.  In  them  the  altera- 
tions of  feldspar  have  also  produced  chlorite  and  kaolin  as  well  as  biotite  and  mus- 
covite. 

22.  Biotite-schist,  from  west  side  of  fault  at  the  uppermost  known  horizon, 
134  Wr.,  from  250  N.,  0  W.,  Sec.  3,  T.  44  N.,  E.  3  W.,  Wisconsin. 

The  rock  is  like  9536  in  21,  except  that  it  is  very  plainly  schistose  and  in  places 
biotite  is  very  prominent. 

In  essential  points  this  section  is  like  4. 

Section  east  of  Mellen  junction. 

23.  Biotitic  and  chloritic  graywacke-slate,  at  a  middle  horizon.  Specimens  12764 
(Slide  5459),  12765  (slide  5460),  NE.  I  of  NB.  4  Sec.  8,  T.  44  N.,  E.  2  W.,  Wisconsin, 
on  the  Wisconsin  Central  Eailroad. 


THE  UPPER  SLATE  MEMBER.  319 

The  rocks  arc.  dark  gray,  lino  grained,  finely  l;inilii:itcil.  In  IIh'im  arc  nnnicioiis 
roiimlisli  spots  of  a  darker  color  Mian  tlic  reinaindiT  of  tho  rock.  Pyrite  is  a])leiififiil 
accessory. 

The  thill  sections  sliow  hotli  tine,  f;raiiicd  and  coarse  grained  materials.  Rather 
small  well  rounded  grains  feldspar  compose  more  than  oncvhalfof  the  coarser  pai'ts, 
while  the  particles  01  this  mineral,  although  of  very  small  size,  are  abundant  in  the 
liner  portions.  The  grains  of  qiuirtz  are  frequently  enlarged.  The  feldspar  conipiises 
orthoclase,  inierocili  e,  and  plagioclase.  Many  of  its  grains  are  much  altered  chlorite, 
biotite,  and  quartz,  resulting  from  their  decomposition.  In  tlie  finer  parts  of  the  sec 
tions  are  plentiful  chlorite,  biotite,  and  finely  crystalline  quartz,  which  may  in  part 
be  firagmental,  and  many  fine  particles  of  a  dark  colored  substance,  which  is  probably 
carbonaceous  material.  In  the  coarser  portions  of  the  rock  much  of  the  chlorite  and 
biotite  are  in  well  defined  flakes.  The  dark  colored  roundish  spots,  seen  niacroscop 
ically  when  examined  in  ordinary  light,  appear  to  be  but  little  different  from  the  finer 
grained  parts  of  the  remainder  of  the  rocks,  but  in  polarized  light  they  almost  com- 
pletely extinguish  the  light. 

24.  Biotitic  graywacke,  from  a  middle  horizon.  Specimen  9582  (slide  3326),  220 
N.,  0  W.,  Sec,  5,  T.  44  N.,  E.  2  W.,  Wisconsin. 

The  rock  is  light  gray,  fine  grained,  quite  massive^  and  breaks  with  a  conchoidal 
fracture. 

The  thin  section  is  comi^osed  largely  of  fragmental  particles  of  quartz  and  feld- 
spar of  varying  sizes,  which  are  set  in  a  finely  crystalline  matrix,  consisting  mostly  of 
quartz.  Biotite  in  minute  folia  is  scattered  between  the  fragments  and  through  the 
fine  matrix.  A  portion  of  it  is  certainly  secondary  to  feldspar,  and  probably  all  of  it 
is  of  this  nature.  The  grains  of  quartz  are  frequently  enlarged.  Much  of  the  finer 
grained  interstitial  quartz  is  probably  secondary. 

25.  Black  biotite-slate  from  a  middle  horizon.  Specimen  9584  (slide  3327)  from 
600  N.,  0  W.,  Sec.  5,  T.  44  N.,  E.  2  W.,  Wisconsin. 

The  rock  is  almost  black,  aphauitic,  cleavable,  and  contains  numerous  black, 
obscure  areas  which  give  it  a  faint  mottled  appearance. 

The  thin  section  consists  of  excessively  minute  particles  f)f  quartz,  biotite, 
chlorite,  kaolin,  and  pyrite.  A  few  grains  of  quartz  somewhat  larger  than  the 
remainder  have  a  fragmental  appearance.  Biotite  is  very  abundant.  Contained  in 
this  fine  material  are  rounded  areas  which  ajii^ear  to  be  almost  completely  decom- 
posed feldspar,  the  minerals  formed^  by  this  alteration  being  the  same  as  those  in 
the  remainder  of  the  section. 

26.  Black  biotite-slate  from  an  upjier  horizon.  Specimen  9593  (slide  3330), 
1950  N.,  1750  W.,  Sec.  4,  T.  44  K,  E.  2  W.,  Wisconsin. 

The  specimen  is  like  9550  in  10  and  9576  in  11,  except  that  the  cleavage  areas 
which  give  the  rock  a  mottled  appearance  are  less  distinct  than  in  them.    Composi- 


320  THE  PENOKEE  lEON  BEARING  SERIES. 

tioii :  Silica,  59-73;  alumina,  22-78;  iron  sesquioxide,  0-1 1 ;  iron  protoxide,  5-98;  man- 
gaiious  oxide,  0-09;  calcium  oxide,  0-53;  magnesium  oxide,  2-9J:;  potassa,  3-4:8;  soda, 
1-41 ;  water,  3-28  =  100-33. 

The  section  of  this  rock  is  much  like  3322  in  10.  The  chief  differences  are, 
that  much  of  the  large  fragmeutal  feldspar  shows  the  distinct  twinning  bands  of 
plagioclase,  and  that  none  of  these  large  feldsi^ars  have  altered  to  such  a  degiee  as  a 
part  of  them  have  in  the  section  referred  to. 

27.  Chloritio  biotite-schists ;  the  uppermost  horizon  shown  at  this  point,  the  rock 
being  mingled  with  the  eruptives  of  the  Keweenaw  series.  Specimens  9586  (slide 
3329),  1980  N.,  540  W.;  9590  (slide  4425),  1850  N.,  640  W.;  9591  (slide  4426),  1940 
N.,  585  W.,  Sec.  5,  T.  44  N.,  R.  2  W.,  Wisconsin. 

The  rocks  are  light  to  dark  gray,  break  with  a  conchoidal  fracture,  and  are 
quite  massive,  although  showing  traces  of  lamination. 

In  the  thin  sections  fragmeutal  quartz  and  feldspar  are  the  most  plentiful  con- 
stituents. Mingled  with  these  minerals  are  much  biotite,  chlorite,  and  musc.ovite, 
the  biotite  being  far  the  most  abuudaut.  Many  of  the  larger  of  the  quartz  grains 
have  taken  a  second  growth,  as  a  result  of  which  their  exteriors  are  minutely  angular. 
Much  of  the  feldspar  is  quite  fresh,  although  in  many  places  it  has  largely  altered  to 
muscovite,  chlorite,  and  biotite.  Less  frequently  the  feldspar  is  replaced  by  sat- 
urating quartz.  While  much  of  the  mica  and  chlorite  are  secondary  to  feldspar,  this 
statement  could  not  with  certainty  be  made  of  all  of  it.  Quite  plentiful  black  par- 
ticles of  ferrite  are  scattered  through  the  sections.  In  the  interstices  is  finely 
crystalline  quartz. 

28.  Muscovitic  biotite-schist.  The  uppermost  horizon  shown  here — higher  than 
the  preceding.  Specimen  2039,  Wis.,  NW.  ^  of  NE.  J  Sec.  6,  T.  44  N.,  R.  2  W.,  Wis- 
consin. 

The  section  has  a  fine  grained  groundmass  of  quartz  and  feldspar,  in  which  are 
abundantly  found  muscovite  and  biotite.  Many  of  the  larger  quartz  grains  include 
numerous  folia  of  mica.  The  feldspar  areas  are  reticulating  and  include  quartz  and 
both  the  micas.  The  muscovite  and  biotite,  besides  being  found  in  the  small  folia 
thus  included  in  the  quartz  and  feldspar,  are  present  in  numerous  large  blades.  This 
section,  taken  by  itself,  would  be  considered  a  completely  crystalline  mica-schist  in 
which  the  interlocking  and  mutual  inclusions  of  the  different  minerals  are  of  the 
most  intricate  sort.  But,  like  the  other  mica-schists  of  the  Up])er  slate,  it  is  believed 
to  be  an  ordinary  clastic  rock  in  which  the  series  of  metasomatic  changes  have  been 
great.  The  large  areas  of  quartz  including  folia  of  mica  represent  clastic  individuals 
of  feldspar.  xVs  these  feldspars  have  altered  to  mica  the  excess  of  silica  has  sepa- 
rated, out  as  quartz,  and  the  areas,  as  stated  above,  which  were  once  orthoclase  or 
plagioclase — in  most  cases  the  foi'mer — are  now  complex,  most  intricately  inteilocking 
areas  of  quartz  and  mica.    Frequently  the  alteration  of  a  single  feldspar  has  resulted 


riii;  riM'Kit  slatk  j\ii:mi;kk.  321 

in  tlic  foruuitiim  of  ;i  siii<flc  iiMlividiiiil  of  (luart,/.,  in  wliich  case  the.  more  or  leas 
(lotiiciuMl  areiis  iiolaiize.  to^ctlicr.  In  otlicr  casos  tlic  dcconiiiositioii  of  a  feldspar  has 
resnltcd  in  the  formation  of  many  grains  of  (puui/,  as  well  as  numerous  folia  of  mica. 
Tiie  proof  of  these  alterations  in  tlie  single  section  is  not  complete,  but  takeu  in  (;on- 
nectiou  with  the  other  mica  schists  of  the  formatiou  it  is  couclusive.  A  reticulating 
area  of  saturating  (piartz  ofteu  contains  detached  cores  of  feldsi)ar,  which  polarize 
as  a  unit.  Sometimes  these  feldsi)ars  liave  not  altered  to  such  an  extent  as  described 
above.  In  almost  every  case  their  rounded  exteriors  are  lost,  l)ut  irregular  areas  of 
consideral)lo  size  ([uite  frequently  remain,  which  include  few  folia  of  biotite  and  little 
saturating  ((uartz.  It  is  possible  that  a  portion  of  the  silica  eomposiug  the  saturating 
ipuirtz  comes  from  extraneous  sources.     (PI.  xxxiv,  Fig.  1.) 

2d.  (Jhloritic  aiul  nuiscovitic  graywacke,  from  a  high  horizon,  higher  than  the 
preceding,  although  not  here  an  uppermost  horizon.  Specimen  3164  (slide  322),  Wis. 
0  N.,  1,000  W.,  Sec.  27,  T.  45  K,  11.  2  W.,  Wisconsin. 

The  thin  section  was  originally  composed  of  rather  small  clastic  i>articles  of 
quartz  and  feldspar.  The  quartz  grains  are  mostly  enlarged.  Secondary  interstitial 
quartz  has  apiieareil.  The  feldsi)ars  have  largely  decomposed,  their  places  now  being 
occupied  by  chlorite,  inuscovite,  saturating  quartz,  and  to  a  small  extent  biotite. 

Section  at  and  west  of  Tylers  forU. 

30.  Black  biotite-slate,  from  a  low  horizon.  Specimen  12770  (slide  5461);  short 
distance  east  of  north  quarter  post.  Sec.  1,  T.  44  N.,  R.  3  W.,  Wisconsin. 

The  rock  is  black,  aphanitic,  finely  laminated,  very  soft,  and  apparently  carbo- 
naceous. 

The  thin  section  shows  an  exceedingly  fine  grained  background,  comjiosed  of 
flinty  and  perhaps  amorphous  quartz,  of  kaolin,  and  of  black  material  which  may  be 
carbonaceous.  Contained  in  this  background  are  numerous  small  fragments  of  quartz 
and  feldspar  and  dark  brown  folia  of  biotite. 

31.  Bioti tic  graywacke  slate,  from  a  low  horizon.  Specimen  12771  (slide  5462), 
SE.  i  of  the  SW.  J  of  Sec.  31,  T.  45  N.,  E.  1  W.,  Wisc^)nsin,  on  the  Wisconsin  Cen- 
tral Eailroad. 

The  rock  is  dark  gray,  very  fine  grained,  cleavable,  and  shows  here  and  there 
lustrous  flakes  of  white  mica. 

The  thin  section  consists  of  very  small  particles  of  quartz,  feldspar,  biotite, 
chlorite,  brown  iron  oxide,  and  apj)arently  some  kaolin.  Much  of  the  quartz  is  frag- 
niental.  The  feldspar  fragments  are  much  altered,  the  resultant  products  being 
chlorite,  biotite,  and  quartz. 

32.  Chlorite  graywacke-slates,  from  a  middle  horizon.  Specimens  12772  (slide 
5463),  12773  (slide  5464),  NE.  J  of  the  ]SW.  \  Sec,  32,  T.  45  'S.,  E.  1  W.,  Wisconsin. 

MON  XIX — ^21 


322  THE  PENUKEE  IRON-BEARING  SERIES. 

The  two  specimens  differ  in  appearance.  The  first  is  almost  blapk,  fine  grained, 
and  finely  laminated.  The  second  is  gray,  coarser  grained,-  and  massive.  Both 
contain  shining  flakes  of  white  mica. 

In  slide  5463  rather  small  fragmental  particles  of  quartz  and  feldspar,  with 
a  few  flakes  of  white  mica,  compose  two-thirds  of  the  section.  The  quartz  grains 
are  often  enlarged.  The  feldspar  comprises  orthoclase,  microcline,  and  plagioclase, 
the  latter  very  plentiful.  All  the  feldspars  have  altered  considerably  to  chlorite, 
seiicite,  biotite,  and  quartz,  chlorite  being  the  most  abundant.  The  remaining  one- 
third  of  tlie  section  is  apparently  composed  of  the  same  minerals  as  the  coarser  parts, 
with  the  addition  of  pyrite,  opaque  iron  oxide,  and  a  little  of  some  carbonate.  Slide 
5463  diff'ers  from  5463  in  that  its  fragmental  quartz  and  feldspar  are  in  much 
f-maller  particles,  in  that  the  latter  is  more  altered;  and  in  containing  more  ferru- 
ginous and  perhaps  some  carbonaceous  material. 

33.  Chlorite  graywacke-slate,  from  a  middle  horizon.  Specimen  12776  (slide 
5466),  near  center  SW.  ^  of  SE.  i  Sec.  29,  T.  45  N.,  R.  1  W.,  Wisconsin,  on- 
Wisconsin  Central  Railroad. 

The  rock  is  precisely  like  12773  in  32. 

The  thin  section  is  in  no  important  point  difterent  from  5463  in  32. 

34.  Chloritic  graywackes,  from  a  middle  horizon.  Sj)ccimens  2108  Wis.  (slide 
387),  1000  N.,  784  W.;  9611  (slide  3341),  1000  N.,  800  W.;  2104  Wis.  (slide  306),  1060 
K,  784  W.;  9613  (slide  3342),  1150  N.,  775  W.,  Sec.  28,  T.  45  N.,  R.  1  W.,  Wisconsin. 

The  rocks  are  light  gray,  rather  fine  grained,  and  cleave  most  readily  along  the 
plane  of  stratification,  although  easily  breaking  across  this  plane  with  a  conchoidal 
or  subcouchoidal  fracture. 

In  thin  sections,  well  rounded  clastic  particles  of  quartz  and  feldspar  are  set  in 
a  fine  matrix,  consisting  generally  of  quartz  and  chlorite,  but  containing  some  kaolin, 
biotite,  and  brown  and  black  material  which  is  taken  to  be  ferrite,  mingled  i)erhaps 
with  pyi'ite  and  organic  matter.  The  fragments  of  quartz  are  frequently  enlarged. 
The  feldspars  are  often  quite  fresh,  but  much  of  this  mineral  is  altered  to  chlorite, 
kaolin,  and  biotite,  with  simultaneous  separation  of  silica. 

35.  Biotitic  graywacke-slate,  from  a  middle  horizon.  Specimen  9627  (slide  3343), 
1550  N.,  0  W.,  Sec.  29,  T.  45  N.,  R.  1  W.,  Wiscousin. 

A  dark  gi'ay  aphanitic  banded  rock. 

The  thin  section  is  exceedingly  fine  grained.  It  consists  of  a  confused  mass  of 
quartz,  feldspar,  chlorite,  biotite,  and  ferrite,  with  perhaps  some  sericite. 

36.  Chloriticbiotite-slates,  from  an  upi^er  middle  horizon.  Specimens  9609  (slide 
4428),  100  N.,  1525  W.;  9610  (sUde  4429),  140  N.,  1540  W.;  2098,  Wis.  (sHde  304), 
south  line  Sec.  21,  at  point  where  intersected  by  Tylers  fork,  Sec.  21,  T.  45  N.,  R. 
1  W.,  Wisconsin. 


TIM']  UPl'KU  SLATE  MEMBER.  323 

The  rocks  are  black,  fine  s'ni'netl  to  apluinitic,  aud  break  readily  across  the 
slaty  clcavajjo  with  a  subconchoidal  fra(^ture. 

Tlie  thill  sections  consist  of  intimately  mingled  minute  particles  of  quartz,  feld- 
spar, biotito,  chlorite,  and  dark  opaque  patches  of  ferrite  or  altered  pyrite.  In  these, 
as  iu  the  previously  described  sections,  the  biotite  and  chlorite  are  in  part  at  least 
plainly  secondary  to  feldspar.  These  slates  are  somewhat  coarse  grained,  and  there- 
fore show  more  plainly  their  fragmeutal  character  than  do  most  of  the  slates  of  the 
district. 

37.  Chloritic  biotite-slate,  from  an  upper  middle  horizon.  Specimen  9608  (slide 
3340),  475  N.,  910  W.,  Sec.  21,  T.  45  N.,  E.  1  W.,  Wisconsin. 

The  rock  is  like  36. 

The  thin  section  is  iiner  grained  than  those  in  36,  but  is  otherwise  like  them. 

38.  Biotite-graywacke,  from  an  upper  middle  horizon,  interstratified  with  9608. 
Specimen  9607  (slide  3339),  550  E".,  910  W.,  Sec.  21,  T.  45  K,  R.  1  W.,  Wisconsin. 

The  rock  is  dark  gray,  of  a  uniform  medium  grain,  massive,  and  breaks  with  a 
subconchoidal  fracture. 

In  thin  section  this  rock  differs  from  387  in  34,  in  that  the  chlorite  is  less  and 
biotite  more  abundant.  They  were  originally  of  the  same  composition.  The  differ- 
ence at  the  present  time  is  due  to  the  fact  that  the  feldspai-s  underwent  different 
alterations  in  the  two  cases.     Pyrite  is  present  in  abundant  small  grains  and  crystals. 

39.  Ghlpritic  graywacke,  from  an  upper  middle  horizon,  interstratified  with  9607 
and  9608.  Specimen  9606  (slide  4427),  715  IST.,  885  W.,  Sec.  21,  T.  45  K,  E.  1  W., 
Wisconsin. 

The  rock  is  like  38. 

The  thin  section  resembles  closely  3341  and  3342  iu  34.  It  differs  from  them 
chiefly  in  containing  a  smaller  proportion  of  tinely  crystalline  interstitial  material, 
and  in  that  the  fragmental  quartz  aud  feldspar  are  in  larger  grains.  The  almost 
complete  decomposition  of  the  feldspar  and  the  resultant  formation  of  chlorite, 
biotite,  muscovite  or  sericite,  and  finely  crystalline  quartz  are  nicely  shown. 

40.  Biotite-slate,  from  an  upper  middle  horizon.  Specimen  9605  (sUde  3338), 
740  ]Sr.,  885  W.,  Sec.  21,  T.  45  K,  E.  1  W.,  Wisconsin. 

The  rock  is  dark  gray,  exceedingly  fine  grained ;  contains  numerous  minute  par- 
ticles of  pyrite,  and  is  given  a  mottled  apxiearance  by  cleavage  surfaces  of  feldspar. 

The  thin  section  consists  chiefly  of  biotite,  quartz,  and  feldspar,  the  latter  min- 
eral now  being  much  less  abundant  than  the  other  two.  The  little  remaining  feldspar 
is  in  "the  last  stages  of  alteration,  its  resultant  products  being  mainly  biotite  aud 
quartz.  The  decomposition  of  each  grain  of  feldspar  produced  many  particles  of  quartz 
and  folia  of  biotite.  In  mauy  cases  polarized  light  is  necessary  to  discover  the  remain- 
ing feldspar  of  a  i)artly  decomposed  individual,  so  closely  do  its  alteration  products 
resemble  iu  appearance  the  remainder  of  the  section.    It  is  quite  probable  also  that  in  • 


324  THE  PENOKEE  IRON-BEARING  SERIES. 

most  cases  the  decomi)osition  of  the  feldspars  has  been  complete,  and  that  all  of  the 
biotite  and  much  of  the  quartz  are  secondary  products.  Scattered  uniformly  through 
the  section  are  rather  plentiful  grains  and  crystals  of  pyrite. 

41.  Chloritic  sericite-schist,  from  an  uppermost  horizon,  mingled  with  Keweenaw 
greenstones.  Specimens  9629  (sUde  4430);  9630  (slide  3345),  120  N.,  150  W.,  Sec.  17, 
T.  45  N.,  R.  1  W.,  Wisconsin. 

The  rock  is  light  gray,  moderately  fine  grained,  banded,  and  breaks  with  a  sub- 
conchoidal  fracture. 

The  thin  section  has  a  groandmass  of  small  blades  and  fibers  of  pale  green  chlo- 
rite and  colorless  sericite,  Avith  a  little  biotite,  which  contains  numerous  small  frag- 
mental  grains  of  quartz  and  fewer  of  feldspar.  The  individuals  of  the  latter  mineral 
are  usually  badly  decomposed,  the  alteration  products  being  chlorite,  sericite,  and  bio- 
tite. In  some  cases  cores  of  feldspar  surrounded  by  these  secondary  products  are 
comparatively  fresh. 

Section  between  Tylers  forh  and  Potato  river. 

42.  Ohloritic  graywacke-slate,  from  a  lower  middle  horizon.  Specimens  12777 
(slide  5467);  12778  (slide  5468),  the  SW.  J  Sec.  24,  T.  45  N.,  R.  1  W.,  Wisconsin,  on 
the  Wisconsin  Central  Railway. 

These  rocks  are  almost  precisely  like  32,  the  only  differences  being  that  12778  is 
slightly  schistose,  while  12773  is  massive  and  white  mica  flakes  are  plenMful. 

In  thin  section  these  rocks  are  almost  precisely  like  those  of  32.  The  only  dif- 
ferences are  that  fragmental  white  mica  (sericite)  is  more,  biotite  less,  and  pyrite  more 
plentiful  than  in  that  number. 

43.  Chloritic  graywacke-slate,  from  a  lower  middle  horizon.  Specimen  12779 
(slide  5469),  NE.  \  of  the  SW.  J  Sec.  24,  T.  45  N.,  R.  1  W.,  Wisconsin,  on  the  Wis- 
consin Central  Railway. 

This  rock  is  like  12773  in  32,  except  that  it  is  of  a  darker  color. 
The  thin  section,  is  like  5463  in  32,  except  that  the  amount  of  black  material, 
either  impure  iron  oxide  or  iron  oxide,  iiyrite,  and  carbonaceous  material  is  greater. 

44.  Biotite-graywacke,  from  an  upper  horizon.  Specimen  9598  (slide  3334),  0  N., 
600  W.,  Sec.  15,  T.  45  N.,  R.  1  W.,  Wisconsin. 

■    The  rock  is  of  a  dark  gray  color,  medium  grained,  uniform  texture,  and  breaks 
wTth  conchoidal  fracture. 

This  section  is  coarser  grained  than  that  of  any  of  the  previously  described  bio- 
titic  graywackes,  but  resembles  them  in  essential  points.  Clastic  particles  of  quartz 
and  feldspar,  well  rounded,  compose  two-thirds  or  more  of  the  mass  of  the  rock.  A 
few  of  the  quartz  grains  are  finely  comjilex,  and  nearly  all  of  them  are  distinctly 
enlarged.  The  alteration  of  feldspar  to  biotite  is  very  nicely  shown.  The  freshest  of 
the  feldspar  grains  are  surrounded  by  and  more  or  less  deeply  intersected  with  sec- 


THE  UPPER  SLATE  MEMBER.  325 


*> 


ondiiry  biotito.  Those  {•niins  yet  retain  their  well  loiiiided  form,  but  in  luauy  cases 
the  (iiifiiuiil  exteriors  of  the  fehlspar  {grains  are  h)st.  Often  the  entire  surfaces  of  the 
fehlspars  inehule  very  numerous  jjartich-s  of  the  biotite,  there  romaininj>-  throufthout 
such  areas  iiere  and  tliere  little  spots  of  feldspar,  wiiicli  tof^etlua'  polarize  as  units. 
The  rather  sparse  matrix  of  the  rock  consists  of  finely  crystalline  (pxartz,  of  feldspar, 
and  of  biotite.  Mucih  of  this  fine  quartz  is  doubtless  a  secondary  infiltration  material 
and  the  biotite  is  believed  to  be  secondary  to  feldspar. 

This  section  is  one  of  the  finest  yet  described  to  show  the  alteration  of  feldspathic 
areas  into  numerous  small  folia  of  biotite.     (PI.  xxxii,  Fig  3.) 

45.  Biotitic  graywaclce,  from  near  the  top  of  the  series.  Specimen  9595  (slide 
3332),  700  N.,  1940  W.,  Sec.  14,  T.  45  N.,  R.  1  W.,  Wisconsin. 

The  rock  is  dark  gray,  rather  fine  grained,  massive,  and  breaks  with  a.  subcou- 
choidal  fracture. 

In  thin  section  medium  sized  clastic  particles  of  quartz  and  feldspar  are  set  in  a 
finer  matrix,  which  consists  of  particles  of  these  minerals  mingled  with  much  biotite 
in  small  folia  and  with  less  plentiful  white  mica  (sericite).  As  in  the  mica-scMsts 
previously  described,  many  of  the  feldspar  j)articles  are  altered  to  biotite  to  a  greater 
or  less  degree. 

Section  at  Potato  river  and  vicinity. 

46.  Chloritic  graywacke-slate,  from  a  lower  middle  horizon.  Specimen  9107 
(slide  2789),  1925  N.,  1990  W.,  Sec.  19,  T.  45  N.,  E.  1  E.,  Wisconsin. 

The  rock  is  dark  gray,  exceedingly  fine  grained,  finely  schistose,  and  shows  upon 
the  cleavage  surface  numerous  minute  particles  of  pyrite. 

In  thin  section  small  fragmental  particles  of  quartz  and  feldspar  are  set  in  an 
exceedingly  fine  matrix,  consisting  chiefly  of  quartz  and  chlorite.  Mingled  with  the 
chlorite  are  flakes  of  sericite  or  kaolin.  Throughout  the  section  are  many  small  areas 
of  dirty  somewhat  altered  pyrite. 

4'7.  Biotitic  and  chloritic  graywacke,  from  an  upper  middle  horizon.     Specimen 
9109  (slide  4418),  1800  N.,  0  W.,  Sec.  13,  T.  45  K,  E.  1  W.,  Wisconsin. 

The' rock  is  dark  gray,  medium  grained,  massive,  and  breaks  with  conchoidal 
fracture. 

The  thin  section  is  almost  precisely  like  that  of  38.  The  particles  of  quartz  are 
distinctly  enlarged;  many  of  them  are  notable  for  the  very  numerous  partly  liquid 
filled  cavities  which  they  contain.     (PI.  xxxii.  Pig.  4.) 

48.  Biotitic  and  chloritic  graywacke,  from  an  upper  horizon.  Specimen  9110 
(slide  2902),  390  K,  1980  W.,  Sec.  7,  T.  45  N.,  R.  1  E.,  Wisconsin. 

The  rock  is  like  47. 

In  thin  section  the  rock  is  almost  precisely  like  that  of  47. 


326  THE  PENOKEE  lEON-BEAEING  SERIES. 

49.  Biotitic  graywacke-slate,  from  an  upper  horizon.  Specimens  9111  (slide 
2790);  9112  (slide  2923),  35  ^.,  940  W.,  Sec.  12,  T.  45  N.,  E.  1  W.,  Wisconsin. 

The  rock  is  dark  gray,  fine  grained,  and  varies  from  massive  to  finely  laminar. 
In  all  essential  points  the  thin  section  is  like  3329  in  27.    As  in  it,  there  is  much 
biotite,  which  is  certainly  secondary  to  feldspar. 

50.  Biotite-slate,  from  near  top  of  series.  SiDCcimen  9113  (slide  2903),  300  N., 
940  W.,  Sec.  12,  T.  45  N.,  E.  1  W.  Wisconsin. 

The  rock  is  dark  gray  to  black,  aphanitic,  finely  laminated,  and  mottled  by 
very  numerous  black  cleavage  areas. 

The  thin  section,  with  the  exception  that  black  opaque  material  and  pyrite  are 
sparsely  present,  is  exactly  like  3322  in  10.     (PI.  sxxiv.  Fig.  4.) 

51.  Feldspathic  quartzites  and  conglomerates  from  top  of  series,  mingled  with 
Keweenawan  greenstones.  Specimens  9115  (slide  4419),  9116  (slide  2905),  470  N.,  30 
W.;  9118  (slide  2906),  9119  (sUde  3298),  470  N.,  65  W.,  Sec.  11,  T.  45  N,,  E.  1  W., 
Wisconsin. 

The  rocks  vary  from  coarse  grained  gray  vitreous  quartzite  to  a  conglomerate 
containing  numerous  pebbles  several  inches  in  diameter. 

In  thin  sections  large  fragmental  grains  of  quartz  compose  a  good  portion  of 
these  rocks.  Mingled  with  the  quartz  is  a  considerable  quantity  of  fragmental  feld- 
spar, also  in  large  individuals.  The  quartzes  are  usually  enlarged,  this  fact  generally 
being  easy  to  discover.  Between  the  fragmental  particles  is  finely  crystalline  quartz 
and  other  accessories  in  large  quantity.  In  slide  2905  the  finely  crystalline  quartz  is 
mingled  with  actinolite,  being  cut  through  and  though  in  every  direction  by  it.  The 
actinolite  needles  are  also  always  included  in  the  enlargements  of  the  clastic  quartz 
grains,  but  never  in  the  cores.  It  is,  then,  plainly  a  secondary  mineral,  and  about  it 
the  infiltrated  quartz  has  crystallized.  Some  of  the  feldspars  have  decomposed,  and 
in  them  is  found,  as  secondary  and  replacing  product,  actinolite  and  quartz.  In 
slides  2906  and  3298  the  feld.^pars  are  badly,  decomposed,  the  secondary  products 
being  kaolin,  large  brilliantly  polarized  flakes  of  muscovite,  and  many  smaller  ones 
of  biotite.  These  minerals  are  also  included  in  the  interstitial  quartz  and  in  slide 
2906,  and  also  in  the  enlargements  of  the  old  quartz  grains.  This  section  nicely 
illustrates  the  micaceous  alteration  of  feldspar,  by  the  decomposition  of  which,  accom- 
panying the  micas,  saturating  quartz  "has  been  formed.  Many  of  the  pebbles  in  the 
conglomerate  are  large,  interlocking,  complex  fragments  of  quartz.     (PI.  xxxii,  Fig.  1.) 

Section  in  the  north  part  of  T.  45  N'.,  K.  1  U.,  \7isconsin. 

52.  Chloritic  graywacke,  from  a  lower  middle  horizon.  Specimen  9123  (slide 
2794),  300  K,  0  W.,  Sec.  8,  T.  45  N.,  E.  1  E.,  Wisconsin. 

The  rock  is  dark  gray,  fine  grained,  massive,  and  breaks  with  conchoidal 
fracture. 


THE   IJPrEK  8LATE  MEMHEH.  327 

111  thill  section  siiiall  i)iirticIo.s  of  (juiirtz,  generally  enlarged  and  sometimes 
finely  i'oiui)lcx,  with  fragments  of  feldsiuir,  iwtli  of  greatly  varying  iiiagiiitudes,  com- 
pose from  ouebalf  to  two-thirds  of  the  rock.  The  feldspar  iiicliuh^s  orthoclase, 
microcliue,  and  plagioelase.  Many  of  the  feldspars  are  inucli  altered,  while  some  of 
them  are  almost  or  quite  decomposed,  the  resulting  products  being  maiuly  (ihlorite 
and  quartz,  although  with  these  minerals  are  found  sericite,  or  kaolin  and  ferrite. 
The  matrix  consists  chiefly  of  finely  ('rystallinc  (piartz,  with  some  amorpho.us  silica, 
mingled  with  Avhich  are  a  considerable  quantity  of  chlorite  and  a  little  kaolin  or  seri- 
cite and  ferrite.  In  the  matrix  are  also  numerous  irregular  patches  of  opafpie 
material,  which  is  in  part  gray  and  in  part  bhick.  These  patches  are  probably  mix- 
tures of  ferrite  aud  partly  altered  pyrite.  The  section  is  that  of  a  flue  grained 
typical  giaywacke. 

53.  Chloritic  graywacke,  from  a  middle  horizon.  Specimen  9124  (slide  2908), 
1200  K,  0  W.,  Sec.  8,  T.  45  N.,  R.  1  E.,  Wisconsin. 

The  rock  is  like  52. 

The  thin  section  is  like  that  of  52. 

54.  Chloritic  graywacke,  ft'om  an  upper  horizon.  Specimen  9126  (slide  2909), 
1000  K,  55  W.,  Sec.  5,  T.  45  N.,  E.  1  E.,  Wisconsin. 

The  rock  is  coarser  grained  than  52  and  53  and  contains  crystals  of  pyrite,  but 
otherwise  is  like  them. 

The  tliin  section  differs  from  those  of  52  and  53  only  in  being  coarser  grained. 

Section  near  east  range  line  of  B.  1  H.,  Wisconsin. 

55.  Magnetitic  clay-slate,  from  base  of  formation.  Specimen  9148  (sUde  2924), 
1100  N.,  40  W.,  Sec.  1,  T.  45  K,  E.  1  E.,  Wisconsin. 

The  rock  is  gray,  flne  grained,  finely  laminated,  and  easily  cleavable  along  the 
lamination.  Composition:  Silica,  52'58;  ahimina,  20'76;  iron  sesquioxide,  12*17;  iron 
protoxide,  4*08;  manganous  oxide,  "21;  calcium  oxide,  -SO;  magnesium  oxide,  1'33; 
potassa,  4-87;  soda,  .37;  lithia,  trace;  water,  3-43: =100-10. 

In  thin  section  the  groundmass  is  very  finely  divided.  It  appears  to  consist  of 
quartz,  feldspar,  kaolin,  with  perhaps  chlorite  aud  biotite.  In  this  groundmass  is 
contained  very  numerous  small  crystals  of  magnetite.  The  analysis  well  bears  out 
the  appearance  of  the  section. 

56.  Magnetitic  clay-slate,  from  near  base  of  formation.  Specimen  9139  (slide 
2911),  1200  N.,  1975  W.,  Sec.  6,  T.  45  N.,  E.  2  E.,  Wisconsin. 

The  specimen  differs  from  55  only  in  being  of  a  darker  color.  Composition : 
Silica,  53'44 ;  alumina,  19-62;  iron  sesquioxide,  11-38;  ii-on  protoxide,  5-35;  manganous 
oxide,  trace;  calcium  oxide,  -42;  magnesium  oxide,  1-58;  potassa,  1-73;  soda,  2-61; 
lithia,  trace;  water,  4-07;  phosphoric  acid,  trace:  =  100-20. 


328  THE  PENOKEE  lEON-BEARING  SERIES, 

The  thin  section  differs  from  that  of  55  only  in  that  its  matrix  and  the  contained 
crystals  of  magnetite  are  both  somewhat  coarser. 

57.  Ohloritic  graywacke,  from  a  lower  middle  horizon.  Specimens  9078  (sUde 
2778);  9079  (slide  2779),  0  N.  to  100  N.,  0  W.,  Sec.  36,  T.  46  N.,  R.  1  E.,  Wisconsin. 

The  rocks  are  like  52  and  53. 

As  in  52,  53,  and  54,  in  thin  section,  fragmental  particles  of  quartz  and  feldspar 
compose  a  large  portion  of  the  rocks.  The  quartz  grains  are  usually  enlarged. 
The  feldspars  comprise  both  orthoclase  and  plagioclase  and  are  frequently  quite 
fresh.  The  abundant  matrices  consist  of  finely  crystalline  quartz,  of  chlorite,  of  seri- 
cite  or  kaolin,  and  of  opaque  brown  or  black  ferrite.  In  a  few  places  small  flakes  of 
biotite  are  seen.  In  fragmental  material  and  siUceous  paste,  as  in  other  particuhxrs, 
these  rocks  are  typical  graywackes. 

58.  Ohloritic  graywacke,  from  a  middle  horizon.  Specimen  9077  (slides  3296 
and  3377),  700  N.,  0  W.,  Sec.  36,  T.  46  N.,  R.  1  E.,  Wisconsin. 

The  rock  is  like  57. 

The  thin  section  is  like  those  of  67. 

Section  betiveen  the  east  range  line  of  B.  1  U.,  Wisconsin,  and  the  west  branch  of  the 

Montreal. 

59.  Ohloritic  graywackes,  from  a  middle  horizon.  Specimens  9068  (slide  2922), 
1115  N.,  1560  W.,  Sec.  28;  9066  (slide  2889),  1040  N.,  160  W.,  Sec.  29,  T.  46  N.,  R.  2 
E.,  Wisconsin. 

The  specimens  are  almost  exactly  like  54. 

The  thin  sections  in  no  essential  points  differ  from  that  of  54. 

60.  Ohloritic  graywackes,  from  an  upper  middle  horizon."  Specimens  9071 
(slide  3295),  1380  N.,  180  W.;  9072  (slide  2890),  1400  N.,  605  W.,  Sec.  29,  T.  46  N.,  R. 
2  E.,  Wisconsin. 

The  rocks  resemble  closely  the  previously  described  graywackes. 

The  thin  sections  are  in  essential  particulars  like  the  chloritic  graywackes  pre- 
viously described.  The  fragmental  grains  of  quartz  are  always  distinctly  enlarged. 
While  many  of  the  feldspar  areas  are  quite  fresh,  others  are  badly  kaolinized  and 
chloritized.  The  abundant  matrix  is  of  the  same  composition  as  in  the  previous 
chloritic  graywackes.  It,  however,  contains  a  little  biotite  and  muscovite.  Here  and 
there  are  black  opaque  areas  which  appear  to  be  ferrite,  but  which  may  contain 
pyrite  or  carbonaceous  material. 

61.  Ohloritic  graywacke,  from  an  upper  middle  horizon.  Specimen  9070  (slide 
4415),  1950  N.,  1540  W.,  Sec.  28,  T.  46  IST.,  R,  2  E.,  Wisconsin. 

The  rock  is  precisely  like  60. 

The  thin  section  is  rather  finer  grained  than  that  of  60;  otherwise  it  is  like  it. 


THE   Ul'l'Elt  SLATD  MEMBER.  829 

Section  ea^t  of  the  went  branch  of  the  Montreal  and  in  the  vicinity  of  the  Montreal  river. 

62,  Graywiiclce-slate,  near  base  of  ronnation.  Siu'c.iiiicii  !K)30  (slide  2917),  530 
N.,  1470  W.,  Sec.  24,  T.  40  N.,  K.  2  E.,  Wisconsin. 

The  rock  is  dark  brown,  tine  grained,  finely  banded,  and  cleaves  readily  along 
the  plane  of  lamination. 

In  tliin  section  fragmental  (juartz  and  feldspar  in  grains  ol'  small  size  compose 
perhaps  oue-half  of  the  mass  of  the  rock.  The  grains  of  feldspar  arc  mostly  mnch 
chloritized  or  kaoliuized.  The  particles  of  qnartz  have  frecjucntly  been  enlarged. 
The  abnndaut  tilling  material  is  chlorite,  finely  crystalline  (piiirtz,  dark  brown  oxide 
of  iron,  and  black  iron  oxide,  a  portion  of  which  appears  to  be  magnetite.  This  rock 
resembles  very  closely  the  fragmental  rocks  found  at  the  base  of  the  Upper  slate  at 
Black  river. 

03.  G-raywacke-slate,  from  a  lower  horizon.  Specimen  9100  (slide  2928),  71.5  N., 
1700  W.,  Sec.  24,  T.  40  N.,  R.  2  B.,  Wisconsin. 

The  rock  differs  from  02  only  in  being  of  a  dark  green  color. 
The  thin  section  differs  from  that  of  02  only  in  that  some  of  the  iron  oxide  is 
magnetite. 

04.  Chloritic  graywacke-slate,  from  a  lower  middle  horizon.  Specimen  9020 
(slide  2882),  1350  N.,  1000  W.,  Sec.  22,  T.  47  N.,  R.  47  W.,  Michigan. 

The  rock  is  dark  greenish-gray,  very  fine  grained,  and  finely  laminated. 

In  thin  section  the  rock  differs  from  the  graywackes  just  described  chieHy  in 
being  much  finer  grained.  There  is  much  fragmental  quartz,  and  feldspar  appears, 
but  it  is  very  fine  grained,  while  the  matrix  is  extremely  so. 

05.  Chloritic  graywacke,  from  a  lower  middle  horizon.  Interstratified  with  04. 
Specimen  9027  (slide  2883),  1350  N.,  1000  W.,  Sec.  22,  T.  47  K,  R.  47  W.,  Michigan. 

The  rock  is  gray,  medium  grained,  massive,  and  breaks  with  conchoidal  fracture. 

In  thin  section  the  rock  is  a  typical  graywacke.  It  differs  from  that  of  54  only 
in  being  slightly  finer  grained  and  in  containing  calcite. 

00.  Chloritic  graywacke,  fi-om  an  upper  middle  horizon.  Spe(;imen  9103  (slide 
2925),  1150  N.,  1100  W.,  Sec.  22,  T.  40  H".,  R.  2  E.,  Wisconsin. 

The  rock  is  like  54. 

In  most  points  this  section  is  like  the  previously  described  graywackes.  Many 
of  the  fragmental  feldspars,  however,  are  large  and  fresh.  They  comprise  both  ortho- 
clase  and  plagioclase,  a  portion  of  the  latter  being  microcline.  The  alteration  of  feld- 
spar to  kaolin  or  inuscovite  is  nicely  illustrated,  some  of  the  feldspar  areas  contain- 
ing very  numerous  flakes. 

07.  Chloiitic  graywacke,  from  an  upper  horizon.  Specimen  9034  (slides  2884 
and  2918),  555  N.,  1445  W.,  Sec.  14,  T.  40  N.,  R.  2  E.,  W;isconsiu. 

The  rock  is  gray,  medium  grained,  massive,  and  breaks  with  conchoidal  fracture. 


330  THE  PENOKEE  lEON-BEARING  SERIES. 

The  thin  sections  closely  resemble  that  of  54.  Very  plainly  fragmental  quartz 
and  feldspar  in  grains  of  medium  size  compose  two-thirds  of  the  sections.  The  qviartz 
grains  are  often  slightly  enlarged.  The  feldspars  are  frequently  fresh  and  often  also 
much  altered,  the  alterations  resulting  in  the  formation  of  sericite  or  muscovite,  chlo- 
rite, and  perhaps  a  little  biotite.  The  matrix  is  like  those  of  the  previously  described 
chloritic  graywackes,  except  that  sericite  or  muscovite  is  here  present. 

68.  Chloritic  and  sericitic  graywacke,  from  near  top  of  series.  Specimen  9032 
(slide  2770),  720  N.,  1280  W.,  Sec.  14,  T.  46  N.,  E.  2  E.,  Wisconsin. 

The  rock  is  like  67,  except  that  it  is  finer  grained. 

The  thin  section  is  finer  grained  than  that  of  67.  It  also  differs  greatly  from  it 
in  that  it  has  a  much  more  crystalUne  appearance,  equaling  in  this  respect  many  of 
the  chlorite-schists  west  of  the  south  end  of  Gogebic  lake.  The  crystalline  appear- 
ance is  made  still  stronger  by  the  fact  that  while  the  quartz  particles  are  exceedingly 
angular,  it  is  with  the  greatest  diflflculty  that  enlargements  can  be  seen  to  cause  this 
angularity — in  most  grains  quite  impossible.  The  mineral  constituents  are  identical 
with  those  of  67,  but  sericite  is  much  more  plentiful  than  in  it,  and  both  the  sericite 
and  chlorite  are  mostly  in  rather  small,  well  defined  leaflets.  That  all  of  these  two 
minerals  are  secondary  to  feldsjiar  can  not  be  shown,  although  a  large  portion  of 
them  certainly  is  of  this  nature.  However,  from  the  association  with  and  likeness  to 
67,  it  can  not  be  doubted  that  both  are  of  like  origin — that  is,  altered  mechanical 
sediments.  It  is  also  certain  that  the  crystalline  appearance  of  67  is  due  to  the  prev- 
alence of  the  sericitic  and  chloritic  alteration  of  feldspar,  combined  with  the  separa- 
tion of  quartz  in  the  interstices  and  the  enlargement  of  the  quartz  particles. 

Section  at  and  west  of  Blach  river. 

69.  Chloritic  clay-slate,  from  a  middle  horizon.  Specimen  9194  (slide  2932),  1200 
N.,  1140  W.,  Sec.  13,  T.  47  E".,  E.  47  W.,  Michigan. 

The  rock  is  gray,  aphanitic,  and  shows  a  slaty  cleavage  which  cuts  across  the 
bedding  plane. 

The  section  is  exceedingly  fine  grained.  It  appears  to  consist  of  a  confused  mix- 
ture of  chlorite,  kaolin,  quartz,  ferrite,  mingled  with  which  are  here  and  there  a  few 
larger  grains  of  quartz  which  appear  to  be  clastic. 

70.  Pyi-itic  clay-slate,  from  a  lower  horizon.  Specimen  12530  (slide  5334),  1000 
N.,  1300  W.,  Sec.  17,  T.  47  E.,  E.  46  W.,  Michigan. 

The  rock  is  light  gray,  very  fine  grained,  and  contains  a  large  amount  of  pyrite, 
both  disseminated  through  the  rock  and  in  large  irregular  masses. 

The  thin  section  consists  of  a  confused  mass  of  minute  particles  of  quartz,  feld- 
spar, chlorite,  kaolin,  ferrite,  and  perhaps  other  clay-forming  minerals.  Contained  in 
this  material  are  abundant  crystals  and  areas  of  pyrite.  ■* 

71.  Chloritic  graywacke,  from  an  upper  horizon.  Specimen  10417  (sMe  4021), 
800  N.,  1340  W.,  Sec.  10,  T.  47  N.,  E.  46  W.,  Michigan. 


TIIK   UPl'Kli  SLATK   I\l  KM  HKIt.  331 

Till'  rock  is  ji'i'ii.Vt  ••''!•  I'alluM-  I'hk^  imiforiii  fj^raiii,  and  iiiassivc. 

Ill  tiic  tliiii  section,  tV;i<iiiu'iils  oC  ((uart/  and  feldspai-  of  varyinj;-  sizes  coniijosc 
two-tiiirds  or  more  of  the  roeU.  Tlie  reldsi)ar  l'ia};ineiits  are  often  i|uitc  IVesli  and 
comprise  orthoclase,  niieroelinu,  and  plagioelase.  Tiio  lillinji-  material  is  finely  erys- 
talliue  sil.it;a,  chlorite,  kaolin,  and  ferrite. 

7li.  Chloritie  f>ray\vaeke,  from  n  lower  middle  horizon.  Specimen  '.)517  (slide 
2980),  325  N.,  715  VV.,  Sec.  10,  T.  17  N.,  Ji.  40  W.,  Michigan. 

The  rock  is  gray,  rather  flue  grained,  massive,  breaks  with  subconchoidal  frac- 
ture, and  contains  large  fragments  of  black  cherty  material. 

In  thin  section,  rather  large  fragmental  particles  of  quartz  and  feldspar,  the  two 
minerals  being  in  about  ecpial  abundance,  compose  the  greater  part  of  the  rock. 
The  grains  of  (piartz  are  usually  quite  widely  enlarged.  A  ])ortion  of  the  feldspar  is 
relatively  fresh,  but  the  greater  part  of  it  is  much  chloritized  and  kaoliuized.  The 
interstitial  material  consists  of  quartz,  chlorite,  kaolin,  ferrite,  with  some  biotite  and 
muscovite. 

73.  Micaceous  graywacke-slate,  from  a  lower  middle  horizon.  Specimen  9518 
(sUde  3090),  325  N.,  715  W.,  Sec.  10,  T.  47  N.,  R.  40  W.,  Michigan. 

The  rock  is  dark  gray,  fine  grained,  finely  laminated,  and  the  cleavage  surface 
shows  numerous  glittering  flakes  of  mica. 

In  thin  section,  rather  small  clastic  particles  of  quartz  and  feldspar  compose 
about  one-lialf  the  bulk  of  the  rock.  The  grains  of  quartz  are  generally  enlarged 
and  the  feldspars  are  usually  much  decomposed.  The  interstitial  material  consists  of 
quartz,  chlorite,  biotite,  muscovite,  and  ferrite.  Folia  of  muscovite  and  biotite  are 
arranged  with  their  longer  axes  in  a  common  direction,  as  though  the  rock  had  been 
subjected  to  squeezing. 

74.  Ohloritic  clay-slate,  from  a  Ioav  horizon.  Specimen  9515  (slide  2^5),  250  N., 
1750  W.,  Sec.  11,  T.  47  N.,  E.  40  W.,  Michigan. 

The  rock  differs  from  09  only  in  that  its  cleavage  and  bedding  correspond. 
The  thin  section  is  like  that  of  09. 

75.  Clay-slate,  from  base  of  formation.  Specimen  9493  (slide  44132),  700  N.,  1015 
W.,  Sec.  12,  T.  47  N.,  E.  40  W.,  Michigan. 

The  rock  is  dark  olive  green  to  black,  aphanitic,  finely  laminated,  cleaves  par- 
allel to  the  bedding  and  also  in  another  direction,  cutting  across  the  first  at  an  obtuse 
angle.     Included  are  particles  and  nests  of  pyrite. 

The  thin  section  is  excessively  fine  grained.  It  appears  to  consist  of  finely  crys- 
tallized quartz,  partly  amorphous  silica,  chlorite,  feldspar,  ferrite,  and  crystals  of 
pyrite.  Quite  a  x^roportion  of  the  quartz  and  feldspar  are,  however,  coarse  enough  to 
show  that  the  rock  is  a  fragmental  one. 


332  THE  PBNOKEE  lEON-BEARlNG  SEEIES. 

SECTION  II.— ORIGIN   OF  THE  UPPER  SLATE  ROCKS. 

From  what  has  preceded,  it  is  evident  that  the  association  of  the  mica- 
schists  and  mica-slates  of  the  Upper-slate  member  with  the  graywackes 
and  graywacke-slates  is  of  a  most  intimate  nature,  both  as  to  corriposition 
and  occurrence.  The  same  set  of  minerals  occur  in  both  classes  of  rock; 
they  are  closely  interstratified  with  each  other.  There  is  an  identical 
change  in  each  class  in  mineral  composition  in  passing  from  east  to  west, 
and  in  any  one  region  the  change  occurs  simultaneously  in  both  classes. 
Finally,  there  is  every  gradation  between  them,  and  the  placing  of  many 
specijfnens  in  one  class  rather  than  in  the  other  is  somewhat  arbitrary. 
It  becomes,  then,  probable  that  the  original  condition-  of  the  graywackes 
and  graywacke-slates  and  the  mica-schists  and  mica-slates  must  in  the  main 
have  been  the  same.  As  some  of  the  graywackes  are  completely  frag- 
mental,  others  somewhat  crystalline,  others  still  more  crystalline,  thus  grad- 
ing into '  the  crystalline  mica-schists  and  mica-slates,  we  will  begin  with 
those  which  are  nearest  their  original  condition  and  trace  the  processes  step 
by  step  in  which  the  original  rock  has  been  changed  into  a  fully  crystal- 
line mica-schist. 

A  general  notion  of  the  graywackes  and  graywacke-slates  has  already 
been  given.  Before  proceeding  to  trace  these  jjrocesses  of  alteration  it  will 
be  necesaftry  to  describe  in  more  detail  the  graywackes  which  are  near 
their  original  condition. 

Macroscopically  the  graywackes  vary  from  tolerably  coarse  grained 
to  aphanitic.  Many  of  them  are  apparently  completely  massive  in  hand 
specimen,  and  such  break  with  conchoidal  fracture,  although  even  these 
in  exposure  show  more  or  less  of  a  cleavage  along  the  bedding  planes. 
Specimens  of  the  graywacke-slates  show  a  decided  tendency  to  cleave 
parallel  to  the  bedding.  In  color  these  rocks  vary  from  light  gray  or  light 
green,  through  various  shades  of  gray  and  green,  to  almost  black.  Under 
the  microscope  the  only  difference  between  the  graywackes  and  graywacke- 
slates  is  one  of  fineness  of  grain.  The  least  altered  varieties  of  them  may 
be  divided  into  two  classes,  one  being  composed  mostly  of  tolerably  large 
well  rounded  particles  of  quartz  and  feldspar  (PL  xxxii),  and  the  other 


THE  UPPER  SLATK  MEMBER.  833 

having'  feldspar  prcdoiniiiant,  little  or  no  (|iiartz  being  present  (PI.  xxxiii, 
Fig.  1).  riie  feldspars  conunonly  comprise  orthoclase,  niicrocline,  and 
plagioelase.  Usually  some  of  the  feldspars  are  altered  to  a  greater  or  less 
extent  to  chlorite,  sericite,  muscovite,  biotite,  and  kaolin.  In  the  inter- 
stices there  may  be  a  small  quantity  of  finely  crystalline  quartz.  In  many 
cases,  however,  the  graywackes  and  graywacke-slates  are  not  so  simple  in 
composition.  Mingled  with  the  larger  particles  of  fragmental  quartz  and 
feldspar  are  finer  particles  of  the  same  sort,  with  other  minerals  (PI. 
XXXII,  Fig.  4).  When  this  finer  silt  is  preponderant  the  graywackes  and 
graywacke-slates  grade  into  the  clay-slates  or  phyllites.  Naturally,  in  a 
belt  in  which  the  above  described  simple  graywackes  and  clay-slates  both 
occur  there  would  be  found  gradations  between  them.  Freqiieiitly  in  the 
same  specimens  we  find  mingled  coarse  and  fine  material.  As  tlie  quantity 
of  finer  material  increases  it  becomes  increasingly  difficult  to  trace  the 
exact  changes  which  have  taken  place  in  the  minerals.  Unless  all  of  the 
material  is  excessively  fine,  i.  e.,  unless  the  rock  passes  into  a  clay-slate  of 
the  finest  possible  sort,  the  processes  of  change  subsequently  described  are 
seen  to  have  taken  place  with  the  larger  particles  of  quartz  and  feldspar  in 
these  fine  grained  slates.  The  graywackes,  besides  showing  great  variation 
in  appearance,  due  to  the  mingling  of  coarser  and  finer  material,  occa- 
sionally contain  so  much  ferrite — brown  iron  oxide — as  to  make  this  sub- 
stance a  chief  constituent.  Pyrite,  some  carbonate  (calcite,  dolomite,  or 
siderite),  and  rarely  carbonaceous  material  are  also  quite  often  present  as 
more  or  less  plentiful  accessories. 

(1)  Quartzose  grayivacke  (PI.  xxxii). — In  the  freshest  of  the  quartzose 
graywackes  many  of  the  particles  of  feldspar  are  as  unaltered  as  in 
ordinary  granite,  but  generally  they  have  decomposed  to  a  greater  or  less 
extent.  This  decomposition  has  ordinai'ily  taken  place  to  a  greater  degree 
near  the  exteriors  of  the  feldspar  particles  than  in  their  centers.  In  the 
particles  in  which  the  decomposition  has  been  somewhat  extended  the 
alteration  has  atfected  the  regularity  of  the  original  fragmental  oval  out- 
lines. Upon  the  other  hand,  even  when  alteration  has  progressed  quite  to 
the  centers  of  the  feldspars,  the  original  outlines  may  at  times  be  quite 
sharp.     The  minerals  which  have  resulted  from  the  partial  alteration,  and 


334  THE  PENOKBE  lEON-BBAlUlsrG  SB1{1ES. 

which  therefore  are  iiiduded  by  or  closely  encircle  the  feldspar,  are  chlorite, 
sericite  or  muscovite  (or  both),  biotite,  probably  kaolin,  and  quartz.  In  the 
grains  which  have  decomposed  to  the  greatest  extent,  the  original  feldspar, 
remaining  and  the  mica  and  quartz  most  intricately  interlock,  so  that  an 
examination  with  a  moderately  high  power  which  covers  only  one  original 
individual  of  feldspar  or  a  part  of  one  gives  the  appearance  of  a  com- 
pletely crystalline  rock  in  which  the  interlocking  is  of  the  most  intricate 
sort;  yet  when  the  same  grain  is  examined  with  a  low  power  its  rounded 
character  is  evident,  and  that  the  area  is  but  an  altered  feldspar  is  manifest, 
while  the  completely  fragmental  character  of  the  rock  as  a  whole  is  plain 
at  a  glance  (PL  xxxii.  Figs.  2  and  3). 

The  fragmental  gVains  of  quartz,  although  now  unusually  sharply 
angular,  often  show  with  perfect  clearness  well  rounded  cores,  their  present 
angularity  being  due  to  a  renewed  growth  subsequent  to  their  deposition 
in  their  present  resting  place.  Commonly  the  majority  of  the  grains  of 
quartz  are  simple,  but  at  times  many  of  tli^m  are  more  or  less  finely  com- 
plex, or  even  of  a  chalcedonic  character.  The  quantity  of  this  kind  of 
quartz  varies  greatly,  rarely  becoming  almost  or  quite  as  abundant  as  the 
simple  quartz  fragments.  In  the  interstices  of  the  rock  is  found  usually 
a  little  finely  crystalline  quartz,  which  is  of  a  secondary  nature,  as 
plainly  so  as  are  the  enlargements  of  the  fragmental  quartz  grains.  In  thQ 
simplest  quartzose  graywackes  and  gray wacke-slates  little  else  is  present. 
In  these  specimens  it  is  evident  that  the  induration  which  has  often  occurred 
is  caused  almost  wholly  by  the  eidargement  of  quartz  fragments  and  the 
separation  of  finely  crystalline  quartz  in  the  interstices,  thus  completely 
filling  the  spaces  which  originally  existed  between  the  loosely  piled  frag- 
ments and  making  the  rock  as  compact  and  strong  as  tliough  it  were  a 
granite.  The  green  and  gray  colors  of  the  rock  (in  the  absence  of  fen-ite) 
are  due  to  the  secondary  products  of  the  feldspar,  chlorite,  biotite,  etc.; 
-green  when  chlorite,  gray  when  muscovite  and  biotite  are  preponderant. 
The  original  conditions  and  secondary  changes  of  the  simplest  of  the  gray- 
wackes are  thus  clear.  They  were  quartz-feldspar  sediments,  mingled  in 
places  with  a  little  clayey  matter,  perhaps  also  with  a  small  quantity  of 
fragmental  mica  and  some  ferrite.     They  reached  their  present  condition 


TlIK   Ul'lMiU  8LATE  MKMliJOK.  '   335 

by  a  secondary  enlargement  of  quartz  fragments;  tlic  deposition  or  forma- 
tion in  situ  of  interstitial  finely  crystalline  quartz,  accompanied  with  a 
micaceous  or  chloritic  alteration  of  the  feldspar — the  first  two  being  pro- 
cesses already  fnlly  described  by  us.' 

Before  following  further  the  series  of  changes  which  explains  the  met- 
amorphosis of  fragmental  sediments  to  mica-slates  and  mica-schists,  it  is 
important  to  recall  the  chemical  changes^  which  occur  in  the  alteration  of 
orthoclase,  raicrocline,  and  oligoclase  to  chlorite,  muscovite,  andbiotite.  The 
average  content  of  silica  of  the  following  minerals  is  taken  from  Dana's 
System  of  Mineralogy :  Orthoclase  and  raicroline,  65  per  cent ;  oligoclase, 
62  per  cent;  muscovite,  45  per  cent;  biotite,  40  per  cent;  chlorite,  25  to 
30  per  cent.  Evidently  where  the  alterations  of  orthoclase,  microcline,  and 
oligoclase  to  muscovite,  biotite,  and  chlorite  have  taken  place  so  extensively 
as  in  the  rocks  under  discussion,  it  is  not  difficult  to  explain  the  presence  of 
the  silica  which  lias  enlarged  the  fragments  of  quartz,  replaced  those  of 
feldspar,  and  separated  as  independent  interstitial  quartz.  One  of  these 
alterations  is  stated  by  Tschermak'  to  occur  as  follows:  "Wenn  man  die 
dreifache  Formel  des  Feldspathes  3  (Kp-AiP;6SiOJ  mit  jener  des  daraus 
entstandenen  Ghmmers  Kp-Aip^.2SiO,+  2  (H  0-Aip;2SiOJ  vergleicht,  so 
ergibt  sich,  dass  von  der  urspriinglichen  Menge  6810^  nur  2SiO^  in  die 
neue  Verbindung  libergehen  und  4SiO^  iibrig  bleiben."  In  further  speak- 
ing of  the  alteration  of  orthoclase  to  muscovite,  Tschermak  also  observes : 
"  Der  neue  entstandene  Muscovit  ist  ofters  auch  von  Biotit  (Magnesiaglim- 
mer)  begleitet." 

For  the  iron  of  the  biotite  and  chlorite  in  the  rocks  under  considera- 
tion, it  is  not  difficult  to  account.  Pyrite,  marcasite,  and  ferrite  are  quite 
widely  present  in  these  rocks  as  accessory  constituents.  Often  the  relations 
of  the  pyrite  or  marcasite  and  biotite  (folia  of  the  latter  surrounding  particles 
of  the  former)  are  such  as  to  lead  to   the  supposition  that  the  former  min- 

'  On  Secondary  Enlargements  of  Mineral  Fragments  In  Certain  Rocks.  E.  D.  Irving  and  C.  E. 
Van  Hise.     Bnll.  U.  S.  Geol.  Survey,  No.  8. 

'^I  insert  freely,  without  quotations,  here  and  in  the  following  pages,  such  parts  of  an  article  bv 
me  on  this  subject,  already  published — Am.  Jour.  Sci.,  3d  series,  vol.  xxxi,  1886,  pp.  453-459 — as  can 
be  used. 

'  Lehrbuch  der  Mineralogie,  zweite  Auflage,  p.  462. 


336  THE  PENOKEE  IRON  BEAEING  SEEIES. 

erals  have  furnished  the  iron  necessary  for  the  transformation  from  feldspar 
to  biotite.     At  all  events,  they  indicate  a  sufficient  supply  of  iron. 

For  a  part  at  least  of  the  magnesium  of  the  biotite  and  chlorite,  it 
seems  that  we  must  look  to  some  source  extraneous  to  the.  feldspar  frag- 
ments; i.  e.,  we  must  regard  it  as  having  been  supplied  by  some  other 
mineral  or  by  percolating  waters.  That  calcium-bearing  and  magnesium- 
bearing  waters  have  been  present  in  these  rocks  is  evident  from  the  occa- 
sional presence  of  secondary  calcite  and  dolomite.  Partial  analyses  of  three 
of  the  biotite-schists  gave  an  average  content  of  MgO  of  2.22  per  cent, 
which  if  entirely  contained  in  the  biotite  would  correspond  to  a  probable 
proportion  of  that  mineral  of  from  10  to  20  per  cent.^ 

(2)  Muscovitic  and  hiotitic  graywacJce  (PL  xxxiii,  Fig.  1). — Macroscop- 
ically,  this  rock  is  gray,  medium  grained,  and  massive.  It  breaks  with  a 
conchoidal  fracture.  Under  the  microscope  large  fragments  of  feldspar, 
with  the  alteration  and  replacement  products  of  the  latter,  compose  the  rock. 
Most  of  the  feldspar  is  orthoclase,  although  both  microcliue  and  plagioclase 
are  present.  Much  of  it  is  quite  fresh,  many  individuals  showing  no  altera- 
tion further  than  a  slight  kaolinization.  Other  feldspar  fragments,  however, 
include  in  each  many  grains  of  quartz,  or  a  single  large  reticulating  quartz 
individual  and  numerous  flakes  of  muscovite  and  biotite.  Here  the  quartz, 
muscovite  and  biotite  are  plainly  replacements  and  alteration  products  of 
the  feldspar.  In  rare  cases  this  alteration  has  proceeded  so  far  as  to  leave 
but  irregular,' partly  replaced  and  altered  cores  of  feldspar  which  are  entirely 
surrounded  with  the  secondary  quartz,  muscovite,  and  biotite. 

The  finer  grained  parts  of  the  rock  are  composed  of  finely  crystalline 
quartz,  a  portion  of  which  may  be  clastic;  of  feldspar,  the  proportion 
being  smaller  than  in  the  coarser  parts,  probably  on  account  of  the  more 
extended  alterations  in  the  small  particles;  and  of  biotite  and  muscovite. 
The  mica  is  here  clearly  also  to  a  very  large  extent  secondary  to  feldspar, 
while  there  is  little  doubt  that  the  small  r^emaining  fraction  of  the  mica  is  of 
the  same  origin.     Scattered  through  the  finer  portions  of  the    section  are 

■  Lehmaim,  in  his  work  on  the  "Entsteliung  der  altkrystallinischeu  Schiefergesteine,"  demon- 
strates the  formation  of  abundant  secondary  biotite  and  other  minerals  as  accompanying  metamor- 
phoses by  folding.     His  work  does  not  state,  however,  from  what  the  biotite  developed. 


THE  UPPER  SLATE  MEMBER.  337 

numerous  small  particles  of  ii  liliuk  substance  which  is  taken  to  be  partly 
altered  pyrite  or  maicasite,  and  perhaps  partly  carbonaceous  material. 
The  induration  in  the  rock  is  due  to  the  formation  in  situ  of  ([uartz,  or  in- 
filtration of  silica,  from  which  finely  cry.stalline  quartz  has  formed,  assisted 
by  the  partial  transformation  of  the  feldspars  into  intei'locking  complex 
areas  of  quartz,  mica  and  feldspar,  which  also  interlock  with  the  material 
of  the  matrix: 

(3)  Biotitic  f/mywacke  (PI.  xxxiii,  Fig.  2). — Macroscopically,  this  rock 
differs  from  (2)  only  in  being"  of  a  darker  gray  color;  and  under  the  inicro- 
scope  also  it  is  much  the  same,  except  that  micaceous  alteration  of  the  feld- 
spars has  been  carried  farther.  Fragments  of  feldspar,  with  its  secondary 
products,  compose  most  of  the  section.  The  alterations  of  feldspar  to  bio- 
tite  and  quartz  are  beautifully  shown.  The  freshest  of  the  feldspar  grains 
are  surrounded  and  more  or  less  deeply  .penetrated  by  secondary  biotite. 
These  grains  yet  retain  their  well  rounded  forms.  However,  in  many  cases, 
the  original  outlines  of  the  grains  of  feldspar  are  lost,  although  often  the 
complex  aggregate  of  resulting  biotite  folia  ai^d quartz,  mingled  with  the  re- 
maining feldspar,  retain  very  perfect  general  roundish  or  oval  forms.  Often 
the  entire  surfaces  of  the  feldspars  include  very  numerous  particles  of  the 
biotite  and  quartz — the  former  usually  much  the  more  plentiful — there  re- 
maining through  such  areas  here  and  there  little  spots  of  feldspar  which  act 
as  a  unit  in  each  area.  With  a  low  power  such  areas  appear  to  be  roundish 
aggregates  of  biotite.  It  is  only  with  a  higher  power  that,  the  remaining 
feldspar  and  its  true  relations  to  the  biotite  are  discovered.  In  this  rock  all 
the  stages  of  the  ])rocess  of  alteration  of  the  feldspars  are  nicely  shown 
from  those  areas  in  which  the  secondary  biotite  forms  but  a  film  around 
the  feldspars  to  those  in  which  no  feldspar  remains,  there  being  in  the 
place  of  the  fragmental  feldspars  interlocking  aggregates  of  biotite  and  quai'tz. 
Even  these  areas  often  so  perfectly  retain  their  general  oval  or  roundish 
character  that,  taken  by  themselves,  they  would  be  regarded  in  the  sections 
as  peculiar  complex  fragments,  which  in  this  condition  had  become  worn  and 
deposited.  Taken  in  connection  with  these  other  grains  present,  there  can 
be  no  doubt  of  their  formation  by  the  alteration  of  feldspar.  As  from  a 
single  large  particle  of  feldspar  many  individuals  of  quartz  and  mica  are 


338  THE  PENOKEE  IRON-BEAEmG  SERIES. 

formed,  the  result  of  the  alteration  is  to  make  the  rock  a  finer  grained  one. 

The  rather  sparse  matrix  of  the  rock  does  not  differ  materially  from 
that  of  (2),  except  that  it  contains  more  mica.  The  induration  is  caused 
by  the  same  processes  as  in  (2). 

(4)  Muscooitic  biotite-slate  (PI.  xxxiii,  Figs.  3  and  4). — Macroscopically, 
this  phase  of  rock  is  mottled  dark  gray  and  black,  quite  massive.  The 
mottlina'  is  due  to  more  or  less  distinct  roundish  areas,  which  show  in  a 
greater  or  less  degree  cleavage.  In  some  of  the  specunens  the  roundish 
areas  are  distinctly  outlined,  and  in  these  cases  the  cleavage  is  eminent. 
In  others,  these  areas  are  indistinctly  outlined,  and  in  such  the  cleavage 
is  less  plain.  Under  the  microscope  the  grains  showing  cleavage  macro- 
scopically are  found  to  be  well  rounded,  partly  altered  feldspars,  mostly 
of  the  species  orthoclase.  These  feldspars  are  set  in  a  groundmass 
which  consists  of  intimately  mingled  minute  grains  of  quartz  and  brown 
folia  of  biotite  with  a  considerable  quantity  of  ferrite.  In  some  specimens 
the  quantity  of  matrix  is  considerable,  even  equal  or  greater  than  the 
known  feldspar  fragments  and  the  material  coming  from  them.  In 
other  cases,  however,  the  feldspar  areas  were  set  very  close  together 
with  room  for  little  other  material.  The  partial  decomposition  of  the  frag- 
mental  feldspar  has  resulted  in  the  formation  of  very  numerous  small  folia 
of  biotite,  fewer  larger  ones  of  muscovite,  and  also  quartz.  In  the  fresher 
feldspar  areas  the  secondary  mica  is  found  most  plentifully  at  or  near  the 
exteriors  of  the  grains,  although  in  almost  every  case  the  alteration  has 
proceeded  to  a  greater  or  less  degrees  quite  to  tlie  centers.  In  some  speci- 
mens most  of  the  original  feldspar  grains  are  yet  cliiefl}^  unaltered,  and  in 
these  the  mottling  and  cleavage,  seen  macroscopically,  is  most  distinct.  In 
other  specimens  nearly  every  feldspar  grain  has  almost  wholly  altered  to 
mica  and  quartz,  and  in  such  specimens,  those  which  are  most  completely 
altered  and  now  therefore  consist  of  a  complex  interlocking  aggregate  of 
quartz  and  mica,  with  little  or  no  remaining  feldspar,  are  with  great  diffi- 
culty separated  from  interstitial  material.  Many  also  are  doubtless  so 
completely  changed  as  to  be  indistinguishable  from  the  matrix.  In  the 
matrix  of  the  rock  it  is  usually  quite  impossible  to  determine  if  any  of 'the 
quartz  is  fragmeutal.     In  some  specimens  some  grains  are  certainly  so,  as 


THE  UPPER  SLATE  MEMBER.  330 

evidenced  by  rouuded  cores  avid  secondary  enlarfi^einents.  The  biotite  of 
tlu'  matrix  is  in  part  plainly  secondary  t«i  feldspar  and  is  iircciscl)-  lik(!  that 
t'ound  in  the  laro-er  feldspars  The  folia  are  deep  l)n)\vn  and  very  strongly 
dicliroic,  and  therefore  probably  bear  a,  hirg-e  percentage  of  iron.  Donbt- 
less  much  of  this  matrix  is  due  to  the  decomposition  of  fragmental  feldspars, 
which  were  smaller  than  those  which  yet  remain  partly  unaltered,  and 
which  have  therefore  completely  altered  to  mica  and  quartz. 

The  feldspar  plainly  shows  this  rock  to  have  been  fragmental,  and  the 
alteration  of  feldspar  to  both  biotite  and  muscovite  upon  a  large  scale  is 
most  beautifully  shown.  The  large  quantity  of  dark  brown  and  black 
ferrite  has  doubtless  furnished  the  iron  required  for  the  formation  of  the 
biotite.  The  peculiar  spotted  appeai-ance  of  the  sections,  the  distinctness  of 
the  spots  varying  with  the  freshness  of  the  feldspar,  viewed  without  the 
microscope,  gives  a  clear  idea,  when  taken  in  connection  with  their  appear- 
ance under  the  microscope,  of  the  processes  by  which  the  rock  reached 
its  present  condition. 

This  'rock  must  as  deposited  have  been  a  tolerably  coarse  grained  one, 
many  of  the  larger,  fresher  feldspars  averaging  about  1  mm.  in  diameter, 
but  the  alteration  of  each  of  these  most  changed  feldspars  in  the  specimens 
to  a  vast  number  of  mica  folia  and  quartz  grains  has  caused  the  rock  to 
become  exceedingly  fine  grained,  there  remaining,  however,  ^Jerfect  proof 
even  in  these  specimens  of  the  original  rouuded  fragmental  character  of 
some  of  the  feldspar. 

(5)  Nearli/  cnjstalline  muscovitic  biotite-schist  (PI.  xxxiv,  Fig.  1). — Macro- 
scopically,  this  rock  is  fine  grained,  grayish,  and  quartzose,  with  small  mica 
flakes  visible.^  Under  the  microscope  the  thin  section  shows  a  fine  grained 
groundmass  of  quartz  and  feldspar  in  which  are  abundant  muscovite  and 
biotite.  Many  of  the  quartz  grains  include  numerous  folia  of  mica.  The 
feldspar  areas  include  quartz  and  both  muscovite  and  biotite.  This  section 
by  itself  if  not  examined  closely  would  be  taken  to  be  of  a  completely 
crystalline  mica-schist  in  which  the  interlocking  and  mutual  inclusions  of 
the  different  minerals  are  of  the  most  intricate  kind,  but,  like  the  other 
mica-schists  of  the  Penokee  series,  it  is  an  ordinary  clastic  rock  in  which 
the  metasonaatic  changes  have  gone  very  far.     The  large  areas  of  quartz, 


340  THE  PENOKEE  lEON  BEARING  SEEIES, 

iiiclu(Jing  folia  of  mica,  are  in  the  places  of  feldspar  fragments.     As  the 
feldspar  has  altered  to  mica  the  excess  of  silica  has  separated  as  quartz. 
Frequently  the  alteration  of  a  single  feldspar  has  resulted  in  the  formation 
of  a  single  ramifying  individual  of  quartz  with  several  or  many  included 
folia  of  mica,  mingled  with  which  are  detached  remnants  of  the  feldspar. 
In  this  rock  in  such  cases  the  mutual  interlocking  of  these  four  minerals, 
muscovite,  biotite,  quartz,  and  feldspar,  could  not  possibly  be  more  inti-i- 
cate  in  any  schist,  gneiss,  or  granite.     In  other  cases  the  decomposition 
of  a  feldspar  has  resulted  in  the  formation  of  many  grains  of  quartz  as 
well  as  numerous  folia  of  mica.     In  yet  other  cases  the  feldspar  areas  have 
not  altered  to  such  an  extent  as  described  above.     In  almost  every  case  the 
rounded  exteriors  of  the  clastic  grains  are  lost,  but  irregular  areas  of  con- 
siderable size  often  remain  which  include  but  few  folia  of  biotite  or  little 
replacing  quartz,  or  both.     The  folia  of  mica  in  this  rock  are  the  largest 
anywhere  found  in  tlie  mica-schists,  many  of  them  being  1  mm.  in  length. 
The  biotite  has  a  remarkably  strong  dichroism,  and  both  biotite  and  mus- 
covite have  well  defined  cleavages. 

(6)  Crystalline  muscovitic  hiotite-schist  (PI.  xxxiv,  Fig.  2). — Macroscopic- 
ally,  this  rock  is  rather  fine  grained.  It  has  a  finely  laminated  structure, 
along  which  the  rock  most  readily  breaks,  although  it  is  massive  enough  to 
break  quite  easily  across  the  direction  of  lamination.  The  individuals  of 
quartz  and  feldspar  are  too  small  to  be  seen  with  the  naked  eye,  but  are 
distinguishable  mth  a  lens.  The  very  abundant  black,  glittering  flakes  of 
mica  are  large  enough  to  be  readily  seen.  A  lens  shows  the  rock  to  con- 
tain a  considerable  quantity  of  pyrite. 

Under  the  microscope  with  a  medium  power  the  sections  show  a 
rather  fine  grained,  apparently  completely  crystalline,  typical  mica-schist. 
The  groundmass  consists  chiefly  of  quartz,  mingled  with  which  is  con- 
siderable feldspar,  both  orthoclase  and  plagioclase.  The  grains  of  quartz 
vary  considerably  in  size,  but  none  are  minute  and  none  large,  the  largest 
being  not  more  than  ^  mm.  in  diameter.  Some  few  of  the  largest,  which 
are  now  very  irregular  in  outline,  contain  rounded  cores,  proving  them  to  be 
fragmental,  and  these  cores  do  not  include  folia  of  mica.  Much  of  the  feld- 
spar present  is   quite  fresh,  many  of  the  plagioclases  exhibiting  sharply 


TllK  UITKE  yLA'JK  IMiaiBEK.  341 

defined  t\vinniiij>-  hands  and  these  may  he  secondary  developments, 
liiotite  in  well  ddined  folia  of  var)in<j-  size,  the  larocr  heino-  ahont 
h  nnn.  in  Icnf^th,  is  very  ahnndant.  Muscovite  is  mncli  less  ahundant. 
The  lolia  of  mica  often  cut  through  the  smaller  <juartz-<>Tains  and  into 
those  of  the  feldspars,  just  as  in  a  typical  cr}stallin(;  schist,  "^riuit  much 
of  the  mica  is  a  secondary  product  can  not  be  proved  from  the  sec- 
tions from  the  locality  of  English  lake,  taken  by  themselves.  A  portion  of 
it  is,  however,  certainly  of  this  nature.  Many  grains  of  feldspar  are  so 
cut  by  folia  of  mica  as  to  make  it  probable  that  the  latter  is  a  secondary 
product,  while  some  of  the  larger  particles  of  feldspar  contain  throughout 
their  areas  folia  of  mica  and  grains  of  quai'tz  which,  with  the  remaining  feld- 
spar, give  an  appearance  of  most  intricate  interlocking,  as  in  no  case  do 
the  grains  of  feldspar  now  show  the  rounded  outlines  which  they  once 
doubtless  had.  This  mica  included  in  the  feldspar  is  precisely  like  that  of 
the  remainder  of  the  rock — the  word  matrix  for  remainder  can  hardl}^  be 
used,  for  in  coarseness  of  grain  the  parts  of  the  rock  which  show  no  trace 
of  fragmental  origin  are  about  the  same  as  the  quartz  and  mica  included 
in  the  feldspars,  and  which,  taken  in  connection  with  the  rocks  previously 
described,  are  regarded  as  proving  that  this  rock,  like  them,  was  once 
altered  sediments.     Quite  immerous  crystals  of  pyrite  are  seen. 

The  rocks  (2)  to  (6),  above  described,  thus  constitute  a  graded  series 
from  the  fresh  feldspathic  graywackes  to  the  crystalline  mica-schists.  The 
gaps  here  left  in  these  typical  descriptions  may  be  filled  in  by  follow- 
ing the  detailed  descriptions  of  the  graywackes  and  graywacke-slates 
and  mica-slateS  and  mica-schists  (pages  309-326).  Here  the  wide  inter- 
val between  the  most  plaiidy  fragmental  graywackes  and  the  most  crys- 
talline mica-schists  is  completely  filled  by  almost  imperceptible  stages, 
and  the  processes  which  caused  the  alterations  are  clearly  indicated;  the 
result  heing  the  production  from  a  completely  fragmental  arkose  rock,  by  meta-  ■ 
somatic  changes  only,  of  a  rock  which  presents  every  appearance  of  complete 
original  crystallization,  and  ivhicJi  tvotdd  be  ordinarily  classed  as  a  genuine  crys- 
talline schist. 

Black  mica-slates  (PI.  xxxiv,  Figs.  3  and  4).— The  series  of  alterations 
above  described  has  also  been  very  important  in  the  production  of  various 


342  THE  PENOKEE  IRON-BE AEING  SERIES. 

black  mica-slates  of  the  Penokee  series.  Macroscopically,  these  slates 
are  all  exceediiig-ly  fine  grained  and  finely  laminated,  cleaving  readily 
along  the  planes  of  lamination.  In  color  they  vary  from  dark  gray  to 
black.  A  lens  shows  many  of  them  to  contain  numerous  small  particles 
of  pyrite.  Very  numerous  roundish  black  areas  are  contained  in  the  fine 
grained,  gray  material  in  many  of  the  specimens.  These  areas  in  sontie 
cases  show  distinct  cleavage  surfaces  and  are  taken  to  be  large  fragmental 
particles.  In  other  cases  they  are  dull  and  bi-eak  without  giving  cleavage 
surfaces,  while  in  yet  other  specimens  these  darker  spots  are  not  found 
at  all. 

Under  the  microscope  the  rocks  which  have  the  mottled  appearance 
described  above  consist  of  two  parts,  a  finely  crystalline  matrix  and 
coarse,  well  rounded  fragmental  feldspars,  which  are  always  altered  to  a 
greater  or  lesser  extent.  This  alteration  is  to  biotite  and  quartz,  many  small 
folia  of  biotite  and  grains  of  quartz  always  being  found  in  a  single  feld- 
spar indi^ndual.  Every  gradation  of  this  change  is  seen,  from  grains  of 
feldspar  which  contain  but  little  biotite  and  quartz  to  those  in  which  the 
remaining  feldspar  is  just  sufiicient  in  quantity  to  enable  one  to  perceive 
that  the  detached  particles  are  parts  of  a  common  individual.  Doubtless 
also  the  alteratio'n  to  biotite  and  quartz  has  often  been  carried  out  com- 
pletely. Accompanying  the  biotite,  thus  secondary  to  the  feldspar,  is  much 
black,  opaque,  partly  altered  pyrite  in  minute  particles.  The  black, 
roundish  spots  seen  macroscopicall}-  are  evidently  the  partly  altered  feld- 
spar fragments.  The  degree  of  this  alteration  as  determined  by  micro- 
scopical study  corresponds  exactly  with  the  appearance  of  the  rock  as  seen  in 
the  various  hand  specimens.  In  the  specimens  in  which  the  feldspar  areas 
are  well  outlined  and  show  clearly  marked  cleavage  siu'faces,  the  biotitic 
and  qviartzose  alteration  has  taken  place  to  but  a  small  extent.  In  the 
specimens  in  which  the  feldspars  are  obscurely  outlined  and  which  lack 
cleavage  the  alteration  has  extended  very  far.  The  matrices  of  these  rocks 
and  the  sections  of  those  hot  containing  large  grains  of  feldspar  are  com- 
posed of  intimately  mingled  quartz,  feldspar,  biotite,  and  pyrite,  with 
perhaps  a  little  organic  matter.^    A  portion  of  the  quartz  is  certainly  frag- 

'Geol.  AVis.,  vol.  iii,  p.  136. 


Tin-]  UIM'HR  SLATK  MlCMl'.Klt.  343 

mental,  as  is  evidenced  l)y  secondaiy  onlar<>-enient.  Tin-  hiotitc  is  all 
believed  to  he  due  to  the  alteration  of  feldspar,  iiiucli  of  it  certainly  being 
of  this  natnrc.  The  matrices  of  the  ditterent  sections  vary  in  coarseness 
and  in  the  relative  proportions  of  iho  viirions  niinerid  constituents,  but  are 
alike  in  all  essential  jjoints. 

We  have  here  plainly  a  series  of  locks  which  are  parallel  to  (4), 
the  chief  differences  between  the  two  sets  of  rocks  being  that  the  black 
mica-slates  are  much  finer  grained,  that  they  contain  more  pyrite,  and  also 
contain  carbonaceous  material.  As  the  unaltered  or  partly  unaltered  feld- 
spars are  as  large  as  in  (4),  it  is  probable  that  the  greater  fineness  of  g-rain 
in  the  black  mica-slates  at  present  is  simply  due  to  the  fact  that  the  feld- 
spars altered  to  smaller  individuals  of  mica  and  quartz  in  them  than  in  (4). 

Source  of  material— It  will  be  later  shown  that  between  the  Penokee 
series  and  the  Southern  Complex  there  is  a  great  structural  break,  the 
clastic  series  having  been  laid  down  b}^  water  action  upon  the  older  crys- 
talline formations.  From  the  description  of  the  Upper  slate,  it  is  evident 
that  the  most  of  its  material  has  been  derived  from  a  set  of  acid  rocks,  its 
two  predominating  constituents  being  quartz  and  acid  feldspar  in  various 
degrees  of  comminution.  There  is  no  reason  to  believe  that  the  Southern 
Complex,  which  stretches  a  great  distance  southward,  was  ever  covered 
throughout  the  Avhole  of  its  extent  by  this  newer  series ;  and  as  the  mate- 
rial of  which  this  upper  belt  is  composed  is  of  precisely, the  character  which, 
would  be  expected  if  derived  from  its  granite,  gneiss,  and  schist,  it  may  be 
considered  as  probable  that  this  is  the  source  of  the  material  of  the 
Upper  slate.  This  hypothesis  ]-eceives  reenforcement  when  the  distribu- 
tion of  the  granitic  and  schistose  areas  is  considered,  and  the  material  which 
the}'  are  able  to  supply  is  compared  with  that  of  the  material  of  the  Upper 
slate  member  at  adjacent  points.  With  the  exception  of  a  few  miles  the 
newer  series  of  rocks  is  directly  in  contact  with  areas  of  green  schists 
as  far  west  as  Penokee  gap.  Without  doubt  the  contiguous  granite  in 
the  we.stern  part  of  Michigan  and  south  of  the  schist  areas  has  also 
furnished  its  quota  of  material ;  so  that  it  has  come,  not  from  the 
schist  alone,  or  the  granite  or  gneiss  alone,  but  from  both  of  them.  In 
passing   westward     toward    Bad   river,    as    has    been    seen,    there    is    a 


344  THE  PBNOKEB  lEOI^-BEAllING  SERIES. 

gi'adual  change  in  the  nature  of  the  original  material  of  the  Upper  slate, 
it  becoming  more  and  more  strongly  feldspathic ;  west  of  Bad  river  prac- 
tically all  of  it  was  feldspar  of  an  acid  character.  By  reference  to 
chapter  i  it  will  be  seen  that  the  Western  granite,  which  reaches  the 
newer  series  in  the  vicinity  of  Penokee  gap,  extends  west  and  south  for  a 
long  distance.  The  granite  contains  an  unusuallj^  small  quantity  of  quartz, 
being  either  a  true  syenite  or  a  granite  in  which  tlie  quartz  is  usually  not  abun- 
dant There  is  clearly  a  connection  between  the  character  of  this  part  of  the 
Upper  slate  and  the  underlying  rock.  The  degradation  of  the  latter,  accom- 
panied by  natural  sorting,  has  furnished  the  nearly  pure  feldspar  material 
which  originall}^  constituted  the  Upper-slate  member  and  made  it  of  such  a 
composition  that  it  was  possible  by  metasomatic  processes  to  produce  from 
it  a  fine  grained  crystalline  mica-schist. 

Summary . — 1.  The  rocks  of  the  Upper-slate  member  are  mechanical 
sediments,  which  have  been  everywhere  altered  to  a  greater  or  less  extent 
by  metasomatic  changes,  and  at  times  the  alterations  have  extended  so  far 
as  to  produce  crystalline  schists. 

2.  In  general,  the  eastern  part  of  the  formation  is  less  altered  than  the 
western  part.  •  Here  the  pre\^ailing  rocks  are  clay-slates,  graywackes,  and 
graywacke-slates.  In  passing  to  the  westward  the  rocks  become  more 
crystalline  in  character,  and  at  the  extreme  west  end  only  mica-slates  and 
mica-schists  are  found. 

3.  While  this  is  true  in  a  general  waj^,  in  the  same  vertical  section 
there  may  be  found  all  phases  of  the  transformation  from  completely  frag- 
mental  rocks  to  crystalline  schists.  As  far  east  as  Sec.  14,  T.  46  N., 
R.  2  E.,  Wisconsin,  a  rock  Avhich  is  very  close  to  a  crystalline  schist  is 
found.  At  the  Penokee  gap  section  the  most  completely  fragmental  gray- 
wacke  of  the  whole  member  occurs,  and  in  this  section  there  are  all  grada- 
tions to  that  but  one  degree  removed  from  the  most  crystalline  phases 
found  in  the  formation.  Even  in  a  single  exposure  in  this  section  are  found 
plainly  fragmental  graywackes  and  mica-slates,  and  micaceous  graywackes 
which  approach  very  close  to  a  mica-schist. 

4.  The  parts  of  the  Upper  slate  which  have  received  large  fragmental 
particles  of  quartz  are  those  in  which  the  clastic  character  is  easiest  to  rec- 


Till':  UPPKK  SLATE  MRMHER.  345 

ognizp,  for  the  grains  of  (jiiartz  iilways  roiuaiu  in  tlioir  entirety.  It  may 
be,  and  intleed  usually  is,  the  case  that  they  have  undergone  a  second  growth 
and  have  thus  become  angular;  l)ut  generalh'  tlic  original  cores  are  easily 
discovered.  In  the  nc!;irl}-  pure  feldspar  sediments,  upon  the  other  hand, 
when  the  feldspar  has  changed  to  other  minerals,  it  is  more  difficult,  and, 
taking  a  specimen  of  the  most  crystalline  mica-schist  by  itself,  impossiljle 
to  make  out  the  original  fraoinental  character  of  the  rock. 

5.  The  more  crystalline  mica-schists  are  derived  from  nearly  pure 
arkoses  and  in  all  cases  the  resultant  schists  are  nuich  tiner  grained  than 
were  the  original  sediments,  for  from  each  fragment  of  feldspar  there  has 
been  produced  several  or  many  individuals  of  mica  and  quartz. 

6.  The  material  for  the  Upper  slate  has  been  derived  from  the  Southern 
Complex,  and  there  is  a  dii'ect  connection  between  the  character  of  the 
rocks  to  the  south  and  those  of  the  slate  belt  adjacent.  The  greater 
part  of  the  belt  has  received  its  material  in  part  from  the  granitic  and  in 
part  from  the  schistose  areas  ;  while  the  part  of  the  belt  west  of  Penokee 
gap  has  received  nearly  all  of  its  material  from  the  syenitic  granite  to  the 
south  and  west. 


CHAPTER    VII. 


By  C.  E.  Van  Hise. 


THE  ERUPTIVES. 

structural  relations.  General  character  of  the  rock.  Comparison  of  Penokee  greenstones  with 
greenstones  of  the  Southern  Complex  and  Keweenaw  series.  Microscopical  character  of  the 
diabases.     Eruptives  in  the  Iron-hearing  member.     Summary. 

Structural  relations. — The  eruptives  of  the  Penokee  series  are  structur- 
ally of  two  classes.  Some  are  sheets,  while  others  are  dikes.  The  field 
relations  of  many  ledges,  however,  are  not  well  enough  exposed  to  show 
to  which  class-  they  belong.  The  exposures  in  T.  44  N.,  R.  5  W.,  Wis- 
consin, are  large,  and  there  seems  to  be  every  indication  short  of  dem- 
onstration that  these  rocks  are  really  interleaved  with  the  sediments 
of  the  Iron-bearing  member.  In  the  east  part  of  T.  47  N.,  R.  46  W., 
Michigan,  and  the  west  part  of  T.  47  N.,  R.  45  W.,  Michigan,  the  iron- 
bearing  belt  has  an  unusual  width.  Here  the  eruptives  are  particularly 
abundant,  as  shown  by  natural  exposure  and  by  test  pitting,  and  the  rela- 
tions of  the  greenstones  and  the  iron-bearing  rocks  are  such  as  to  indicate 
that  to  some  extent  the  greenstones  are  interleaved,  but  there  is  not  suffi- 
cient evidence  of  this  to  enable  one  to  make  the  assertion  without  qualifica- 
tions. There  is  no  absolute  proof  whether  these  interleaved  greenstones 
are  contemporaneous  volcanic  outflows  which  interrupted  the  deposition  of 
the  iron  belt,  or  subsequent  intrusions.  All  the  evidence  at  hand  is  of  a 
lithological  character,  and  it  points  to  subsequent  intrusions  rather  than  to 
contemporaneous  flows.  The  rocks  are  medium  grained,  holocrystalline, 
nonamygdaloidal,  and  do  not  contain  minerals  in  two  generations ;  proper- 

346 


THE   ERUPTIVK8.  347 

ties  which  are  ordinarily  takou  as  indicating-  that  the  rocks  possessing 
them  have  solidified  at  depth. 

The  presence  of  very  numerous  dike-rocks  in  the  iron-bearing  belt  has 
become  apparent  through  exploring  aiid  mining  operations.  These  dikes 
are  much  altered  and  soft,  and  therefore  they  do  not  outcrop,  so  that  their 
existence  would  never  have  been  otherwise  suspecte<l.  In  almost  every 
mine  in  the  whole  district,  and  in  man}-  test  pits,  dike-rocks  have  been  en- 
countered. These  vary  greatly  in  thickness,  running  i'vom  but  a  few 
inches  to  90  feet.  The  positions  of  these  dike-rocks  are  given  in  detail  in 
the  discussion  of  the  origin  of  the  ores ;  and  here  it  is  only  necessary  to 
say  that  in  general  they  cut  across  the  iron  formation  nearly  at  right  angles, 
and  therefore  were  probabl}'  formed  before  the  uplifting  of  the  Penokee- 
Gogebic  series.  In  quite  a  number  of  cases  they  have  been  traced  into 
the  underlying  quartzite.  Frequently  the  places  of  passage  of  the  dike- 
rocks  from  the  iron  formation  into  the  quartzite  show  faulting  to  a  greater 
or  less  degree.  This  faulting  along  the  Hue  of  dikes  is  illustrated  by  PI. 
XXXI,  Fig.  6. 

In  the  Upper  slate  member  the  greenstones  outcrop  with  sufficient  fre- 
quency to  show  that  they  are  quite  plentiful,  but  it  has  not  been  possible 
to  determine  whether  they  are  interleaved  or  are  in  the  nature  of  dikes. 
It  is  not  at  all  unlikely  that  both  dike-rocks  and  sheets  occur.  Indeed  it 
would  almost  certainly  be  the  case  that  some  material  from  the  nu- 
merous dikes  which  pass  through  the  iron  belt  would  succeed  in  intruding 
itself  between  the  layers.  A  large  exposure  on  the  Wisconsin  Central 
Raih-oad,  in  the  NE.  i  of  the  NE.  i  of  Sec.  8,  T.  44  N.,  R.  2  W.,  Wiscon- 
sin, gives  indications  that  a  certain  amount  of  interleaving  has  taken 
place,  the  greenstones  and  slates  being  mingled,  however,  in  a  confused 
manner.  The  outcrop  occm-s  just  north  of  the  railroad,  and  in  passing 
eastward  mixed  greenstone  and  slate  appear.  There  are  several  belts  of 
almost  pure  slate  contained  in  the  greenstone,  but  after  a  time  the  slate  is 
continuous.  If  the  eruptive  is  a  great  dike,  it  may  be  that  slate  fragments 
have  been  caught  by  it ;  but  it  appears  more  probable  that  it  has  squeezed 
itself  between  the  layers  of  the  slate  adjacent  to  the  contact.  The  line 
between  the  Upper  slate  member  and  the  overlying  Keweenaw  series  is 


348  -     THE  PEHOKEE  IRON-BEARING  SERIES. 

very  irregular,  and  along  this  contact  line  at  a  number  of  places  the  slates 
and  greenstones  are  mixed  in  the  same  confused  manner  as  in  the  outcrop 
just  mentioned.  On  the  ground  it  is  difficult  at  these  places  to  say  at 
what  point  ,the  Penokee  series  ends  and  the  overljdng  eruptives  of  the 
Keweenaw  series  begin.  The  slates  are  cut  by  masses  of  greenstone  which 
seem  to  be  at  times  interljedded  and  at  other  times  in  the  nature  of  dikes. 
These  relations  are  what  would  be  expected  along  the  line  of  contact  of  a 
fragmental  series  and  an  overlying  erujjtive  one. 

General  character  of  the  rock. — The  igneous  matei'ial  of  the  Penokee  series 
has  all  unmistakably  been  at  the  time  of  its  intrusion  one  species  of  rock, 
diabase.  Occasionally  the  diabases  vary  toward  or  into  a  gabbro,  but  this 
is  exceptional,  and  all  plainly  belong  together.  The  degree  of  alteration  is 
very  great  at  times,  large  masses  of  rock  being  totally  decomposed ;  but 
even  the  most  altered  phases  so  frequently  retain  their  diabasic  structure 
and  can  be  traced  into  a  comparatively  fresh  rock  that  the  conclusion  is 
reached  that  they  all  are  altered  diabases.  The  eruptives  in  the  central 
part  of  the  Iron-bearing  member  (the  ore-producing  part)  are  those  which 
are  most  altered.  The  greenstones  in  the  Upper-slate  member  are  much 
fresher  upon  the  whole  than  those  contained  in  the  iron  belt ;  while  occa- 
sionally there  is  here  found  a  fresh  olivine  diabase  in  which  even  the  olivine 
is  mostly  unaltered.  As  there  is  every  reason  to  believe  that  the  rocks  here 
found  are  contemporaneous  with  the  dikes  contained  in  the  iron  belt  (are 
in  fact,  in  all  probability,  in  many  cases  but  continuations  of  them),  it  would 
seem  to  follow  that  the  great  difference  in  degree  of  alteration  in  the  two 
cases  is  due  to  the  difference  of  characters  of  the  rocks  in  which  the  dikes 
are  contained.  This  difference  in  alteration  is  interesting  in  connection  with 
what  has  previously  been  said  in  reference  to  the  origin  of  the  ores.  It  was 
noted  that  the  iron-bearing  belt  (particularly  in  the  part  which  bears  ores) 
is  much  more  readily  penetrable  by  percolating  waters  than  the  slate  belt. 
The  lower  horizon  iron  rocks  have  been  subject,  at  least  near  the  present 
surface,  to  a  long  series  of  changes  which  have  been  shown  to  be  due  to 
percolating  waters.  The  ready  penetrability  of  the  Iron-bearing  member 
by  percolating  water  has  also  been  favorable  to  the  alteration  of  the  inclosed 
greenstones.     On  the  other  hand,  the  Upper  slate,  composed  of  a  series  of 


THE  EEUrTIVES.  349 

clayey  rocks,  penetrable  with  great  difficulty  if  at  all  b)-  percolating  water, 
contains  greenstones  which  are  relatively  little  altered. 

Comparison  of  Penokee  greenstones  with  f/reenstoncs  of  the  Southern  Com- 
plex and  Keweenaw  series.— The  rocks  underlying  the  Penokee  series — the 
Southern  Complex — contain  here  and  there  exposures  wliicli  in  all  essential 
respects  are  like  the  diabases  of  that  series.  However,  parts  of  the  basic 
eruptives  of  the  older  succession  are  massive,  profoundly  altered  green- 
stones and  schistose  rocks  which  are  believed  to  have  been  greenstones, 
but  if  so  they  have  undergone  extreme  metaniorpliisni.  That  there  should 
be  eruptives  here  which  do  not  differ  in  degree  of  alteration  from  those 
contained  in  the  Penokee  series  is  what  would  be  expected,  for  through 
these  underlying  rocks  must  have  passed  the  dikes  which  traversed  the 
Penokee  series  itself,  and  doubtless  the  fresher  greenstones  represent  parts 
of  the  same  dikes  which  are  found  in  the  higher  series. 

The  Keweenawan  eruptives  differ  from  those  contained  in  the  Peno- 
kee series  in  their  great  variety  and  also  in  their  manner  of  alteration. 
The  immediately  overlying  rock  is  in  turn  a  coarse  grained  gabbro,  a 
diabase,  a  diorite  which  at  times  is  schistose  or  quartzose,  a  melaphyre,  an 
augite-porphyrite,  a  porphyrite,  and  an  amygdaloid.  Not  only  do  all 
phases  of  these  basic  rocks  here  occur,  but  in  the  same  locality  several 
may  be  found  closely  associated.  In  short,  nearly  all  of  the  phases  of  the 
basic  eruptives  of  the  Keweenaw  series  occur,  and  in  the  sudden  altei-ations 
from  one  phase  to  another  and  their  intricate  mingling  they  are  typical  of 
the  series  to  which  they  belong.' 

The  KcAveenawan  eruptives  above  the  Penokee  series  are  the  volcanic 
equivalents  of  the  Penokee  dikes  and  sheets.  Along  the  contact  line  of  the 
two  series  the  eruptives  are  sometimes  intricately  related.  These  facts 
strongly  suggest  that  the  Penokee  dikes  are  the  pipes  through  which  the 
volcanics  passed.  The  identical  lithological  character  of  the  dikes  and 
sheets  suggests  that  the  former  fed  the  latter.  We  are  thift  led  to  the 
hypothesis  that  all  of  these  eruptives  belong  to  a  single  period,  the  Ke- 
weenawan. 

'Rolaud  D.  Irving:  Monograph  U.  S,  GeoJ.  Survey,  vol.  v,  Copper-bearing  Epclss  of  Lake 
Superior, 


350  THE  PENOKEE  lEON-BEARlNG  SERIES. 

Microscopical  character  of  the  diabases. — The  diabases  usually  have  a 
well  developed  ophitic  structure,  the  augites  being  of  large  size  and  includ- 
ing many  somewhat  idiomorphic  lath-shaped  plagioclases.  In  the  diabases 
iii  which  this  structure  reaches  the  extreme  the  feldspars  have  a  tendency 
toward  two  generations,  there  being,  aside  from  the  smaller  lath-shaped 
plagioclases,  larger,  somewhat  porphyritic  appearing  ones.  The  rocks  vary 
from  ophitic  diabases  to  a  true  gabbro,  all  grades  of  variation  being 
observed.  In  the  passage  from  diabase  to  gabbro  the  feldspars  become 
broader;  the  pyroxene  includes  less  of  feldspar,  imtil  in  the  coarsest  grained 
rocks  the  sti'ucture  is  granitic  (granular)  and  the  contained  pyroxene  takes 
on  the  diallage  cleavage.  The  gabbro  occurs  in  only  a  few  localities  and 
is  of  little  importance  as  compared  with  the  diabases. 

The  original  minerals  are  apatite,  magnetite,  olivine,  plagioclase,  and 
monoclinic  and  orthorhombic  pyroxene.  The  latter  occurs  only  in  one  ex- 
posure, and  in  the  most  widespread  phase  of  rock  the  only  important  orig- 
inal minerals  are  magnetite,  plagioclase,  and  augite.  The  order  given  is 
that  of  crystallization.  In  some  of  the  rocks  this  succession  can  be  made 
out  with  a  good  deal  of  sharpness,  each  mineral  present  having  nearly 
completed  its  crystallization  before  the  succeeding  one  began  to  separate. 
This  is  particularly  true  of  the  ophitic  diabases  and  becomes  less  and  less 
true  in  passing  toward  tlie  gabbros. 

In  the  minerals  present  and  their  relations  there  is  nothing  particularly 
different  from  other  occurrences  of  diabase  in  the  Northwest  with  the 
exception  that  an  orthorhombic  pyroxene  is  found.  The  points  of  most 
interest  are  found  in  connection  with  the  alteration  which  the  rocks 
have  undergone.  As  has  before  been  said,  they  vary  from  an  almost 
perfectly  fresh  condition  to  one  in  which  none  of  the  original  minerals 
remain.  The  rock  in  which  the  alteration  has  gone  farthest  is  contained 
in  the  Iron-bearing  member,  and  as  this  peculiarly  altered  rock  is  different 
from  the  ordinary  altered  rocks  of  the  series  it  will  be  separately  considered. 

The  apatite  is  but  sparsely  present  in  most  of  the  rocks,  but  is  occa- 
sionally found  in  quite  plentiful  large,  well  formed  crystals.  It  is  always 
the  first  mineral  to  crystallize  and  has  the  usual  well  developed  crystal  out- 
lines. 


THE  ERTJPTIVES.  361 

Mag-uetite  or  inenaccanite  is  always  quite  abundant.  It  occurs  in 
small  crystals  or  rod-like  areas  in  the  finer  griiiiicd  diabase  to  grate-like 
forms  in  the  opliitic  diabases,  and  in  large  soli<l  grains  in  the  gabbro. 
Usually  it  has  to  a  greater  or  less  degree  crystal  outlines.  Very  frequently 
it  is  altered  to  gray  leucoxene,  proving  that  it  was  originally  menaccanite 
or  titaniferous  magnetite.  Some  of  the  grate-like  mingled  magnetite  and 
leucoxene  areas  are  very  regular.  More  ordinarily  the  magnetite  or  menac- 
canite is  but  little  altered.     It  is  very  often  Jissociated  with  secondary  biotite. 

The  olivine,  as  usual,  is  found  in  roundish  granules.  It  is  present  in 
comparatively  few  of  the  rocks,  and  these  the  fresher  ones,  but  in  one  case 
is  very  abundant.  It  may  have  been  a  constituent  of  the  much  altered 
phases,  but  if  so  it  has  left  no  evidence  of  its  presence.  It  is  always 
altered  to  a  certain  extent,  the  most  usual  resultant  product  being  pale 
gi-een  material,  which  is  taken  to  be  serpentine,  but  which  in  some  cases 
may  be  chlorite. 

Plagioclase  is  one  of  the  two  most  abundant  minerals  of  the  rock. 
Only  in  a  single  exposui'e,  that  containing  the  orthorhombic  pyroxene,  is 
it  subordinate  in  quantity.  It  is  always  striated  and  is  found  in  small  lath- 
shaped  to  large,  broad  individuals. 

The  feldspars  of  three  of  the  freshest  diabases  were  separated  from  the 
other  constituents  and  analyses  made.  In  only  one  of  these  cases  was 
there  two  feldspars  present.  No.  1  (specimen  12880)  is  from  a  diabase  from 
the  southeast  corner  of  Sec.  13,  T.  47  N.,  R.  46  W.,  Michigan;  No.  2  (speci- 
men 9656)  is  from  a  gabbro  from  the  center  of  the  south  half  of  Sec.  14,  T. 
44  N.,  R.  4  W.,  Wisconsin;  No.  3  A  and  B  (specimen  9108)  are  different 
feldspars  from  an  olivine  diabase  from  the  NE.  ^  of  Sec.  13,  T.  45  N.,  R. 
1  W.,  Wisconsin.  The  analyses  are  by  Mr.  Thomas  M.  Chatard  of  the 
chemical  laboratory  of  the  United  States  Geological  Survey. 


352 


THE  PENOKEE  IRO]Sr  BEARING  SERIES. 

Analyses  of  feldspars. 


HoOat  105° 

HjO  at  red  heat. 

SiOj 

AUO3 

rejOs 

FeO 

MnO 

CaO 

MgO 

K2O 

Na,0 


119 
51-18 
27-00 

3-19 
(*) 

•17 
11-70 

1-92 
■41 

3-48 


100-24 


■03 

-54 

51-99 

29-32 

1-23 

(*) 

trace. 

12-60 

■63 

■28 

2-91 


99.53 


3A 


-13 

•64 

56-15 

20-05 

1-98 

(*) 

•13 

8-70 

■54 

1-56 

4-79 


100-67 


■95 

01-65 

19-91 

2-28 

(*) 

trace. 

4-12 

■61 

5^72 

4^74 


99^98 


'  Undetermined. 


According  to  Zirkel's  Mineralogie,  p.  682,  AljAiio  has  the  following 
composition:  SiO^,  51-34;  AlA,  31-20;  CaO,  13-67;  NaA  3-79.  In  No. 
1  it  will  be  seen  that,  if  the  alumina  and  ferric  oxide  are  put  together, 
the  calcium  and  magnesium  counted  as  the  protoxide  base  equivalent  to 
calcium,  and  the  sodium  and  potassium  be  put  together,  this  analysis 
corresponds  almost  exactly  to  the  above  theoretical  calculation.  Therefore 
it  may  be  considered  as  very  close  to  AliAuo,  which,  under  the  present 
accepted  nomenclature,  makes  the  feldspar  labradorite. 

According  to  the  same  authority  AljAug  has  the  following  composi- 
tion: SiOs,  53-01;  AI0O3,  30-06;  CaO,  12-36;  NajO,  4-57.  The  composition 
of  AliAn,  has  just  been  given.  Combining  percentages  in  the  same  way 
as  before,  it  will  be  seen  that  No.  2  is  intermediate  in  composition  between 
AlaAuj  and  AliAua,  which  again  makes  the  feldspar  labradorite. 

From  the  same  authority  AUAug  has  SiOj,  57-37;  AI2O3,  27-12;  CaO, 
8-92;  NaaO,  6-59.  Making  allowance  for  the  water  contained  in  3A,  and 
combining  as  in  the  previous  case,  it  is  seen  that  this  analysis  corresponds 
very  closely  to  this  theoretical  composition,  which  would  place  this  feldspar 
at  the  acid  end  of  the  andesine  series.  The  analysis  of  3B,  and  particu- 
larly the  large  amount  of  potassium  which  it  contains,  would  seem  to 
indicate  that  we  have  here  probably  one  of  the  anorthoclase  series,  and  it 
is  possible  that  the  percentage  of  1-56  per  cent  of  potassium  in  3A  is 
accounted  for  by  a  mingling  of  anorthoclase  and  andesine.  It  is  to  be 
noticed  that  it  is  in  the  olivine  diabase — that  is,  the  rock  which  has  an 


THE  ERUPTIVKR.  353 

ahiimhincu  of  a  basic  iiiiiieial — that  the  most  acid  feldspar  is  found.  These 
determinations  are  from  tlie  freslier  rocks.  Usually  the  feldspar  is  con- 
siderably decom[)osed,  while  not  infrequently  some  unaltered  plagioclasc 
remains.  The  most  usual  alteration  of  the  feldspars  has  resulted  in  giving 
it  a  gray  tiu'bid  appearance,  which  is  taken  to  l)e  a  kaoliuitic  decomposi- 
tion. Fi-equently  the  inclosed  flakes  are  large  enough  to  give  brilliant 
]iolarization  colors.  Next  to  this  kaoliuitic  decomposition  in  frequency  is  a 
cliange  to  green  pleochroic  needles,  which  are  amphibole,  probable  variety 
smaragdite.  Independent  needles  of  this  kind  uot  only  occur  in  great 
abundance  throughout  the  feldspar,  but  others  have  penetrated  it,  coming 
from  the  enlargement  of  augite  and  paramorphic  hornblende.  In  some 
cases  the  feldspar  is  penetrated  in  every  direction  by  these  needles, 
so  that  but  little  of  the  original  material  remains.  At  times  a  green  chlo- 
ritic  substance  is  contained  within  the  feldspars,  within  which  is  developed 
epidote.  Not  infrequently,  also,  small  blades  of  secondary  biotite  are 
found  included. 

The  pyroxene  is  generally  of  the  augitic  variety,  although  in  the 
rocks  which  approach  gabbros  it  takes  on  a  diallage  parting.  It  has  a 
slight  pleochroism,  varying  from  colorless  to  veyy  pale  reddish  brown.  In 
no  section  is  the  pyroxene  entirely  unaltered,  while  in  many  sections  there 
only  remain  here  and  there  detached,  cores  of  the  original  mineral,  and 
not  infrequently  it  has  entirely  decomposed.  The  most  common  alteration 
is  to  ordinary  hornblende.  The  angle  c :  C  of  the  secondary  hornblende  is 
rather  high,  being  in  some  cases  more  than  20°.  Its  pleochroism  is  normal. 
Ordinarily  C  is  greenish  blue,  b  bluish  green,  and  a  pale  bluish  green  to  al- 
most colorless.  Less  frequently  C  is  yellowish  green,  b  greenish  yellow,  and 
a  a  light  yellow.  Rarely  the  alteration  is  to  typical  basaltic  hornblende. 
Here  C  is  dark  brown,  b brown,  and  a  a  light  yellow.  The  absorption  is  C  ^b 
>  a.  The  hornblende  is  usually  paramoi'phic,  a  single  large  individual  of 
hornblende  resulting  from  one  of  pyroxene.  The  rare  diallagic  variety  of 
pyroxene  has  generally  altered  to  a  green  fibrous  amphibole,  which  is  taken 
to  be  smaragdite.  But  the  most  interesting  fact  in  connection  with  the 
change  of  augite  to  amphibole  is   that  the  areas  now  found  are    at  times 

larger  than  were  the  original  augites;  that  is,  a  growth  subsequent  to  the 
MON  SIX 23 


354  THE  PENOKEE  lEONBEAEING  SERIES. 

consolidation  of  the  rock  has  occun-ed.  This  growth  is  much  better  iUus- 
trated  by  the  diabases  of  the  eastern  area  and  its  description  is  deterred  to 
chapter  viii. 

The  series  of  alterations  has  not  ended  in  the  change  from  pyroxene 
to  amphibole.  The  secondary  amphibole  has  to  qviite  an  extent  altered  to 
biotite.  The  biotite  is  as  clearly  secondary  to  amphibole  as  the  amphibole 
is  to  augite.  In  certain  rocks  in  the  Northwest  the  alteration  of  horn- 
blende to  biotite  occurs,  definite  crystallographic  relations  obtaining 
between  the  two;  but  in  the  Penokee  rocks  the  alteration  of  each  of  the 
amphibole  individuals  has  resulted  in  the  formation,  not  of  a  single  biotite, 
but  in  many  small  folia.  All  of  the  biotite  is,  however,  not  derived  from 
the  amphibole,  for  a  considerable  quantity  of  that  mineral  is  found  sur- 
rounding the  magnetite  areas,  and,  when  the  latter  occurs  in  grate-like 
forms,  filling  the  spaces  between  the  bars.  This  biotite  is  also  plainly 
secondary  and  is  in  the  nature  of  a  reaction  mineral,  the  magnetite  furnish- 
ing the  oxide  of  iron  necessary  for  its  formation,  while  the  surrounding 
minerals  have  furnished  the  remaining  constituents.  Magnetite  areas 
inclosed  wholly  within  feldspar  are  in  some  cases  surrounded  by  such 
secondary  biotite,  and  it  wofild  then  seem  that  the  plagioclase  has  largely 
furnished  the  material  for  the  formation  of  the  biotite. 

All  the  minerals  of  the  diabases  have  now  been  considered,  except  the 
rare  rhombic  pyroxene.  This  has  its  usual  fine  fibrous  rectangular  cleav- 
age, and  characteristically  includes  very  immerous  minute  black  particles 
along  its  parting.  It  is  somewhat  feebly  pleochroic  and  is  probably 
bronzite  or  hyperstheue.  It  has  a  tendency  to  occur  in  idiomorphic 
six-sided  forms,  showing  apparently  two  macropinacoids  and  the  four 
planes  of  the  domes.  It  is  quite  frequently  surrounded  by  reaction- 
ary or  intergrowth  rings,  which  consist  in  part  of  minute,  brilliantly  polariz- 
ing particles.  The  rhombic  pyroxene  is  in  strong  contrast  with  the  more 
abundant  colorless  or  pale  brown  augite.  The  augite  has  no  black  inclu- 
sions, does  not  ordinarily  show  rectangular  extinction,  and  has  a  coarsely 
prismatic,  nearly  rectangular  cleavage,  instead  of  a  finely  fibrous  rectan- 
gular one. 


TTir:  KRT^PTivES.  355 

Eruptiovs  in  the  [ron-haarinr/  member. — The  oruptives  contaiued  in  tlie 
iron  formation  ari'  mostly  iii  tlie  tonii  of  dikt'S.  Tlii«  is  particularly  true  of 
its  ceiiti-al  part,  but  in  T.  47  N.,  U.  45  W.,  Midiigau,  there  arc  eruptives 
which  are  a])pareutly  iuterleaved.  Tlie  great  abundance  of  these  dikes  in 
the  parts  of  the  iron  formation  which  have  been  cut  by  mining,  and  their 
relation  to  each  other  as  well  as  to  the  containing  formation,  are  shown 
by  Pis.  XXX  and  xxxi. 

Macroscopically,  the  dikes  vary  in  extent  from  a  coarse  diabase  to  a 
very  fine  grained  rock.  The  fact  that  the  greenstones  included  in  the 
iron  formation  are  as  a  whole  altered  to  a  much  greater  extent  than  the 
eruptives  elsewhere  in  the  Penokee-Gogebic  series  has  been  noted  in  a 
previous  chapter.  Even  those  which  are  very  much  altered  retain  the 
characteristic  structure  of  a  greenstone,  although  in  some  cases  the  decom- 
position has  gone  so  far,  or  else  the  rocks  were  originally  so  fine  grained,  that 
they  are  now  aphanitic.  The  greenstone  structure  is  usually  so  pronounced 
that  the  writer  in  the  field  had  little  doubt  as  to  the  real  nature  of  these 
rocks,  even  before  they  were  traced  into  the  less  altered  phases,  and  of  course 
before  a  microscopical  study  of  them  was  made.  In  several  mines  the  same 
dike  in  different  places  is  a  comparatively  little  altered  rock,  which  is  prop- 
erly a  diorite,  and  a  completely  altered  one,  which  is  a  typical  soapstone. 
The  great  Colby  dike  presents  a  good  instance  of  this,  its  major  portion 
being,  however,  extremely  altered.  The  color  varies,  depending  upon  the 
degree  of  alteration,  from  the  dark  greenish  gray  of  the  ordinary  diabase 
of  the  Peiiokee-Gogebic  series,  through  various  shades  of  dark  green  and 
light  green,  to  almost  snow  white.  The  more  common  phases  of  the  rock 
are  dirty  greenish  white.  All  are  colored  various  shades  of  brown  and  red 
upon  their  surface  and  along  the  cracks  by  iron  oxides,  and  oftentimes 
these  stains  have  penetrated  the  solid  dikes  to  a  considerable  distance.  As 
a  result  of  the  decomposition  the  diabases  have  become  very  soft,  so  that 
they  can  readily  be  scratched  by  the  finger  nail,  and  a  specimen  may  be 
broken  to  pieces  in  the  hands.  This  softness  has  not  resulted,  however, 
in  making  them  porous  or  less  compact  than  when  unaltered.  Their  non- 
penetrability  by  water  is  clearly  shown  by  the  fact  that  the  interiors  of 
the  dikes  are  in  the  main  unstained  by  iron  oxide,  which  has  colored  red 


356  THE  PENOKEE  IEOjST  BEARING  SERIES. 

everything  else  adjacent.  The  rocks  have  a  strong  soapy  feel,  and  because 
of  this,  and  the  fact  that  they  are  somewhat  hke  rocks  in  other  districts  asso- 
ciated with  iron  ore,  the  miners  have  given  them  the  name  of  soapstones. 
The^  soapstones  in  this  district  never  have  the  schistose  structure  and  seri- 
citic  appearance  presented  by  the  soapstones  of  the  Marquette,  VermiHon 
lake,  and  Menominee  districts.  This  difference  may  be  and,  indeed,  proba- 
bly is,  due  to  the  fact  that  in  the  latter  the  rocks  have  been  subject  to  pow- 
erful dynamic  forces. 

A  microscopical  study  shows  that  in  the  less  decomposed  phases  of  rock 
the  character  of  the  alteration  is  not  materially  different  from  that  already 
described  as  occumng  in  the  diabases  of  other  parts  of  the  series ;  that  is, 
the  augite  is  merely  altered  to  amphibole  and  the  feldspar  has  been  to 
some  degree  affected  by  gray  decomposition.  In  other  cases  the  alteration 
has  resulted  in  the  formation  of  green  chlorite  in  the  place  of  the  augite, 
the  feldspar  being  affected  the  same  as  in  the  previous  case.  When  the 
decomposition  has  proceeded  farther  there  appears  quite  often,  in  consider- 
able quantity,  a  brilliantly  polarizing  material  which  is  taken  to  be  a 
zeolite. 

When  the  decomposition  has  gone  very  far  even  the  magnetite  is 
mostly  or  wholly  pei'oxidized.  The  place  of  the  augite  and  feldspar  is 
largely  taken  by  a  colorless  or  pale  yellow  material.  Between  crossed  nicols 
this  substance  is  often  wholly  dark  or  dull  gray  and  does  not  change  its 
appearance  by  rotation.  At  other  times  it  polarizes  as  an  obscure  aggregate 
whose  color  and  appearance  are  somewhat  like  those  of  serpentine,  although 
analyses  indicate  that  that  mineral  is  not  present.  In  all  but  the  most  altered 
phases  some  chlorite  and  apparently  a  little  secondary  quartz  are  present. 
Abundantly  scattered  through  most  of  the  sections  are  particles  of  limon- 
nite,  hematite,  and  sometimes  magnetite.  When  the  rock  is  so  altered  that 
none  of  the  original  minerals  remain,  the  diabasic  structure  frequently  is 
imprinted  upon  the  homogeneous  almost  amorphous  material.  In  the  some- 
what rare  extreme  degree  of  alteration,  however,  all  definite  structure  has 
been  obliterated,  and  such  material,  if  examined  by  itself,  gives  no  indication 
whatever  of  its  origin.  That  these  soapstones  are  altered  diabases  is  shown 
by  the  facts:  that  in  certain  cases  the  same  dikes  exhibit  little  altered  and 


THE  EltUl'TlVEti. 


357 


extremely  altered  phases;  that  the  great  majority  of  the  soapstoues  retain 
a  distiuet  diabasic  structure ;  and  that  the  alterations  of  the  minerals  from 
theij"  fresh  condition  to  comi)lete  decomposition  are  traceable  in  all  its 
stages.  With  the  decay  of  the  rock  there  has  come  about,  as  would  be 
expected,  a  very  considerable  change  in  composition. 

The  analyses  below  are  by  Mr.  Thomas  M.  Chatard  of  the  chemical 
laboratory  of  the  United  States  Geological  Survey.  No.  1  (specimen  12880) 
is  a  fresh  diabase,  from  Sec.  13,  T.  47  N.,  R.  46  W.,  Michigan.  The  felds- 
par of  this  diabase  is  labradorite,  No.  1  of  page  352  being  from  this  rock. 
No.  2  (specimen  12878)  is  from  the  same  dike,  where  the  alteration  has  ex- 
tended to  a  middle  stage ;  and  No.  3  (specimen  12966)  is  typical  soap  rock 
from  the  Aurora  mine,  in  the  NE.  4  of  the  SW.  ^4  Sec.  23,  T.  47  N.,  R.  47 


W.,  Michigan. 


Analyses  of  diabases  and  soapstone. 


HjOatioso 

H2O  at  red  lieat. 

COj 

S03» 

P20S 

SiOj 

TiOj 

ALiOs 

Fe^Oa .^.... 

CrjOs- 

FeO 

NiO(CoO) 

MnO 

BaO 

CaO 

MgO 

K2O 

Na,0.. 


•15 

2-34 

•38 

•03 

•13 

47-90 

•82 

15^  60 

3-69 

trace. 

S^41 

•10 

■17 

■05 

9^99 

8^11 

•23 

2^05 


100-15 


3-12 

8-25 

1-89 

-06 

-16 

46-85 

1-12 

22-62 

5-12 


100-21 


■29 

13-54 

■38 


•14 

41^60 

3^79 

f  37-20 

3^21 


1^58 

-30 

•08 

2^54 

-08 

•10 

trace. 

1^25 

-23 

2-01 

-02 

2-66 

100-85 


*  SO3  calculated  from  BaO  foand,«3  this  latter  protaWy  exists  as  BaSoj. 
t  AI2O3  is  probably  a  little  liigh  owing  to  alkali  retained  by  titanic  acid. 

The  analyses  indicate  that  in  the  decomposition  of  these  rocks  the 
minerals  become  hydrated;  that  the  silica  lessens  in  quantity;  that  the  rela- 
tive proportion  of  alumina  is  largely  increased ;  that  the  calcium,  magne- 
sium, and  iron  protoxide  are  almost  wholly  removed ;  and  that  the  relative 
proportion  of  titanic  oxide  is  increased,  this  probably  being  due  to  the  fact 


358  THE  PENOKEE  lEON-BEAEING  SERIES. 

that  all  of  the  titanium  which  was  originally  present  in  the  magnetite  re- 
mains in  the  decomposed  rock.  The  percentage  of  silica  which  is  taken 
away  is  really  larger  than  is  indicated  by  the  difference  between  1  and  3, 
since  in  the  latter  so  large  a  proporton  of  water  is  present.  Pure  kaolinite, 
H4AI2 Si, Og  contains  SiOo,  46-50;  AI0O3,  39-66;  H^O,  13-94.  Disregarding 
the  small  amount  of  impurities  in  No.  3,  its  composition  corresponds  very 
closely  with  this  mineral.  It  seems  plain  that,  as  a  result  of  the  leaching 
action  to  which  the  lower  part  of  the  iron  formation  is  subjected,  the  dikes — 
that  is,  augite-plagioclase-magnetite  rocks — are  so  changed  that  their  compo- 
sition is  very  close  to  that  of  the  mineral  kaolinite ;  although  it  is  possible 
that  this  average  composition  is  due  to  several  important  minerals  rather 
than  to  a  single  one. 

That  the  diabase  dikes  high  up  in  the  Penokee  series  are  really  con- 
tinuations of  the  dikes  which  cut  nearly  at  right  angles  the  underlying  iron 
formation  there  can  hardly  be  a  doubt.  The  contrast  between  the  two  is  a 
striking  instance  of  the  influence  of  environment  upon  the  decomposition 
of  a  rock.  The  diabases  inclosed  by  the  impervious  Upper  slate  have 
been  kept  in  a  well  preserved  condition  through  the  ages  which  have 
elapsed  since  their  intrusion,  and  some  of  them  are  remarkably  fresh. 
Other  parts  of  the  same  dikes  in  a  formation  which  contains  evidences  of 
having  been  long  subject  to  the  action  of  percolating  waters  have  been 
completely  decomposed.  It  thus  appears  that  in  this  case  environment  has 
been  a  far  more  important  element  than  age  in  the  preservation  of  the  rock. 

Summary. — The  Penokee  eruptives  are  of  two  classes.  Dikes  cutting 
the  formations,  and  interbedded  sheets,  which  are  probably  intrusions  of  the 
same  age  as  the  dikes. 

The  eruptives  are  diabases,  which  occasionally  pass  over  into  gabbros. 

Diabases  in  every  respect  like  those  of  the  Penokee  series  are  found 
both  in  the  Southern  comj)lex  and  in  the  Keweenaw  series.  These  are  all 
presumably  of  the  same  age;  that  is,  Keweenawan. 

The  Penokee  diabases  are  in  all  respects  tj^pical  rocks  of  their  class. 

In  one  case  a  rhombic  pyroxene  of  some  interest  is  present. 

While  the  diabases  of  the  Upper  slate  member  are  often  quite  fresh, 
these  rocks  have  generally  undergone  an  extensive  series  of  alterations ; 


TIIK  EKUPTIVES.  359 

the  feldspars  lia\  ing  altered  to  or  been  replaced  by  kaolin,  chlorite  or  sma- 
ragdite ;  and  the  pyroxene  havinir  passed  over  into  liornblende,  biotite  or 
chlorite. 

The  alterations  have  extended  farthest  in  that  part  of  the  Iron-bearing 
member  containing-  the  g-reat  bodies  of  ore ;  that  is,  in  tliose  parts  of  the 
formation  which  have  been  subject  to  the  action  of  percolating  waters. 
This  alteration  has  gone  so  far  that  often  none  of  the  original  minerals 
remain.  All  traces  of  the  original  structure  of  the  rock  may  even  be  lost, 
although  frequently  the  diabasic  structure  remains  imprinted  upon  the 
homogeneous  almost  amorphous  material  which  results  from  the  complete 
alteration  of  the  rock. 

The  strong  contrast  in  the  characters  of  the  diabases  in  the  Upper 
slate  and  Iron-bearing  members  shows  that  environment  may  be  a  more 
important  element  than  age  in  the  preservation  of  a  rock. 


CHAPTER  VIII. 


By  0.  E.  Van  Hise. 


THE  EASTERN  AREA. 

Introduction. 

Section  I.     The  Iron-bearing  member. 

Distribution.     Petrographical  character.     Mingled  fragmental  and  nonfragmentai  sedimentation. 
Probability  of  ore  deposits  in  the  eastern  area.     Tabulation  of  petrographical  observations. 
Section  II.     Fragmental  rocks  south  of  the  Greenstone-conglomerated. 

Geographical  distribution.     Petrographical  character.     Tabulation  of  petrographical  observa- 
tions. 
Section  III.     The  Greenstone-conglomerates. 

Distribution.     General   characteristics.     Origin  of    the  Greenstone-conglomerates.     Tabulation 
of  petrographical  observations. 
Section  IV.     Fragmental  and  ferruginous  rocks  north  and  east  of  the  Greenstone-conglomerates. 

Geographical  distribution.     Surrounding  rocks.     Continuation  of  the  belt  east  and  west.     Struc- 
ture of  the  belt.     General  petrographical  cliaracter.     Mingled  fragmental  and  nonfragmental 
sediments.     Coarsely  fragmental  rocks.     Tabulation  of  petrographical  observations. 
Section  V.     The  Greenstones. 

The  main  area.     The  area  in  Sees.   20,  29,   and  30,  T.  47  N.,  R.  43  W.,  Michigan.     The  area  in 
Sees.  24,  13,  14,  and  15,  T.  47  N.,  R.  44  W.,  Michigan. 
Section  VI.     Stratigraphy. 

Litholoidcal  evidence  as  to  equivalence  with  the  main  Penokee  area.  Stratigraphical  eviclenc 
as  to  equivalence  with  the  main  Penokee  area.  Relations  of  the  belts  of  the  eastern  area  to 
each  other.  Great  width  of  parts  of  the  eastern  area.  The  southern  dips.  Sequence  of  events. 
Mingled  fragmental  and  nonfragmental  sediments.     Summary. 

INTRODUCTION. 

The  area  east  of  the  center  of  T.  47  N.,  R.  44  W.,  Michigan,^  or 
roughly  east  of  the  Little  Presque  Isle  river,  differs  from  the  simple  suc- 
cessions described  in  the  previous  chapters  in  many  important  points.  As 
will  be  seen  later,  the  differences  are  due  to  the  fact  that  this  area  was  the 
center  of  o-reat  contemporaneous  volcanic  activity.  Consequently  the  suc- 
cession includes  large  thicknesses  of  volcanic  tuffs  and  lava  flows.     These 

360 


THE  EASTERN  AREA.  361 

beds  are  not  itaralloli'd  hy  niiy  tlmt  an!  found  in  the  western  area.  P^'ur- 
tlier,  this  voU-anic  material  has  greatly  disturbed  the  normal  succession  of 
belts  in  the  district,  so  th.it  it  is  difficult  to  certainly  correlate  the  forma- 
tions east  of  the  Pres(|ue  Isle  with  those  west  of  it.  Another  point  in 
which  this  area  differs  from  the  western  area  is  that  in  one  place  the 
relations  of  the  horizontal  Eastern  sandstone  to  the  Penokee  series  can  be 
made  out.  The  subject  is  divided  into  the  following  sections:  The  Iron- 
bearing  member;  the  fragmental  rock  south  of  the  greenstone-conglomer- 
ates; the  greenstone-conglomerates;  the  fragmental  and  ferruginous  rocks 
north  and  east  of  the  greenstone-conglomerates;  the  greenstones;  stratig- 
raphy. 

SECTION  I.— THE  IRON-BEARING  MEMBER. 

Distribution. — The  rocks  naturally  and  artificially  exposed  east  of  the 
Little  Presque  Isle  river  which  can  certainly  be  referred  to  the  Iron-bear- 
ing member,  .although  quite  numerous,  do  not  form  as  continuous  a  belt  as 
do  similar  rocks  to  the  westward.  Through  the  east  part  of  T.  47  N.,  R 
44  W.,  Michigan,  and  the  west  mile  of  T.  47  N.,  R  43  W.,  Michigan,  the 
gaps  between  the  exposures  are  in  each  case  about  a  mile,  while  in  one 
instance  a  gap  of  2  miles  occurs.  East  of  this  latter  point  prospecting  has 
shown  rocks  of  the  Iron-bearing  member  to  be  practically  continuous  for 
about  4  miles;  i.  e.,  from  the  southwest  part  of  Sec.  20  to  the  northwest 
part  of  Sec.  23,  T.  47  N.,  R.  43  W.,  Michigan.  East  of  the  latter  point  no 
rocks  are  found  which  certainly  can  be  refeiTed  to  the  Iron-bearing 
member.  ^ 

In  the  easternmost  section  of  the  western  area  of  the  Iron-bearing  belt 
the  exposures  spread  over  a  horizontal  distance  greater  than  at  any  other 
locality  in  the  whole  Penokee  series,  extending  as  they  do  from  the  north- 
east part  of  Sec.  21  to  the  northwest  ^jart  of  Sec.  15,  T.  47  N.,  R.  44  W., 
Michigan,  a  distance  of  nearly  a  mile.  The  most  of  the  exposures  of  this 
section  are  south  of  a  huge  ridge  of  basic  eruptives,  some  of  them  being  in 
contact  with  this  rock.  North  of  this  eruptive,  and  about  a  half  mile  north 
of  the  nearest  exposure  of  iron-bearing  rocks  to  the  southward,  are  ex- 
posed by  a  test  pit  rocks  which  undoubtedly  belong  to  the  Iron-bearing 
member.     The    only   other  locality  where  the  thickness  of  the  belt   ap- 


362  THE  PENOKEE  IRON-BEAEIIfG  SERIES. 

proaches  that  hei'e  attained  is  west  of  Sunday  lake,  and  here  again  the 
apparent  thickness  is  in  part  and  perhaps  largely  due  to  eruptives.  For 
the  distance  between  these  two  points  the  Keweenaw  series  directly  overlies 
the  Iron-bearing  membei'.  In  the  eastern  area  the  rocks  belonging  to  the 
Iron-bearing  member  constitute,  as  far  as  at  present  known,  a  narrow  belt, 
narrower  than  anywhere  to  the  west.  The  causes  of  this  change  from 
extreme  width  to  extreme  narrowness  will  be  discussed  later. 

Petrograpliical  character. — The  locations  of  the  exposures  belonging  to 
the  iron  formation  are  found  upon  PI.  xrii  and  indicated  in  the  tabulations 
to  follow.  The  kinds  there  found  include  nearly  every  phase  of  rock 
characteristic  of  the  Iron-bearing  member  west  of  the  Presque  Isle.  This 
likeness  of  the  rocks  east  and  west  of  this  stream  is  such  that  no  question 
can  be  entertained  as  to  their  identity  of  character  and  origin.  A  general 
discussion  of  the  original  nature  aiid  subsequent  modifications  of  the  rocks 
of  the  Iron-bearing-  member  has  been  given  in  another  place  and  need  not 
be  here  repeated.  It  is,  however,  worthy  of  note,  that  in  some  of  the 
ferruginous  cherts  in  T.  47  N.,  R.  43  W.,  Michigan,  are  found  very  numer- 
ous small  geodic  cavities  which  are  lined  with  quartz  crystals.  That  this 
quartz  is  of  a  secondary  nature,  or  at  least  has  been  rearranged  since  the 
rock  was  originally  formed,  can  hardly  be  doubted.  The  close  association 
of  siderite,  magnetite,  hematite,  actinolite,  and  quartz  is  finely  shown  by 
one  exposure.  The  relations  here  are  such  as  to  indicate  that  the  mag- 
netite has  formed  directly  from  siderite,  as  has  hematite  so  extensively  in 
the  iron-bearing  belt  to  the  west.  Further,  the  actinolite  appears  where- 
ever  quartz  is  found,  while  the  quartz  present  in  the  section  is  in  irregular 
veinlike  forms  cutting  across  the  lamination.  The  conclusion  is  that  this 
quartz  is  secondary,  and  that  at  the  time  it  formed  a  portion  of  the  silica 
in  solution  united  with  the  bases  present — calcium,  magnesium,  and  iron — 
to  form  the  actinolite.  We  have  here,  then,  another  reenforcement  of  the 
argument  given  for  the  derivation  of  the  actinolitic  slates  from  an  original 
clierty  carbonate. 

Mingled  fragmental  and  nonfragmental  sedimentation. — The  one  impor- 
tant point  in  which  the  iron-bearing  rocks  east  of  the  Little  Presque  Isle 
differ  from  those  to  the  westward  is  that  thej^  are  interstratified  with  a 


TIIK   EASTEIJN  AREA.  3(53 

greater  or  less  (luaiitity  of  mechanical  sediments.  This  minf^lino-  of  fni'j;- 
mentiil  and  nctnfragmental  sediments  has  occnrrcfl  so  extensively  in  the 
eastern  area,  that  a,  new  color  is  us(h1  npon  Pis.  ii  and  xiii  to  desi<Jiiatc 
this  additional  phase  of  rock.  'I'liis  color  is  intermediate  between  the 
yellow  of  the  Qnartz-slate  member  and  the  l)rown  of  the  Iron-bearing 
mend)tir.  It  is  thus  chosen  because  the  ledg-es  so  marked  contain  a  lary-e 
amount  of  Ixtth  fragmental  and  nonfragmental  material,  and  are  therefore 
intermediate  between  the  two  iiiembers.  This  new  characteristic  of  the 
rocks  of  the  series  is  noted  as  far  we^t  as  in  the  west  part  of  Sec.  17,  T. 
47  N.,  R.  44  W.  Here  are  cherts  and  jaspers  which  contain  a  varying  and 
even  a  large  amount  of  fragmental  quartz  and  feldspar  (PI.  xxxv,  Fig.  1). 
Apparently  interstratified  with  these  rocks  are  clay -slates  which  are  largely 
raechaiiical  sediments,  although  it  is  difficult  or  impossible  to  determine 
exactly  what  proportion  of  the  very  fine  material  in  these  slates  is  frag- 
mental and  what  nonfragmental. 

In  Sees.  19,  20,  21,  22,  and  23,  T.  47  N.,  R.  43  W.,  Michigan,  the  iron- 
bearing  rocks  have  been  located  in  many  places  by  explorations,  but  the 
numerous  test  pits  there  sunk  have  as  often  struck  the  soft  chloritic  slate 
as  the  iron-bearing  rocks  proper.  The  cross-cuts  from  these  test  pits 
clearly  show  that  in  the  distance  north  and  south  through  which  the  iron- 
bearing  rocks  occur  there  are  also  several  or  many  intermediate  layers  of 
clay-slates  or  other  partially  fragmental  rocks,  and  in  some  cases  there  is 
interstratified  with  the  iron-bearing  rocks  a  vitreous  quartzite.  The  succes- 
sion of  rocks  in  Sec.  21  is  as  follows:  At  the  south  are  well  developed 
exposures  which  belong  to  the  feldspathic  quartz-slates,  the  exposures  con- 
taining both  the  characteristic  variegated  slates,  and  above  these  next  to 
the  iron  formation  a  vitreous  quartzite.  To  the  north  follow  a  series  of  inter- 
laminated  cherty  ore  formation  rocks  and  chloritic  slates  of  some  thickness. 
Farther  to  the  north  in  a  test  pit  is  shown  vitreous  quartzite,  the  particles 
of  which  are  chiefly  large  grains  of  quartz  which  have  had  a  second 
growth.  Still  farther  to  the  north  is  again  found  lean  cherty  ores,  and 
again  north  of  these,  after  passing  an  interval  of  some  distance  which  is 
unexplored,  is  found  a  wide  belt  of  ferruginous  fragmental  slates.  Where 
the  slates. beleng  in  the  succession  will  be  discussed  later;  but  disregarding 


364  THE  PBNOKEE  IRON- BEARING  SERIES. 

tlie  last  mentioned  rock,  there  is  here  found  several  interlaminated  belts  of 
fragmental  and  nonfragmeutal  sediments  at  a  horizon  which,  from  its  rela- 
tions to  the  Quartz-slate  member  and  the  character  of  a  portion  of  the 
rocks  contained,  must  be  considered  as  belonging  to  the  iron-bearing  for- 
mation. 

In  the  southern  part  of  Sec.  20,  T.  47  N.,  R.  43  W.,  Michigan,  iron- 
belt  rocks  occur  over  a  distance  of  about  one  quarter  of  a  mile  north  and 
south.  The  first  explorations  were  carried  on  in  the  extreme  southern 
part  of  the  section.  Here,  resting  upon  a  basic  eruptive  flow,  is  a  con- 
glomerate the  matrix  of  which  is  largely  nonfragmental.  This  peculiar 
conglomerate  is  considered  fully  in  another  connection  and  need  not  at 
present  be  noted  further.  '  Above  this  conglomerate  there  is  within  a  few 
paces  of  one  another  ankerite,  chloritic  and  quartzose  siderite,  and  actino- 
litic  quartz  rock.  This  occurrence  well  illustrates  the  mingling  of  frag- 
mental and  nonfragmental  sediments,  the  quartz  of  the  quartzose  siderite 
being  fragmental.  Also  here,  in  striking  juxtaposition,  are  unaltered  car- 
bonate and  completely  altered  actinolitic  slates.  Other  rows  of  test  pits, 
running  north  and  south,  extend  for  some  distance  east  and  west  from 
the  east  line  of  the  section  to  three-quarters  of  a  mile  west  of  that 
line.  Each  of  these  rows  shows  interlaminated  layers  of  purely  non- 
fragmental sediments,  which  are  typical  feiTuginous  cherts  and  mixed  ores, 
and  layers  of  chloritic  slates,  which,  while  very  fine  grained,  are  in  some 
cases  cei'tainly  partly  fragmental,  and  are  believed  to  be  mainly  a  finely 
divided  mechanical  sediment.  A  westward  extension  of  this  belt,  as  judged 
by  its  course  to  the  east  (PI.  xiii),  would  carry  it  through  the  extreme 
south  part  of  Sec.  19  and  the  north  part  of  Sec.  30,  T.  47  N.,  R.  43  W. 
This  expectation  of  a  westward  extension  of  the  belt  is  further  indicated 
by  exposures  of  a  westward  continuation  of  the  basic  eruptive  flow  found 
in  Sees..  18  and  29.  The  only  outcrops  of  rock,  however,  which  are  here 
known  to  occur  cannot  be  referred  to  the  Iron-bearing  member,  but  must 
be  placed  in  the  intermediate  phase  of  rock  referred  to  above,  being 
about  one-half  a  fragmental  and  one-half  a  nonfragmental  sediment.  At 
the  exposures  in  the  north  part  of  Sec.  30  the  mechanical  sediment  is  pre- 
ponderant, while  in  the  exposures  in  Sec.  1 9  the  rock  is    a  ""conglomerate 


THE  EASTEKN  AREA.  365 

wliicli  fouttiius  tVaj^iiionts  of  jaspur,  ([uartz,  and  feldspar  iiiterlaiiiiiiated 
with  narrow  seaiu.s  of  jasper  wliicli  appear  to  he  iioiifragmental.  It  is 
of  course  possil)le  that  the  iron-bearing-  rocks  proper  occur  in  Sees.  1!) 
and  30,  but  expUirations  have  not  yet  developed  the  fact.  So  far  as  we 
are  able  to  judge  by  present  knowledge  the  belt  which  in  Sees.  20  and 
29  is  largely  a  nonfragmental  sediment  becomes  to  the  west  in  the  next 
sections  largely  a  mechanical  sediment. 

ProhaUlity  of  ore  deposits  in  the  eastern  area. — We  are  now  in  a  position 
to  judge  as  to  the  probability  of  finding  iron  ore  in  paying  quantities 
ixi  the  eastern  area.  The  position  of  the  ore  deposits  in  the  main  area  to 
the  west  and  the  conditions  which  favor  their  concentration  have  been 
discussed.  It  will  be  remembered  that  they  in  general  rest  upon  a  frag- 
mental  foot-wall  quartzite,  and  that  the  concentration  has  been  possible 
because  of  the  penetrability  of  the  layers  of  the  formation  by  percolating 
waters  combined  with  the  imijervious  character  of  the  belts  of  rock  north 
and  south  of  the  Iron-bearing  member,  as  well  as  the  presence  in  a  pecu- 
liarly favorable  condition  of  numerous  impervious  dikes.  In  the  eastern 
area  it  is  evident  that  none  of  these  conditions  are  found.  In  the  first 
place  the  original  iron  carbonate,  instead  of  being  deposited  in  a  single 
homogeneous  belt  800  feet  in  thickness,  is  scattered  through  a  much  greater 
thickness  of  rock,  part  of  which  is  more  largely  a  fragmental  than  non- 
fragmental sediment.  Second,  the  different  layers  of  the  iron-bearing  rock 
which  are  purely  nonfragmental  are  separated  from  one  another  by  almost 
impervious  layers  of  chloritic  and  clay-slates  or  eruptive  outflows;  so  that 
it  is  not  possible  for  the  concentration  to  have  gone  on  as  a  unit,  as  in  the 
western  area.  Instead  of  having  a  single  impervious  basement  for  the  non- 
fragmental rocks  we  have  several  or  many  such  basements.  In  each  one 
of  these  narrow  belts  there  could  have  been  but  a  scant  supply  of  iron 
carbonate  from  which  iron  ore  deposits  could  have  formed.  It  would  thus 
follow  that  if  concentration  has  taken  place  in  these  belts  the  deposits 
there  formed  would  be  of  small  size.  Again,  many  of  the  rocks  of  the 
iron-bearing  belt  in  the  eastern  area  belong  to  the  actinolitic  slate  type. 
It  has  been  already  noted  that  in  the  main  area,  where  the  ore  formation  is 
mainly  of  this  type  of  rock,  no  ore  deposits  have  been  discovered.     Finally, 


366  THE  PENOKEE  IRON-BEAEING  SERIES. 

at  the  base  of  the  belt  itself  we  have  no  proof  that  there  is  a  continuous 
layer  of  feldspathic  quartz-slates,  as  is  the  case  to  the  west. 

It  is,  then,  improbable  that  the  eastern  area  will  in  the  future  be  found 
to  contain  large  ore  deposits  although  small  ore-bodies  may  be  found. 
This  improbability,  arrived  at  inductively,  is  still  further  reenforced  by 
the  fact  that  extensive  explorations  throughout  its  whole  extent  have  failed 
to  find  a  single  workable  ore  deposit. 

TABULATION   OF   PETKOGRAPHICAL   OBSEKVATIONS.' 

1.  Magnetitic,  sideritic,  aud  actinolitic  slate.  Specimens  12696  (slide  5416), 
12697,  (slide  5417),  1000  N.,  18.50  W.,  Sec.  23,  T.  47  K,  R.  44  W.,  Michigan. 

The  rocks  are  dark  gray  to  black,  mostly  fine  grained,  laminated,  the  lighter 
colored  belts  being  coarser  grained  than  the  others. 

In  thin  sections  a  backgronnd  of  finely  crystalline  interlocking  quartz  contains 
abundant  magnetite,  siderite,  actinolite,  and  some  hematite.  Both  the  magnetite  and 
siderite,  in  certain  belts,  exclude  the  other  minerals.  The  light  gray  belts  seen 
macroscoi)ically  are  very  largely  siderite.  These  sections  illustrate  the  simultaneous 
occunence  of  quartz,  magnetite,  hematite,  actinolite,  and  siderite,  as  well  as  any  thin 
sections  from  the  Penokee  district.  The  magnetite  and  actinolite  are  closely  associated, 
as  is  usual  in  the  actinolitic  slates;  also  in  the  almost  solid  masses  of  siderite; 
wherever  quartz  occiu's,  actinolite  is  abundantly  present.  The  quartz  is  mostly  in 
vein-like  forms  as  though  it  were  a  secondary  mineral.  This  association  of  actinolite 
with  the  quartz  suggests  that  the  entrance  of  silica  was  necessary  in  order  that  the 
actinolite  might  form.  The  quartz  in  slide  5417  differs  from  that  found  in  any  other 
section  of  the  series  in  that  the  individuals  are  several  times  longer  than  broad.  The 
elongation  of  the  grains  cdrrespouds  to  the  banding  of  the  rock.  This  arrangement 
and  interlocking  of  the  quartz  grains  are  those  of  a  much  squeezed  crystalline  schist. 

2.  Magnetitic  actinolite  rock.  Specimen  10403  (slide  4494),  1164  N.,  1779  W., 
Sec.  23,  T.  47  N.,  R.  44  W.,  Michigan. 

In  thin  section,  an  interlocking  felted  mass  of  actinolite,  stained  red  by  hema- 
tite, contains  numerous  crystals  and  areas  of  magnetite,  the  projecting  edges  of 
which  show  crystal  outlines.     A  little  finely  crystalline  quartz  is  also  included. 

3.  Ferruginous  actinolite-schist.  Specimens  9354  (sl'de  3308),  9355  (slide  3119), 
9356  (Slide  3268),  300  N.,  1500  W.,  Sec.  25,  T.  47  N.,  R.  44  W.,  Michigan. 

The  rocks  are  dark  green,  fine  grained,  banded,  and  magnetitic. 

'  The  numbers  of  specimens  and  slirles  are  those  of  the  collection  of  the  Lake  Superior  division. 
Locations  are  given  from  the  southeast  corner  of  the  sections  iu  steps  of  2,000  per  mile. 


TiTK  i;astkhn  area.  3(57 

Actinolite,  chlorite,  qiuirt/,,  iiiiignetito,  heinutitc,  iiml  hiowii  iron  oxide  ar«  all 
found  in  tlie  thin  .sections,  bnl  iron  staineil  actinolite  is  |M-('d(nniniinl.  Tlii^  alteration 
of  aetinolite  has  foiined  tiie  chlorite.  The  sections  resemble  closely  those  ui' "t'-i  in  the 
iron  member  west  of  the  Presciue  Isle,  p.  244. 

4.  Hematitic  and  magnetitic  actiuolito-schist.  Specimeus  9201  (slide  ;i011), 
1)292  (slide  ;U07),  1270  K.,  177.")  W.,  Sec.  30,  T.  47  N.,  11.  43  W.,  Michigan. 

These  rocks  arc  tiner  grained  than  those  of  3,  but  are  otherwise  like  them. 

Tiie  minerals  present  and  their  relations  are  the  same  as  in  the  previous  iiumber. 
The  ])redominant  actinolite,  which  is  largely  arranged  in  sheaf-like  forms,  is  every- 
where heavily  stained  with  brilliant  red  hematite.    Magnetite  iii  crystals  is  abundant. 

5.  Hematitic  cherts.  Specimens  12581  (slide  5341),  500  N.,  1000  W. ;  12582 
(slide  5342),  533  N.,  810  W.,  Sec.  20,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rocks  are  composed  of  alternate  belts  of  white  or  gray  chert  and  of  red 
or  brown  iron  oxide. 

In  thin  sections  a  groundmass  of  finely  crystalline,  with  peihaps  some  amorphous 
silica,  contains  both  hematite  and  limonite,  which  are  largely  concentrated  in  roughly 
Ijarallel  belts. 

6.  Ankerite.  Specimen  9264  (slide  4487),  75  N.,  600  W.,  Sec.  20,  T.  47  N.,  R.  43 
W.,  Michigan. 

The  rock  is  light  gTay,  fine  grained,  massive,  and  breaks  with  conchoidal  frac- 
ture. It  sparingly  contains  pyrite  in  small  crystals  and  thin  layers  of  almost  pure 
magnetite.  The  weathered  surface  is  brown,  from  the  presence  of  peroxide  of  iron. 
The  following  is  an  analysis  made  by  Mr.  W.  F.  Hillebrand:  SiOs,  3-16,  TiOz,  none; 
AljOs,  0-08;  Fe^Os,  0-93,  FeO,  15-18;  MnO,  1-15;  CaO,  2G-65;  MgO,  11-01;  H2O,  0-54; 
CO^,  41-10;  P2O5O-O6,  CI,  trace;  FeS,  0-34.    Total,  100-20. 

The  thin  section  consists  almost  wholly  of  an  interlocking  mass  of  finely  but 
perfectly  crystalline  ankerite.  Contained  as  accessories  in  this  are  small  crystals  of 
pyrite,  magnetite,  and  small  clusters  of  radiating  needles  of  much  altered  actinolite. 

7.  Actinolitic  chert.     Specimen  9265  (slide  4488),  75  N.,  600  W.,  Sec.  20,  T.  47 
K,  R.  43  W.,  Michigan. 

The  rock  is  light  olive  green,  tine  grained,  and  massive. 

The  thin  section  consists  chiefly  of  the  two  minerals,  quartz  and  actinolite. 
The  quartz  makes  a  finely  crystalline  interlocking  groundmass,  in  which  the  actinolite  is 

■  contained  in  clusters  of  small  radiating  blades.    A  little  brown  ferrite  and  a  few 

■  crystals  of  magnetite  are  seen.  The  section  is  cut  by  a  vein  which  is  composed  mostly 
of  comparatively  coarsely  crystalline  quartz,  including  almost  indefinitely  numerous 
minute  long  needles  of  actinolite,  which  are  arranged  parallel  to  one  another. 

8.  Quartzose  siderite^    Specimens  9266  (slide  4489),  9267  (slide  ,4490),  100  N., 
600  W.,  Sec.  20,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rocks  are  dark  green,  rather  fine  grained  and  finely  laminar. 


368  THE  PENOKEE  lEOI^-BEAEING  SERIES. 

The  thin  sections  contain  a  matrix  of  siderite,  with  some  quantity  of  finely  crys- 
talline quartz,  in  which  are  bnried  plentifully  rather  small  fragmental  grains  of  quartz 
and  numerous  areas  of  pale  green  fibrous  chlorite.  A  little  brown  iron  oxide  and 
magnetite  are  found.  These  sections  are  intermediate  between  the  nonfragmental 
rocks  of  the  iron-bearing  belt  and  the  fragmental  slates,  and  might  with  almost  equal 
propriety  be  classed  with  one  division  as  with  the  other. 

9.  Hematitic  chert.     Specimen  7402  (slide  1865),  1320  N.,  615  W.,  Sec.  21,  T.  47 
IS.,  R.  43  W.,  Michigan. 

The  rock  is  red,  aphanitic,  and  breaks  with  a  conchoidal  fracture. 

The  thin  section  is  composed  mostly  of  finely  crystalline  quartz.  This  includes 
hematite  in  finely  disseminated  particles  and  in  areas  of  considerable  size.  The  iron 
oxide  and  chert  are  arranged  to  quite  an  extent  in  more  or  less  perfect  concretions. 

10.  Siliceous  hematites.  Specimens  7384  (slide  1850),  1000  N.,  75  W.,  9250  (slide 
4483),  1210  N.,  450  W.,  Sec.  21,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  a  cherry  red  and  black,  banded,  nearly  pure  hematite. 

In  thin  section,  a  continuous  ramifying  mass  of  hematite  contains  numerous 
complex  cherty  areas. 

SECTION  II.— FRAGMENTALT  ROCKS  SOUTH  OF  THE  GREENSTONE-CONGLOM- 
ERATES. 

Geographical  distribution. — South  of  the  Iron-bearing  member,  through- 
out all  the  main  area  and  constituting  one  of  the  most  characteristic 
features  of  the  Penokee  series,  is  the  Quartz-slate  member.  The  eastern- 
most exposure  of  these  slates  in  this  area  is  in  the  north  part  of  Sec.  21, 
T.  47  N.,  R.  44  W.,'  Michigan.  East  of  this  point  there  is  a  break  of  about 
4  miles,  which,  so  far  as  at  present  known,  is  not  bridged  by  a  single 
exposure  of  a  jilainly  fragmental  rock  referable  to  this  belt.  In  the  south- 
west part  of  Sec.  19,  T.  47  N.,  E.  43  W.,  the  first  exposure  east  of  the 
Little  Presque  Isle  is  found  which  can  be  regarded  as  the  equivalent  of 
the  Quartz-slate  member.  From  this  point,  running  somewhat  north  of 
ea,st  to  the  northwest  part  of  Sec.  23,  T.  47  N.,  R.  43  W.,  are  quite  numer- 
ous exposures  south  of  and  mingled  with  the  iron-bearing  belt  which  have 
in  places  great  likeness  to  or  even  lithological  identity  with  the  Quartz- 
slate  formation  to  the  westward.  In  Sec.  21,  T.  47  N.,  R.  43  W.,  just  east 
of  the  Presque  Isle  river  and  in  the  east  part  of  the  section,  occur  large 
exposures  of  slates  and  quartzites  which  in  every  way  are  lithologically 
like  the  feldspathic  quartz-slates  of  the  main  area.     Not  only  are  these 


'riiio  EASTKitN  ai;f-a.  3H9 

rocks  similar,  l)ut  their  arraii<^eni(^nt  with  roforeiu-e  to  cacli  other  is  the 
same  as  those  to  the  west.  The  soutiienmidst  layers  are  g'reeii  and  hrowti 
varieg-atetl  slates  interleaved  with  layers  ut'  line  grained  vitreous  t'eldspatliic 
quartzite,  just  as  shown  by  the  lower  layei-s  of  the  (juartz-slates  at  the 
tvpical  localities  of  Tylers  fork,  Potato  river,  and  the  wesr  branch  of  the  ■ 
Montreal.  Above  these  variegated  slates  are  thick  beds  of  coai'se  vitreous 
quartzite  which  outcrop  in  bold  exposure.  A  very  shoi-t  distance  north  of 
the  quartzites  test  pits  have  shown  that  the  iron-belt  i-ocks  occur.  Thus 
far  at  this  point  we  have  the  typical  Penokee  succession,  l)ut  test-pitting 
shows,  as  heretofore  explained,  that  the  iron-bearing  rocks  are  mingled  to 
a  g'reater  or  less  extent  with  fras>'mental  material.  East  and  west  of  Sec. 
21,  T.  47  N.,  R.  43  W.,  Michigan,  are  fragmental  rocks,  which  are,  however, 
not  a  separate  belt  below  the  iron  formation,  but  are  intercalated  with  its 
nonfragmental  sediments. 

Petrofjrajiliical  cliaracter. — It  is  not  practicable  to  separate  these  frag- 
mental rocks  into  sharply  detined  divisions,  as  the  different  phases  grade 
into  each  other  by  insensible  degrees.  They  can  be  somewhat  arbi- 
trarily divided  into  quartz-slates  including  quartzite,  and  ferruginous  feld- 
spathic  quartz-slates.  As  to  this  first  division  nothing  more  need  be  said, 
as  they  are  precisely  like  rocks  of  the  same  name  in  the  Quartz-slate  mem- 
ber to  the  west.  The  second  division  comprises  the  following  varieties: 
Ferruginous  and  feldspathic  (juartzite,  sometimes  conglomeratic,  jasper- 
conglomerate,  and  ferruginous  chlorite-slates. 

The  ferruginous  feldspathic  quartzites  differ  from  tlie  quartzites  found 
in  the  main  quartz-slate  area  to  tlie  west  in  that  tljey  contain  a  very 
large  amount  of  iron  oxide,  which  is  mostly  hematite,  but  mingled  with 
limonite  and  some  magnetite.  In  some  specimens  the  hematite  is  so 
abundant  as  to  form  a  continuous  ramifying  slieet  in  which  is  buried  tlie 
worn  fragments  of  quartz  and  feldspar.  These  fragments  at  several 
exposures  are  so  large  as  to  class  the  rock  as  a  conglomerate. 

The  exposures  shown  by  the  test  pits  in  the  soutli  part  of  Sec.  20,  T.  47 

N.,  R.  43  W.,  Michigan,  are  of  peculiar  interest  because  of  their  relations  to 

the  underlying  eruptive,  and  the  great  likeness  under  the  microscope   of 

some  of  them  to  a  large  part  of  the  greenstone-conglomei'ate,  to  be  later 

MON  XIX 24 


.370  THE  PENOKEE  lEON-BEAEING  SEEIES. 

described.  Just  south  of  the  place  of  their  occurrence  is  a  high  east  and 
west  ridge  of  diorite-porphyrite,  which  is  a  part  of  a  flow  outcropping  at 
various  places  for  a  distance  of  two  miles  east  and  west.  At  "the  northern 
foot  of  this  hill,  dipping  to  the  north,  is  the  jasper-conglomerate,  having  a 
slaty  matrix.  The  specimens  from  the  test  pits  a  few  paces  to  the  north 
are  the  nonfragmental  sediments  of  the  iron  belt.  In  the  matrix  of 
this  "jasper-conglomerate  are  very  numerous  irregular  compact  frag- 
ments, which  contain  tabular  plagioclases  and  which  ajjpear  to  be  frag- 
nients  of  the  fine  grained  basic  eruptive  just  to  the  south.  These 
fragments  are  often  vaguely  defined;  they  are  extraordinarily  irregular  in 
form;  they  are  very  much  altered.  Besides  these  complex  .fragments  there 
are  found  quite  numerous  angular  pai-ticles  of  feldspar  of  moderate  size. 
These  may  have  been  furnished  by  the  porphyritic  plagioclases  of  the 
tmderlying  porphyrite.  These  two  varieties  of  fragments  are  cemented  by 
a  groundmass  which  consists  largely  of  cherty  silica.  This  silica  is  around 
and  between  tlie  fragments  in  narrow  belts,  just  as  it  is  found  in  a  wide- 
spread phase  of  the  greenstone-conglomerate.  Contained  in  the  above 
matrix  are  very  numerous  angular  blood-red  jasper  pebbles  of  varying 
sizes,  some  of  them  being  large  enough  to  be  classed  as  bowlders.  These 
bright  red  jasper  pebbles  give  the  rock  a  very  striking  appearance.  The 
only  essential  difference  between  this  conglomerate  and  certain  phases 
of  the  greenstone-conglomerates  is  in  the  presence  of  these  jasper  pebbles, 
and  whether  it  ought  to  be  classed  here  or  with  the  greenstone-conglom- 
euates  is  a  somewhat  doubtful  question  It  is  placed  here'  because  it  is 
certainly  a  water-deposited  fragmental  rock,  and  is  also  certainly  below 
the  rocks  of  the  iron-bearing  belt. 

The  third  phase  of  rock  is  the  chloritic  and  clay-slates,  which  are 
plentifully  interstratified  with  the  nonfragmental  iron-bearing  sediments  in 
Sees.  20,  21,  22,  and  23,  T.  47  N.,  R.  43  W.,  Michigan.  Macroscopically, 
these  rocks  are  soft,  green  or  brown,  aphanitic,  finely  lannnated  ones. 
Their  constituents  are  difiicult  to  make  out  with  certainty.  Some  of  the 
chief  ones  are  quartz,  chlorite,  sericite,  brown  ii-on  oxide,  pyrite,  and 
perhaps  kaolin.  How  far  these  rocks  are  fragmental  and  how  far  nonfrag- 
mental sediments  it  is  difficult  to  determine,  so  fine  grained  are  they.     In 


THE  EASTERN  AREA.  871 

a  ixM-tion  of  them  some  of  tlic  (|uai-t/.  is  in  small  roimdisli  areas  wliieji 
have  an  unmistakable  tVaoineiital  character.  Furtlier,  all  of  the  character- 
istics of  the  rocks,  both  in  liaiid  specimen  and  thiu  section,  are  those  of 
compact  clayey  sediments,  which  in  all  probability  they  are. 

TABULATn)N    OF    PKTROGRAPHICAL   OBSERVATIONS.'- 

1.  Ferruginous  and  feldspatliic  conglomerate.  Specimen  0205  (slide  3012),  150 
N.,  1750  W.,  Sec.  10,  T.  47  N.,  K.  4;5  W.,  Michigan. 

The  rock  is  dark  gray  and  massive,  and  the  matrix  is  iiiedium  grained.  The 
contained  pebbles  are  Jasper,  white  quartz,  and  green  schist.  They  are  mostly  small, 
although  occasionally  one  10  inches  in  diameter  is  found. 

A  thin  section  from  the  matrix  shows  fragmeiital  particles  of  quartz  aud  feld- 
spar of  quite  uniform  size,  the  former  composing  perliai)s  four-flfths  of  the  section. 
The  grains  of  quartz  are  often  enlarged,  and  some  of  them  are  liuely  complex.  The 
feldspar  is  orthoclase,  nucrocline,  and  plagioclase.  Its  grains  are  In  part  fresh,  and  in 
part  also  much  kaoliuized  or  partly  altered  to  chlorite.  In  the  interstices  are  finely 
crystalline  quartz,  dark  brown  iron  oxide,  and  greeu  chlorite.  • 

2.  Ferruginous  and  feldspatliic  quartzites.  Specimens  7430  (slide  1801),  7437 
(shde  1802),  1050  N.,  1420  W.;  0285  (slide  2062),  0286  (slide  2063),  1075  N.,  1350  W.; 
0287  (slide  3008),  1040  N.,  1300  W.,  Sec.  30,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rocks  are  dark  gray  to  black,  medium  grained,  and  vary  from  massive  to 
schistose,  the  darker  ('olored  specimens  containing  much  oxide  of  iron. 

In  each  of  the  thiu  sections  the  mineral  constituents  are  the  sa  ae  as  in  the  pre- 
vious number,  the  only  difference  between  the  various  sections  being  relative  propor- 
tions of  the  minerals  contained.  Slides  1891  and  2962  have  darlv  brown  ferrite  in  a 
continuous  ramifying  sheet  in  wliich  the  other  minerals  are  buried.  In  slides  2063 
and  3008  the  oxide  of  iron  is  muclr  less  in  quantity,  while  the  fragments  of  feldspar 
and  quartz  in  them  are  abundant.  The  grains  of  fragmental  (j[uartz  are  frequently 
enlarged. 

3.  Sericitic  graywacke.  Specimen  9284,  (slide  2961),  300  N.,  500  W.,  Sec.  19,  T. 
47  N.,  E.  43  W.,  Michigan. 

The  rock  is  dark  gray,  fine  grained,  schistose,  and  cleaves  readily  along  the 
plane  of  schistosity. 

Rather  small  clastic  particles  of  quartz  and  feldspar,  the  former  preponderant, 
compose  three-fourths  of  the  thin  section.  The  interstices  are  filled  with  finely  crys- 
talline quartz,  kaolin  or  sericite,  chlorite,  dark  brown  ferrite,  and  iilentiful  grains  of 
black  lustrous  galenite. 

'The  mimbers  of  speciiiieus  aud  slides  are  those  of  the  collection  of  the  Lake  Superior  division. 
"Locations  are  giveu  from  the  southeast  corner  of  the  sections  in  steps  of  2,000  per  mile. 


372  THE  PENOKEE  INON-BEARING  SEEIES. 

4.  Jasper-conglomerate.  Specimens  9261  (slide  3005),  9263  (slide  4486),  7418 
(slide  1877),  GOK,  600  W.,  Sec.  20,  T.  47  N.,  R.  43  W.,  Michigan. 

The  matrix  of  the  rock  is  dark  green,  fine  grained,  finely  laminated,  has  a 
greasy  feel,  and  contains  uuineroiis  medium  sized  grains  of  a  cleavable  mineral.  This 
matrix  is  quite  thiclcly  studded  with  fragmeuts  of  red  jasper,  some  of  which  are  from 
6  to  8  inches  in  diameter. 

The  thin  "sections  are  composed  of  finely  crystalline  and  coarsely  fragmental 
parts.  The  fragmental  portions  comprise  large,  somewhat  rounded  areas  of  a  finely 
crystalline  basic  eruptive  and  medium  grains  of  feldspar,  the  former  variety  of  frag- 
ments being  more  abundant.  These  complex  basic  areas  contain  cldorite,  biotite, 
tabular  feldspars,  ferrite,  and  epidote,  and  are  almost  precisely  like  the  basic  erup- 
tive which  is  developed  upon  a  large  scale  just  to  the  southward.  The  feldspar  frag- 
ments are  in  part  ortlioclase  and  in  part  plagioclase.  The  abundant  fine  grained 
cementing  material  consists  of  clierty  silica,  sm  ill  flakes  of  chlorite,  a.nd  few  of  biotite. 
The  pebbles  of  the  conglomerate  are  typical  red-banded  jaspers.  The  rocks  are 
apparently  intermediate  between  fragmental  ones  and  those  of  the  nonfragmental 
iron-bearing  belt.  jSTanfragmental  sedimentation  has  began,  but  it  is  yet  acc'om- 
panied  with  mechanical  sedimentation. 

5.  Olay-slate.  Specimen  12580  (slide  5340),  460  :N'.,  1010  W,  Sec.  20,  T.  47  K, 
E.  43  W.,  Micliigan. 

The  rock  is  a.  light  green,  aphanitic,  finely  laminated,  soft  slate. 

The  thin  section  consists  of  intimately  mingled  finely  crystalline  quartz,  chlorite, 
sericite,  and  iron  oxide.  Much  of  this  quartz  is  fragnreiital.  The  sericite  is  a  light 
greenish-yellow,  somewhat  brilliantly  polarizing,  and  is  arranged  in  parallel  rows  of 
flakes  which  extinguish  rectangularly.  If  other  constituents  are  present,  they  are 
too  obscure  to  be  recognizable. 

6.  Feldspathic  (luartzites.     Specimens  7383  (slide  1849),  900  N.,  460  W.;  9244 
(slide  4479),  9245  (slide  4480),  1100  N.,  460  W.,  Sec.  21,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rock  varies  from  flesh  color  through  greenish-gray  to  dark  brown ;  is 
rather  fine  grained,  almost  massive,  and  breaks  with  a  subconchoidal  fracture. 

The  thin  section  is  composed  largely  of  clastic  particles  of  (juartz  and  feldspar, 
the  former  much  the  more  abundant  and  often  enlarged.  Tlie  feldspars  are  orthoclase, 
microcline,  and  plagioclase.  The  rather  abundant  interstitial  material  is  finely  crys- 
talline quartz,  chlorite,  kaolin,  and  iron  oxide. 

7.  Quartzite.     Specimen  7385  (slide  1851),  900  N.,  750  W.,  Sec.  21,  T.  47  N.,  R 
43  W.,  Michigan. 

The  rock  is  a  greenish-gray,  medium  grained,  compact  vitreous  quartzite. 

The  thin  section  is  mainly  composed  of  enlarged  fragmental  grains  of  quartz. 
The  induration  is,  however,  mostly  due  to  finely  crystalline  interlocking  interstitial 
quartz,  mingled  with  which  is  chlorite  and  brown  oxide  of  iron. 


THE  EASTKIiN  AIMCA.  J^73 

8.  FiMTiiR-inoiis  and  clildritic  slate.  Spcciiiiciis  7100  (sliilc  l,S(i;;),  1 1  10  N.,  ".;;() 
W.;  ims  (slide  USI),  1170  N.,  r>10  W.,  See.  L'l,  T.   17  N.,  If.  l::  W.,  Miehi-aii. 

The  roek  is  of  a  peculiar  liislroiis  lnowiiisli  fipeeii  coloi-,  iiuiltled  witli  irrcj;iilar 
pat<-lies  oCa  dull  hrielv-ied  eiilor,  is  finely  laiiiiiiaied  and  very  soil. 

In  thin  section,  a  quart/,  harkgromid  e(Mitains  abundant  |»ale  j^reen  ehlmite, 
liniwu  iron  oxi(h',  red  hematite,  and  numerous  scales  of  a  luillianMy  ])ohiiiziufi-  min- 
eral which  istakeu  to  be  serieite. 

it.  dhloiitic  and  biotitic  slate.     Six'cimeu  73S(;  (slide    :i045),  050  N.,  S(Mt  \V., 
See.  131,  T.  47  N.,  U.  43  W.,  Miehijiiiu. 

Ill  tliin  section,  a  tinely  crystalline  (|iiartzose  iiioundmass  contains  a  felted,  mass 
of  fibrous  somewhat  iron  stained  ehloiiteand  biotite,  the  libers  of  which  are  arranged 
in  a  common  direction,  and  give  the  rock  a  strong  schistose  character.  Scattered 
through  this  fine  material  are  a  few  small  grains  of  jdainly  fragmental  quartz. 

10.  Ferruginous  and  chloritic  slates.  Specimens  7;!!>!)  (slide  1S(;2),  1  l.jO  N.,  520 
W.;  7401  (shde  1S(!4),  1200  N.,  4S0  W.;  0249  (slide  4482),  1210  N.,  450  W.,  Sec.  21,  T. 
47  N.,  R.  43  VV.,  Michigan. 

The  rocks  are  dark'greeu,  thinly  foliated,  and  have  a  soft  greasy  feel. 

In  thin  section  the  rocks  differ  from  S  chieHy  in  that  they  have  no  liydromica, 
and  contain  (juite  plentifully  small  grains  of  plainly  fragmental  (juartz,  wbicli  stand 
out  sharply  from  tlie  groundmass.     Both  hematite  and  limonite  are  plentiful. 

11.  Ferruginous  quartzite.  Specimens  9252  (slide  4484),  9253  (slide  4485),  1315 
N.,  570  VV.,  Sec.  21,  T.  47  N.,  li.  43  W.,  Michigan. 

The  rocks  are  greenish  to  reddish  gray,  massive,  and  vitreous. 

The  thiu  sections  are  almost  wholly  composed  of  interlocking  large  grains 
of  quartz.  Films  of  iron  oxide  are  found  in  the  interstices  and  Itetweeu  the  cores 
and  enlargements  of  the  quartz  grains. 

12.  Feldspathic  quartzite.  Specimen  12588  (slide  5343),  1312  N.,  1790  W.,  Sec. 
22,  T.  47  ]Sr.,  R.  43  W.,  Michigan. 

The  rock  is  gray  to  pink,  fine  grained,  vitreous. 

The  thiu  section  is  chiefly  composed  of  rather  small  originally  well  rounded 
particles  of  quartz  and  feldspar  of  remarkably  uniform  size.  The  quartz  is  several 
times  as  abundant  as  the  feldspar.  The  grains  are  sometimes  iiuely  complex  and 
often  plainly  enlarged.  The  feldspar  comprises  ortlioclase,  micro(;line,  and  plagioclase, 
all  quite  fresh,  although  a  few  of  the  grains  have  been  affected  by  decomposition. 
Between  the  clastic  jjarticles  are  tound  tinely  crystalline  silica,  numerous  small  well 
defined  brilliantly  polarizing  flakes  of  kaolin  or  serieite,  and  a  few  particles  of  ferrite. 
The  section  also  contains  a  few  grains  of  each  of  the  minerals  zircon  and  tourmaline, 
the  former  at  times  showing  its  characteristic  zonal  structure.  These  minerals  are 
rounded,  and  must  be  classed  as  fragmental  constituents  rather  than  as  indigenous  in 
this  rock. 


374  THli  PENOKEE  IROH-BEARING  SERIES. 

SECTION  III— THE  GREENSTONE-CONGLOMERATES. 

Distribution. —  The  greenstone-conglomerates  are  restricted  to  tlie  area 
represented  in  PI.  xiii,  occurring  nowhere  except  in  T.  47  N.,  R.  43  W., 
and  R.  44  W.,  Michigan.  It  will  be  seen  that  their  westernmost  appearance 
is  in  the  extreme  northeast  part  of  Sec.  16,  T.  47  N.,  R.  44  W.,  Michigan.. 
In  the  northwest  part  of  Sec.  14,  T.  47  N.,  R.  44  "W.,  Michigan,  are  areas 
which  are  mapped  as  detached  from  the  main  mass  of  conglomerate  because 
they  are  not  known  to  be  connected  with  it.  From  the  exposure  in  the  south 
part  of  Sec.  15,  T.  47  N.,  R.  44  W.,  a  belt  of  the  conglomerate  continues 
eastward,  rapidly  ,widening,  and  in  Sees.  24  and  25,  T.  47  N.,  R.  43  W., 
reaches  its  maxium  width,  1|  miles.  Continuing  eastward,  the  belt  quite 
rapidly  narrows,  and  its  last  appearance  is  in  the  northwest  part  of  Sec.  23, 
T.  47  N.,  R;  43  W.,  Michigan.  The  exposures  of  eruptive  rocks  in  Sees. 
13  and  14,  in  the  west  part  of  Sec.  23  and  in  the  northeast  part  of  Sec.  24, 
T.  47  N.,  R.  44  W.,  Michigan,  have  all  the  characteristics  of  surface  flows; 
that  is,  they  contain  minerals  of  two  generations,  are  often  amygdaloidal, 
and  have  a  groundmass  which  is  frequently  parti}'  amorphous  and  is  always 
finely  crj^stalline.  These  exposures  are  believed  to  be  more  closely  allied 
to  the  o-reenstone-comfflomerates  than  to  the  diabases  of  the  Penokee  series. 

General  characteristics. — The  term  "greenstone-conglomerate"  is  applied 
to  the  rock  of  this  area  because  it  is  a  fragmental  one,  in  which  nearly  all 
the  fragments  are  from  basic  eruptives,  as  is  also  the  major  portion  of  the 
matrix  in  which  these  fragments  are  set.  The  term  "agglomerate"  would 
convey  a.  false  impression.  The  rocks  covered  by  the  term  greenstone- 
conglomerate  include  agglomerates  and  water-deposited  elastics,  the  detritus 
of  which  is  chiefly /rom  greenstones,'  as  well  as  gradations  between  these  two 
extremes.  Macroscopically,  the  rocks  vary  from  an  aphanitic  slate-  to  a 
coarse  conglomerate.  At  times  the  fine  grained  clay-slates  and  the  con- 
glomerates are  intimately  mingled,  but  in  general  the  slate  exposures  are 


'The  woi'd  jrreenstone  is  used  here  in  its  old  sense,  to  cover  all  the.  basic  eruptives  of  the  district. 

'A  rock  remarkably  similar  to  many  ot  these  conglomerates  is  <lescribed  by  Giimbel  under  the 
tennSchalsteiu.  (Grundziige  der  Geologic,  pp.  196,  197.)  The  matrices  and  finer  fragmects  of  some  of 
the  roclcs  in  question  present  a  schistose  structure  very  similar  to  that  figured  by  him.  The  minerals 
now  present  .are  identical  with  those  contained  in  his  schalstein,  and  his  description  could  be  applied 
almost  exactly  to  them. 


TTIK  KASTHliN   AKKA.  375 

apart  tVom  the  cong-lonKM-Mtc  oiu's.  'riicsc  slates  nn-  more  lrc(|U('iitl\'  found 
in  the  southern  than  in  the  northern  part  of  the  heh.  The  color  of  the 
rocks,  whetiier  tine  graiiUMl  oi'  con'^lonieratic,  is  some  shade  of  <)Teen  or 
gTayish-<4Teen.  Sometimes  the  rocks  show  a  schistose  structin-e,  althoiig-ji 
in  o'cneral  they  are  quite  massive.  The  couo'lomeratic  phases  are  the  pre- 
dominant ones,  and  kei-e  the  fragments  are  ver}-  abundant,  so  as  to  leave 
but  little  room  for  a  matrix.  Nearh'  all  f>f  the  pel)liles  and  bowlders  are 
of  a  single  variety,  pale  greenish-gray,  aphanitie,  and  massive.  In  some 
localities  aphanitie,  dark  reddish-brown,  felsitic-looking  pebbles  are  quite 
numerous,  although  they  never  become  the  predominant  varietv.  Very 
rarely  worn  white  quartz  pebbles  are  found.  The  pebbles  are  usually  more 
resistant  then  the  matrix,  and  consequently  proti'ude  upon  the  weathered  sur- 
face in  n(Klnlar  or  maminillar)^  forms.  In  the  greater  number  of  the  expo- 
sures fracture  takes  place  through  matrix  and  pebble  with  about  equal  ease. 
In  those  exposures  in  which  the  matrices  are  much  altered,  and  therefore 
schistose,  fracture  occurs  around  the  pebbles  to  a  greater  or  less  extent, 
depending  upon  the  degree  of  alteration.  The  lines  of  separation  between 
the  pebbles  and  matrices  in  the  fresher  rocks  are  quite  sharp,  but  in  those 
which  are  more  altered  they  are  vague ;  and  as  there  is  often  but  a  slight 
difference  in  the  color  and  texture  of  the  pebbles  and  matrices,  when  the, 
rocks  are  much  altered  it  is  difficult  to  separate  one  from  the  other  on  a 
fractured  surface,  although  the  difference  is  clearly  seen  upou  the  weathered 
surface. 

A  stud}-  of  some  fifty  thin  sections  shows  that  the  matrices  are  of  many 
phases,  which  vary  into  each  other  by  imperceptible  gradations.  At  times 
the  matrix  appears  to  be  a  fine  grained  eruptive,  which  contains  fragments 
of  essentially  the  same  material.  This  phase  of  matrix  is  described  under 
the  greenstones  of  the  eastern  area  and  the  description  will  not  l)e  here 
repeated.  'An  important  fragmental  kind  is  composed  of  the  fine  debris 
from  material  like  the  contained  pebbles,  combined  with  finely  crystalline 
quartz ;  that  is,  it  is  a  recomposed  'greenstone,  and  is  often  schistose. 
The  minerals  most  frequently  found  in  this  phase  of  matrix  are  quai-tz,  tabu- 
lar plagioclases,  chlorite,  epidote,  titanite  and  leucoxene,  actinolite,  and,  as 
accessories,  oxide  of  iron,  a  carbonate,  and  occasionally  renmants  of  augite. 


37G  THE  PENOKEE  IRON -BEAEmG  SEKIES. 

The  proportions  of  these  minerals  vary  widely  in  the  different  sections.  In 
one  variety  a  quartzose  backgTound  subordinate  in  quantit}-  contains  fine 
debris  from  the  basic  eruptives;  that  is,  tabular  plagioclases,  actinolite, 
and  chlorite,  all  of  the  minerals  being  mingled  in  the  most  confused  manner, 
but  generally  showing  a  laminated  structure.  In  another  variety  of  frag- 
mental  matrix-  of  about  equal  importance  to  the  last  the  background  is 
composed  almost  wholly  of  quite  pure  finely  crystalline  and  chalcedonic 
quartz,  in  which  are  well  outlined  sharp  areas  of  intermingled  chlorite  and 
epidote.  Between  these  two  main  varieties  there  are  various  gradations. 
In  some  of  the  sections  the  matrix  and  fine  fragmental  material  have  a 
pretty  well  defined  stratiform  arrangement;  in  others,  they  vary  from  this 
regular  stratiform  character  to  a  most  extraordinary  irregular  arrangement, 
the  fragments  being  extremely  angular,  and  the  matrix  between  running 
around  and  through  them  in  the  most  lawless  fashion.  Words  fail  to  give 
any  proper  conception  of  this  strange  appearance,  but  some  idea  of  it  will 
be  obtained  by  reference  to  PI.  xxxv,  Figs.  2,  3,  and  4.  Of  less  impor- 
tance than  the  foregoing  are  the  black  chlorite-slates  and  black  calcareous 
slates.  These  phases  are  usuall)'  nonconglomeratic,  and  are  precisely  like 
the  slates  of  the  fragmental  belt  north  and  east  of  the  greenstone-cc^ng-lom- 
erates.  Some  of  them  contain  unmistakalole  fragmental  quartz  mingled 
with  fine  grained  clayey  material.  Others  contain  a  good  deal  of  some 
carbonate,  oxides  of  iron,  and  finel}'  crystalline  quartz ;  in  other  words,  are 
like  the  mingled  nonfragmental  and  fragmental  water-deposited  sediments 
of  the  eastern  area. 

The  fragments  of  the  conglomerates  vary  from  large  bowlders  to 
single  individuals  of  one  mineral.  The  pebbles  are  usually  immerous,  and 
often  so  thickly  set  as  to  give  little  room  for  a  matrix,  although  the}'  vary 
in  abundance  and  are  sometimes  absent.  It  has  already  been  stated  that 
the  great  majority  of  the  pebbles  are  of  two  well  defined  types.  •.  The  com- 
mon light  green  or  grayish-green  pebbles  are  orjiinarily  fine  grained  fo 
aphanitic  porphyrites,  which  are  quite  often  amygdaloidal.  Their  back- 
ground varies  from  glassy  to  holocrystalline.  When  glassy,  they  have 
been  almost  wholly  devitrified,  being  changed  into  a  pale  green  or  light  gray 
nonpolariziug  or  very  feebly  polarizing  aggregate.     The  common   re(;og- 


Till',  i;ast1':i;x  ai;ka.  :^77 

nizablc  iniiicrals  ctmlaiiUMl  in  tlic  L;lass\  \ari('tics  arc  tahiilar  plai^'ioclascs, 
Ifiiciixciic,  and  litaiiitc,  cliliiritc,  ami  ciiidntc,  Tlic  IioIoctn  stallinc  \arict>'  of 
pcWhlc  is  soiiH'tiiiu's  fresh  ciKHi^li  In  lie  distiiictl \-  rcicogniizcil  as  a- diabase- 
|)(ir|)liNi'it(';  tlic  more  altered  ones  would  he  classed  as  porphyrites. 
These  ha\e  as  cliief  coustitueiits  the  iniuerals  aliove  mentioned  with 
tli(^  a<l(liti<)n  of  a-ctinolite  or  hornblende,  and  occasionalh'  augite  and 
nieiiaccanite,  with  almost  always  more  or  less  of  secondary  (piartz.  The 
chlorite  and  epidote  are  in  large  measure  the  result  ol'  the  alteration  of  au 
original  p\roxene  mineral,  although  in  part  they  are  derived  from  the 
decomposition  of  feldspar,  and  apparently  the  actinolite  comes  from  the 
same  sources.  The  quartz  is  believed  to  lie  always  secondary.  When  the 
pebbles  are  ol'  an  amygdaloidal  character,  the  amygdules  are  usually  of 
quartz  or  calcite  or  epidote,  or  two  or  all  three  of  these  combined.  These 
augite-i)orphvi'itesand  porphyrites  resemble  (dosely  the  rocks  described,  by 
Irving  as  diabase-porphyrite  and  ash-bed  diabase  in  the  Keweeamw  series.-^ 

The  second  variety  of  pelibles,  the  red  felsitic-looking  kind,  differs 
onl)^  from  those  just  described  in  that  tlie  constituents  are  deeply  stained 
with  oxide  of  iron.  They  are  then  simply  ferruginous  porphyrites.  It  thus 
appears  that  almost  all  of  the  larger  fragments  of  the  greenstone-conglom- 
erates are  in  their  character  basic  eruptives.  Besides  the  fragments  thus 
derived  from  liasic  eruptives  there  are  rarely  found  well  rounded  pebbles 
of  white  quartz.  These  pebbles,  althougli  so  infrequent,  are  important  as 
bearing  upon  the  probable  origin  of  the  rocks.  In  one  case  there  is  in  a 
section  a  rounded  quartz  area  which  is  complex,  and  which  is  composed  of 
simple  quartz  grains  now  interlocking  by  enlargement.  This  pebble  was 
then  derived  from,  a  sandstone,  which  was  changed  to  a  quartzite  either 
prior  to  or  shvce  its  deposition  in  its  present  place.  Aside  from  the  classes 
of  fragments  mentioned  there  is  not  infrequently  present  in  the  matrix 
large  simple  well  rounded  grains  of  quartz  and  feldspar,  These  quartz 
grains  are  often  enlarged.  They,  like  the  quartz-pebbles  just  referred  to 
would  seem  to  indicate  that  the  rock  is  .water-deposited. 

Origin  of  the  Greenstone-conr/loinemtes. — ^^The  one  feature  in  common 
which  nearly  all  exposures  of  the  area  classed  iinder  this  term  have  is  the 

'  R.  D.  Irving:  Copper-beariug  rocks  of  lake  Superior;  Mouograph  U.  S.  Geol.  Survey,  vol.  V, 
1883,  pp.  77-87. 


378  THE  PENOKEE  IRON-BEAEING  SEEIES. 

pi-eseiice  of  fragments  of  a  basic  eruptive,  although  a  few  exposures  of  a 
.schistose  nonconglomeratic  material-  are  found.  These  fragments,  as  has 
been  seen,  A^ar}-  from  those  which  are  well  rounded  to  those  which  have 
the  extreme  of  angularity.  Basic  eruptive  fragments  of  this  sort  may 
have  been  derived  from  various  possible  sources,  those  which  are  well 
rounded  having  been  probably,  although  not  certainly,  subjected  to  water 
action,  but  this  does  not  necessarily  tell  their  source.  It  has  been  seen 
that  south  of  the  iron-bearing  belt  of  the  formation  is  a  layer  of  amygda- 
loidal  porphyrite;  also,  very  closely  associated  with  the  conglomerates 
themselves  in  the  northern  portion  of  the  area  are  other  large  exposures  of 
a  shnilar  rock.  These  massive  rocks  are  remarkably  like  many  of  the 
pebbles  contained  in  the  greenstone-conglomerates.  It  is,  then,  possible 
that  the  fragments  of  the  greenstone-conglomerates  have  been  derived  by 
the  deo-radation  of  older  or  contemporaneous  eruptive  outflows,  or  it  is 
possible  that  they  are  the  direct  ejecta  from  a  vqlcanic  vent,  or  (and  this  is  . 
most  likely  the  case)  the  pebbles  have  in  part  come  from  both  of  these 
sources. 

The  greenstone-conglomerates  and  breccias,  which  have  a  matrix 
essentially  like  an  altered  greenstone  and  most  irregular  fragments  of 
basic  eruptives  witli  difficult)"  separated  from  the  matrix,  have  a  remark- 
able likeness  to  the  schistose  fine  grained  porphyrites  Avith  which  tliey  are 
closely  associated  in  the  field.  This  material  certainly  can  not  have  resulted 
from  the  breaking  down  of  a  solidified  rock  by  water  action.  The  frag- 
ments are  precisely  like  in  their  shape  and  character  to  the  ejecta  of  a 
volcanic  vent.  How  these  fragments  and  the  matrix  in  which  they  are 
contained  became  mingled  it  is  impossible  to  sa)'  with  certainty.  The 
brecciation  may  be  merely  that  of  an  onflowing  bed  of  lava  which  con- 
tinues its  forward  movement  after  it  has  reached  so  viscous  a  condition 
that  it  becomes  fractured  in  every  direetion  by  its  motion,  caused  by  a 
force  in  the  rear,  or  this  mixture  may  be  the  ejecta  from  a  cone  falling  upon 
a  lava  stream  flowing  down  its  side.  The  first  explanation  is  regarded 
as  the  more  probable.  "Whichever  hypothesis  is  true,  that  this  material 
is  essentially  the  same  in  composition,  as  the  surrounding  surface  lava 
flows  is    certain,  for    take  awaj-   the  fragments  and  it    would  be    impos- 


THE  I':astkkn  area.  37y 

sible  to  (Hsting'uisli  tlic  matrix  iVdin  the  iiiorc  sdustosc  and  altered  di'  the 
flows.  Ill  those  i)hases  of  the  gTeenstoue-coiiijloiiierjite  in  which  there  is 
ahiiost  no  matrix  at  all  and  those  in  which  the  matrix  apjiears  hut  to  be 
flne  debris  of  the  same  kind  as  tlie  pehhles  and  how  Iders — thai  is,  com- 
posed of  angular  fragments  of  very  greatly  varying  sizes  which  ha\e  l)een 
closely  packed  together  and  subsequently  cemented  1)\-  alterations  <.f  the 
materials  contained  with  the  addition  of  interstitial  (|uartz — it  seems  prob- 
able that  the  rock  is  merely  a  heap  of  volcanic  tuff.  The  volcanic  material 
may  have  fallen  upon  the  sides  of  the  cone  or  farther  away  have  been 
deposited  in  water;  in  which  latter  case  the  debris  would  have  been  sub- 
jected to  water  action.  In  those  cases  in  which  the  matrices  exhibit  the 
extraordiuar)-  irregular  and  concretion-like  areas,  A'arying  into  roughlj- 
stratiform  deposits,  it  would  seem  that  the  ashes  had  accumulated  under 
or  at  least  had  been  subject  to  the  leaching  action  of  water. 

In  the  phase  of  conglomerates  in  which  finel}'  crystalline  quartz 
becomes  an  important  constituent  it  would  seem  that  the  rocks  had  origi- 
nated by  the  mingling  of  volcanic  material  with  nonfraginental  sediments 
under  water.  Whether  the  contained  fragments  ^fell  directly  upon  the 
water  from  a  volcanic  vent  or  the  water  was  the  agent  which  broke  down 
this  material  from  recently  formed  lava  can  not  positivel)-  be  deter- 
mined, but  the  rounded  forms  of  a  portion  of  the  pebbles  and  the  irregular 
forms  of  others  wordd  seem  to  indicate  that  both  processes  may  have 
furnished  a  share  of  the  pebbles.  That  this  class  of  conglomerate  was 
deposited  under  the  surface  of  water  is  furthei-  indicated  b}-  the  fact  that 
in  this  nonfragmental  quartzose  background  there  is  not  infrequently  con- 
tained well  rounded  particles  of  quartz  and  feldspar,  the  former  of  which 
have  been  enlarged,  and  in  a  few  cases  these  rounded  quartzose  fragments 
attain  the  magnitude  of  pebbles.  That  this  peculiar  mingled  fragmental 
and  nonfragmental  material  is  of  the  origin  indicated  is  further  evidenced 
by  the  jasper-conglomerates  in  the  extreme  southern  part  of  the  SE.  ^ 
of  Sec.  20,  T.  47  N.,  R.  43  W.,  Michigan.  The  associations  of  this  rock 
are  explained  in  chapter  vm.  It  was  there  seen  that  the  conglomerate 
gradually  passes  upward  into  the  nonfi-agmental  sediments — chert  and 
cherty  iron  carbonate — of  the  iron  belt.     There  is  little  doubt  that  it  is  a 


380  THE  PENOKBE  IRON-BE ARIISTG  SERIES. 

mingled  nouclastic  and  clastic  sediment.  If  this  is  the  origin  of  this  jasper- 
conglomerate  it  is  a  strong  indication  that  the  greenstone-conglomerates, 
which  differ  from  it  only  in  the  absence  of  jasper  fragments,  are  of  a  like 
character.  It  follows  from  the  above  that  the  class  of  conglomerates  which 
contain  a  preponderating'  amount  of  nonfragmental  material  are  closely 
allied  to  the  rocks  of  the  Iron-bearing  member  of  the  Penokee  series.  The 
contained  frag'ments  of  basic;  eruptives  is  the  chief  point  in  which  they  are 
unlike  this  iron-bearing  belt.  Tliis  phase  of  the  greenstone-conglomerates  ■ 
is  still  more  nearly  like  in  essential  character  the  ferruginous  and  frag- 
mental  rocks  north  and  south  of  the  greenstone-conglomerates,  the  chief 
difference  between  the  two  classes  of  rocks  lieing  that  the  complex  basic 
eruptive  fragments  take  the  place  of  the  simple  quartz,  feldspar,  and  clayey 
fragments  of  these  belts.  It  has  been  noted  that  closely  mingled  with  the 
greenstone-schists  and  greenstone-conglomerates  are  a  few  rocks  which  are 
essentially  like  the  ferruginous  and  fragmental  I'ocks  to  the  north  and  east. 
It  would  appear  that  in  these  cases  for  a  short  time  mingled  fragmental 
and  nonfraii'mental  sedimentation  has  occurred  without  having'  received 
basic  eruptive  debris.-*  In  these  iuterlaminated  rare  varieties  of  rock  we 
have  still  another  link  between  the  greenstone-conglomerates  and  the 
Ordinary  ferruginous  and  fragmental  belt  to  the  north  and  east. 

From  the  foregoing  it  ^vould  appear  that  in  the  g'reenstone-conglomerate 
area  and  vicinity  we  have  g-radations  from  a  rock  which  is  a  purely  basic 
eruptive  amygdaloidal  flow  to  those  Avhich  are  simply  mingled  clastic  and 
nonclastic  sediments.  Intervening  between  these  widely  separated  phases 
are  the  brecciated  rocks,  which  appear  to  have  had  a  lava  base  and  contain 
fragments  of  essentiallv  the  same  material  ;  rocks  which  appear  to  be  tuffs 
alone  ;  rocks  in  which  tuff  has  fallen  \i\)on  water  and  has  become  mingled 
with  water-formed  detritus  and,  varying  from  this  by  a  gradual  lessening 
of  the  tuff,  rocks  which  are  simply  mingled  fragmental  and  nonfragmental 
water-deposited  sediments. 

Whether  all  of  the  above  conclusions  as  to  the  origin  of  tlie  different 
phases  of  this  rock  are  true  or  not,  it  is  certain  that  this  area  lias  been  the 
center  of  great  volcanic^  activity.  This  is  evidenced  by  the  great  masses 
of  greenstone  in  Sees.  26  and.  27,  T.  47  N.,   R.  44  W.,   Michigan,   by  the 


TIIK  EASTERN   AlfEA.  881 

surface  porphyritic  How  in  Sees.  -Jf)  iiiid  .".0,  T.  17  N.,  !>.  4o  W.,  ]\Iicliifr}ni, 
and  by  the  lar^i-e  eriiptive  exposures  in  Sees.  11,  If),  and  IC,  'V.  47  N.,  K. 
44  W.,  Ariclijo-jui,  as  widl  as  l)y  tlic  ;4Teat  mass  of  oi-ccnstonc-coiij^'lomenite 
itself  Tliis  voleanie  activity  is  an  exceptional  tiling-  in  tlic  Tenokee  sue 
oessi<in,  an<l  in  it  is  l)elieved  to  lie  tlie  key  wliicli  explains  tlie  \vr\  excep- 
tional characters  of  the  series  in  'V.  47  N.,  R.  43  and  44  W.,  Michigan. 

TABULATION   OF    PETKOGBAPIIICAL   OBSEKVAl'TONS.' 

1.  dhloritic  slate.  Specimen  12«S0  (slide  5410),  from  1655  N.,  60  W.,  Sec.  16,  T. 
47  N.,  R.  44  W.,  Michigan. 

The  rock  is  a  dark  green,  fine  graiued,  schistose  one,  containing  large  vaguely 
outlined  areas  which  seem  to  be  of  a  couglomeratic  character. 

The  thin  section  seems  to  be  that  of  a  fine  grained  porphyritc  whicli  lias  ouce 
had  a  glassy  background.  The  background  has  almost  wholly  devitriticd.  At  one 
side  of  the  section  it  consists  of  a  turbid  gray  aniorplums  aggregate.  This  gradnally 
changes  into  green  nonpblarizing  chlorite  in  passing  towards  the  other  side  of  the 
section.  The  background  contains  many  minute  tabular  i)lagioc]ascs,  gray  areas  of 
leucoxenc  (whieh  here  and  there  include  particles  of  unaltered  nienaccunite),  and 
some  finely  crystalline  ipiartz.  The  section  contains  also  a  fewporiihyiitic  crystals  of 
plagioclase  and  intersecting  ^■eins  of  quartz. 

2.  Greenstone-conglomerafes.  Specimens  0360  (slide  3032),  425  N.,  12.50  W.; 
9.369  (slide  .3035),  365  N.,  1325  W.,  Se.-.  15,  T.  47  N.,  E.  44  W.,  Michigan. 

The  matrix  of  the  conglomerate  is  a  mottled  green  color,  fine  grained,  and  quite 
massive,  although  traces  of  a  schistose  structure  arc  shown.  The  larger  part  of  the 
contained  i)ebblcs  are  of  a  pale  green  coha-,  aplianitic,  massive,  and  often  with  illy 
defined  outlines.  A  few  pebbles  are  of  a  dark  brown  color,  well  rounded  and  clearly 
defined. 

In  thin  section  the  fragments  are  seen  to  be  from  basic  eruptives.  A  few  of 
them  are  well  rounded,  but  they  are  mostly  extraordinarily  irregular  and  sharply 
angular  in  form.  In  places  they  are  so  closely  packed  together  that  their  outlines  are 
with  difliculty  deterniiued.  As  a  result  of  these  peculiarities  the  sections  present  a 
striking  appearance,  as  if  made  up  of  a  jiileof  various  kinds  of  particles  having  the 
greatest  possible  irregnhirities  of  outline.  The  majority  of  the  fragments  have  a 
gray  partially  devitrifleil  glassy  background,  in  which  are  set  small  tabular  crystals 
of  plagioclase;  in  others  there  seems  to  have  been  an  amorphous  background,  but  it 
is  now  wholly  devitrified,  being  composed  of  chlorite,  gray  feebly  i)olarizing  material 
and  somewhat  brilliantly  polarizing  minute  needles.    The  basic  fragmeuts  a:  e  ('hiefly 

'  The  uumbors  ul'  siitciiiieiis  mikI  slides  are  those  of  the  eolleetioii  of  file  Lake  Superior  division. 
Locations  are  given  from  the  southeast  corners  of  the  sections,  m  stejis  of  2,0f0  per  wile. 


382  THE  PENOKEB  IRON-BEAEING  SERIES. 

cemented  by  interlocking  quartz  and  feldspar,  the  latter  comprising  both  orthoclase 
and  plagioclase.  The  feldspar  in  this  cementing  material,  unlike  that  contained  in 
the  fragments,  is  usually  fresh.  This  dilference  maybe  due  to  a  more  acid  character. 
Contained  in  one  of  the  sections  are  quite  numerous  areas  of  a  colorless  mineral  which 
shows  cleavage  in  two  directions,  gives  somewhat  brilliant  interference  colors,  and  is 
taken  to  be  pyroxene.  Quite  frequently  roundish  cores  of  this  mineral  have  a  border 
of  fibrous  material  which  terminates  by  a  gradual  fraying  out.  A  few  large  frag- 
ments of  an  undetermined  mineral,  which  has  very  feeble  double  refraction,  are  seen. 
(PI.  XXXV,  Figs.  3  and  4.) 

Section  ill  and  near  west  half  of  Sees.  14  and  23^  T.  47  N.,  B.  44  W.,  Michigan. 

3.  Actiuolite-chlorite-schists.  Specimens  9377  (slide  3037),  1330  N.,  25  W.;  9378 
(slide  3038),  1425  IST.,  15  W.,  Sec.  22,  T.  47  N.,  E.  44  W.,  Michigan. 

The  rocks  are  dark  lustrous  green,  fine  grained,  and  finely  laminated. 

The  sectioiis  are  mostly  composed  of  pale  green  nonpolarizing  chlorite,  needles 
of  actinolite  and  biotite,  finely  crystalline  and  chalcedonic  quartz,  and  roundish 
granules  and  clusters  of  granules  of  yellowish-gray  titanite.  In  places  tlie  silica  is  so 
abundant  as  to  constitute  a  matrix  for  the  remaining  minerals,  while  in  other  parts  of 
the  sections  the  chlorite,  biotite,  and  actinolite  almost  entirely  exclude  the  quartz. 
The  actinolite  is  in  needles,  which  intersect  and  often  occur  in  radiating  clusters. 
Those  included  in  the  quartz  are  usually  quite  fresh,  but  those  embedded  in  the  chlo- 
rite have  altered  to  a  large  extent  to  biotite,  all  of  which  mineral  is  secondary  to  and 
most  often  pseudomorphous  after  the  actinolite. 

4.  Ferruginous  chlorite-slate,  north  of  3.  Specimen  9341  (slide  3023),  1500  N., 
1550  W.,  Sec.  23,  T.  47  N.,  E.  44  W.,  Michigan. 

The  rock  is  dark  greenish-gray,  aphanitic,  and  has  a  well  developed  slaty  cleav- 
age. 

The  section  is  exceedingly  fine  grained.  It  consists  of  pale  green,  fibrous,  non- 
polarizing  chlorite,  finely  crystalline  and  amorphous  silica,  and  of  minute  opaque  or 
nearly  opaque  particles  of  ferrite,  with  a  little  biotite. 

5.  Oalciticand  chloritic  slate  interstratified  with  4.  Specimens  9342  (slide  2979), 
9343  (slide  3116),  1500  N.,  1550  W.,  Sec.  23,  T.  47  N.,  E.  44  W.,  Michigan. 

The  rocks  are  light  grayish-green,  fine  grained  and  vary  from  slaty  to  schistose. 

The  thin  sections  consist  of  pale  green  nonpolarizing  chlorite  and  calcite  or  other 
carbonate,  and  a  finely  crystalline  chtjlcedon'ic  and  amorphous  silica,  with  also  appar- 
ently some  kaolin  or  sericite.  The  carbonate  is  particularly  abundant,  and  gives  the 
sections  an  appearance  closely  allied  to  some  of  the  sideritic  cherts  of  the  iron-bear- 
ing belt. 

6.  Greenstone-conglomerates,  interstratified  with  4  and  5.  Specimens  9338 
(slide  3114),   9339  (slide  3L15),   1500    N.,  1550  W,;  7452  (slide  1906),  1500  N.,  1710 


T!1K   KASTEKN  Ah'EA.  383 

W.;  7433  (slides  2048  iind    L'O-l't),  l.JOO  N.,  U>r>i)  W.,    Sec.    2:?,  T.  47  N.,  K.  44  W., 
Michi.i;:Mi. 

Tlie  rocks  are  like  2.  Upon  the  weatliered  surface  t lie  more  resistiiiit  i)ebbles 
Itrotrude  with  oval  outlines.  When  tlie  rock  is  freshly  fractured  tlie  jiale  yrayish- 
fireeu,  rounded  pebbles  are  well  delincd  in  their  darker  colored  matrix. 

In  thin  section  the  smaller  frai^inents  and  (heir  relations  to  the  backj;j;round  are 
precisely  as  in  2.  This  background  is  liner  grained,  ami  a])pears  to  consist  of  very 
finely  crystalline  quartz,  mingled  with  the  finest  sort  of  debris  from  material  like 
that  composing  the  fragments.  This  background  and  the  smaller  fragments  are 
arranged  in  a  roughly  stratiform  May,  which  must  indicate  stratification  or  a  Howage 
structure.  The  sections  contain  abundant  larger  fragments,  which  vary  in  character 
from  those  having  an  almost  wholly  devitrified  glassy  background  containing  much 
altered  tabular  plagioclases  to  those  which  have  a  holocrystalline  base  in  which  is 
found  many  comparatively  fresh  augites  and  jxirphyritic  crystals  of  plagioclase. 
The  plagioclase  has  often  wholly  altered  to  white  uuca  or  has  been  rejilaced  by  (-alcite. 
Since  the  above  pebbles  contain  augite,  they  are  properly  augitei)()ipliyiites. 

7.  Greenstoue-conglonierates  north  of  (i,  near  middle  of  belt.  Specimens  '.1337 
(slide  3022),  75  N.,  1500  W.;  7465  ; slide  1910),  7400  (slide  1917),  7407  (slide  lOlS),  140 
]Sr.,  1500  W.;  9330  (slide  3021),  270  N.,  1500  W.;  7408  (slide  1919),  290  N.,  1500  W., 
Sec.  14,  T.  47  N.,  R.  44  W.,  Michigan. 

In  places  the  conglomerate  is  free  from  pebbles,  and  is  then  dark  grayish- 
green,  fine  grained,  and  schistose.  The  pebbles,  when  present,  are  all  of  the  green 
massive  variety  described  in  2.  They  vai;y  in  size  from  those  so  small  as  to  be  lost  in 
the  matrix  to  those  several  inches  in  diameter.  The  conglomerate  is  thickly  studded 
■with  the  pebbles,  which  are  best  seen  upon  the  weathered  surface. 

The  matrices  of  the  sections  are  of  two  varieties;  in  one,  finely  crystalline^  quartz 
is  the  predominant  constituent,  in  the  other  it  is  subordinate  in  quantity.  In  this 
latter  phase  with  the  quartz  there  is  mingled  very  abundant  chlorite,  much  aiiiorphous 
gray  material,  and  many  partly  altered  tabular  plagioclases.  The  phase  of  the  ma- 
trix rich  in  (juartz  also  carries  these  minerals,  although  in  less  quantity.  Contained 
in  both  varieties  of  matrix  are,  sciattered  somewhat  sparsely,  large  well  rounded 
fragmental  grains  of  feldspar  and  quartz,  the  latter  being  sometimes  bunched  into 
areas  of  some  size,  the  constituent  grains  of  which  are  usually  enlarged.  Tlie  peb 
bles,  as  in  the  previously  described  conglomerates,  are  fragments  of  altered  basic 
eruptives.  (Jommonly  this  alteration  has  extended  very  far,  so  that  they  are  now  com- 
posed of  chlorite,  epidote,  and  partly  decomposed  tabular  plagioclases  as  chief  con- 
stituents, and  with  these  rather  abundant  titanite  and  some  secondary  quartz.  The 
pebbles  vary  from  quite  well  rounded  to  those  as  irregular  in  form  as  in  2.  ^hiny  of 
them  are  cut  into  or  even  apparently  dissevered  by  the  ramifying  quartzose  matrix. 
The  well  rounded  character  of  the  (piartz  grains  in  a  matrix  of  nonfragmental  quartz 
would  seem  to  be  evidence  that  these  rocks  were  formed  under  water. 


384  THE  PENOKEE  IRONBEAEING  SEETES. 

8.  Greenstone-conglomerate  uortli,  of  7.  Specimen  9331  (slide  2977),  1040  K, 
1500  W.,  Sec.  14,  T.  47  ISl.,  "R.  44  W.,  Michigan. 

The  matrix  of  the  conglomerate  i.s  dark  mottled  green,  fine  grained,  and  S(;his- 
tose.     It  is  studded  with  pebbles  and  bowlders,  some  of  the  latter  being  of  large  size_ 

The  thiu  section  is  cut  from  the  matrix.  A  continuous  rauiifyiug  mass  of  non- 
polarizing  green  chlorite  and  cherty  and  chalcedonic  quartz  contains  numerous 
roundish  com])lex  areas  of  epidote,  many  small  areas  of  titanite,  few  much  altered 
crystals  of  feldspar,  and  occasionally  large  areas  of  calcite,  these  latter  apparently 
replacing  feldspars.  There  is  nowhere  any  evidence  of  fragmental  material.  Parts 
of  the  section  are  quite  like  an  altered  eruptive,  and  it  thus  appears  probable  that  the 
matrix,  as  well  as  the  fragments  of  this  rock,  is  almost  wholly  or  wholly  a  volcanic 
product. 

Section  along  and  near  the  east  line  of  Sees.  14,  23,  and  26,  T.  47  N.,  B.  44  W.,  Michigan: 

0.  Greenstone-conglomerates.  Specimens  9358  (slide  3029),  400  IST.,  1525  W.; 
9357  (slide  4932),  450  j!^.,  1700  W.,  Sec.  25,  T.  47  N.,  E.  44  W.,  Michigan. 

The  rocks  are  in  nowise  different  from  6,  except  that  they  show  a  more  decided 
schistose  structure. 

The  thin  sections  show  no  difference  between  matrix  and  pebbles.  The  back- 
ground appears  to  consist  of  amorphous  gray  material,  in  which  are  included  exceed- 
ingly tinely  crystalline  chlorite,  ([uartz,  epidote,  perliaps  a  little  actinolite,  and  here 
and  there  nnich  altered  porphyritic  crystals  of  feldspar.  A  few  large  roundish  areas 
of  quartz  or  calcite,  or  both,  of  an  amygdaloidal  character  arc  seen.  So  far  as  the 
sections  go,  one  would  call  this  rock  a  schistose  greenstone,  and  yet  the  conglomeratic 
appearatu'e  of  the  exposure  is  marked. 

10.  Chlorite-slate,  north  of  9.  Specimen  10413  (slide  4018),  800  K,  1625  W.,  Sec. 
25,  T.  47  N.,  E.  44  W.,  Michigan. 

The  rock  is  dark  green  to  black,  aphanitic,  very  finely  laminated,  and  readily 
cleavable. 

In  thin  section  an  exceedingly  finely  crystalline  quartzose  background  contains 
abundant  chlorite  and  small  particles  of  black  and  dark  brown  ferrite.  The  arrange- 
ment of  the  chlorite  and  iron  oxide  corresponds  with  the  lamina',  of  the  rock.  Con- 
tained in  the  fine  grained  material  are  some  larger  grains  of  quartz  which  are  plainly 
fragmental. 

11.  Greenstone-conglomerate,  north  of  10.  Specimen  7459  (slides  2050  and 
2051),  345  N.,  0  W.,  Sec.  23,  T.  47  N.,  E.  44  W.,  Michigan. 

The  thin  sections  of  this  rock  do  not  differ  essentially  from  those  of  6. 

12.  Greeustone-cougloraerate;  north  of  11.  Specimen  9312  (slides  2970  and 
2971),  9313  (sUde  4929),  190  N.,  20  W.,  Sec.  14,  T.  47  N.,  E.  44  W.,  Michigan. 


THE   lOASTEHN   AREA.  385 

The  matrix  of  tlic  rock  is  daiU  iiiottiiMl  ;,^■^^(•Il,  fine,  };iaiiic(l,  mid  sciiistose.  The 
prcviiiling  pebbles  are  the  Siitne  as  those  in  7.  OiH'iisionally  a  wliite  (luartz  pebble  is 
seen. 

The  thin  sections  are  in  essential  respects  like  those  of  7,  liavinj;-  a  (jnartzose 
matrix.  Large  frajjiuents  of  extraordinary  irregularity  are  contained  in  a  reticulating 
groundniass  consisting  mostly  of  finely  crystalline  (juartz.  The  (juartz  runs  around 
and  through  the  fragments  in  such  a  manner  as  to  suggest  tluit  it  is,  in  large  pait  at 
least,  secondary  and  has  dissevered  the  contained-fragments.  This  is  rendered  prob- 
able by  the  fact  that  the  irregular  outline  of  one  fragment  is  often  the  reverse  of 
the  outline  of  the  adjoining  one.  The  fragments  are  composed  of  green  nonpolariz- 
ing  chlorite,  much  altered  tabular  plagioclases,  roundish  granules  of  titanite,  and 
abundant  epidote,  the  latter  being  sometimes  concentrated  into  large  irregular  com- 
plex areas.     (PI.  xxxv,  Fig.  2.) 

Section  east  and  west  of  the  line  between  Sec.  19,  T.  47  JV.,  E.  43  W.,  and  Sec.  24,  T.  47. 

K,  B.  44  W.,  Michigan. 

13.  G-reenstone-conglomerate.  Specimen  9350  (slide  4931),  20  N.,  475  W.,  Sec 
24,  T.  47  N.,  E.  44  W.,  Michigan. 

The  sj)ecimen  does  not  differ  from  those  of  2. 

The  tWn  section  combines  the  characteristics  of  those  of  2  and  12.  The  pebbles 
vary  in  size  and  are  closely  packed  together,  so  that  the  matrix  is  very  sparse.  It 
consists  of  the  line  debris  of  the  pebbles  mingled  with  finely  crystalline  quartz. 
A  portion  of  the  j)ebbles  have  the  gray  amorpnous  background  so  common  in  2,  con- 
tained in  which  are  many  somewhat  altered  crystals  of  plagioclase.  Pebbles  of 
another  class  are  similar  to  those  in  12,  iu  that  they  contain  a  large  amount  of  epidote 
and  chlorite. 

14.  Greenstone-conglomerate.  Specimen  9349  (sMde  3027),  425  'N.,  500  W.,  Sec. 
24,  T.  47  N.,  E.  44  yi.,  Michigan. 

The  specimen  is  not  different  from  the  previously  described  greenstone- 
conglomerates. 

The  thin  section  is  cut  from  a  schistose  portion  which  .was  taken  to  be  the 
matrix,  and  is  plainly  a  much  altered,  very  fine  grained,  amygdaloidal  porphyrite. 
The  background  now  consists  of  gray  amorjihous  material,  much  altered  plagioclases, 
and,  as  secondary  materials,  chlorite,  epidote,  and  small,  brilliantly  polarizing  flakes. 
The  amygdules  are  chiefly  chalcedonic  quartz,  but  frequently  associated  with  it  are 
chlorite  and  calcite. 

15.  Greenstone-conglomerate.  Specimen  9300  (slide  3013),  850  N.,  1750  W.,  Sec. 
19,  T.  47  N.,  E.  43  W.,  Michigan. 

The  matrix  of  the  conglomerate  is  dark  green,  rather  coarse  grained,  and 
schistose.  The  pebbles  do  not  differ  from  those  of  the  previously  described  con- 
glomerates. 

MON  XIX 25 


386  THE  PENOKEE  lEON-BEAEING  SEEIES. 

The  thin  section  is  like  those  of  13,  except  that  the  qiiartzose  background  is 
finer  grained. 

16.  Greenstone-conglomerate.  Specimen  7451  (slides  1905  and  2153),  900  N., 
850  W.,  Sec.  34,  T.  47  N.,  E.  44  W.,  Michigan. 

The  rock  varies  from  dark  grayish  green  to  dark  green,  is  schistose,  and  weathers 
to  a  dirty  yellowish  brown  color. 

In  thin  section  the  matrix  of  the  conglomerate  is  very  fine  grained  and  schistose. 
It  consists  of  finely  crystalline  quartz  and  the  debris  of  basic  eruptives,  including 
chlorite,  epidote,  and  brilliantly  polarizing  needles  arranged  in  parallel  lines.  The 
fragments  vary  from  those  of  large  size,  which  are  very  plainly  altered  porpliyrites,  to 
minute  altered  particles  which  are  lost  in  the  contained  matrix.  Epidote  is  plentiful 
in  all  parts  of  the  section. 

17.  Greenstone-conglomerates.  Specimens  7447  (slide  1900),  1935  N.,  1000  W.; 
9311  (slide  4928),  1950  K,  1350  W.,  Sec.  24,  T.  47  K,  E.  44  W.,  Michigan. 

The  rock  is  dark  mottled  green,  roughly  schistose,  and  contains  numerous  small, 
vaguely  outlined  pebble-like  areas. 

These  sections  do  not  difter  materially  from  those  of  12. 

With  the  naked  eye  the  well  rounded  outlines  of  the  pebbles  are  clearly 
distinguished  fi'om  the  matrix  in  wlii(;h  they  are  contained.  The  background 
microscopically  consists  of  quartz  (much  of  which  is  chalcedonic),  chlorite,  epidote, 
and  gray  amorphous  material.  The  large  pebbles  have  a  background  consisting  in 
about  equal  proportion  of  gray  material  and  pale  green  nonpolarizing  viridite,  which 
contains  greatly  altered  plagioclases  and  finely  crystalline  secondary  quartz. 

Exposures  in  the  east  part  of  Sec.  20,  T,  47  N.,  R.  43  W.,  MicMgan. 

18.  Greenstone-conglomerates.  Specimens  9242  (slide  2949),  950  N.,  625  W.; 
9239  (slide  4925);  9241  (slide  2948),  920  N.,  920  W.,  Sec.  20,  T.  47  K,  E.  43  W., 
Michigan. 

The  rocks  are  like  2,  except  that  the  matrix  is  distinctly  schistose. 

The  thin  sections  are  like  those  of  2  so  far  as  the  fragments  are  concerned,  but 
the  matrix  consists,  aside  from  finely  crystalline  quartz,  of  the  fine  debris  derived 
from  the  basic  fragments. 

19.  Greenstone-conglomerate.  Specimen  7415  (slide  1874),  1075  N.,  35  W.,  Sec. 
20,  T.  47  N.,  E.  43  W.,  Michigan. 

The  specimen  is  like  2. 

The  section  is  from  a  pebble  of  the  conglomerate.  It  is  a  fine  grained,  amygda- 
loldal  j)orphyrite,  having  a  gray,  nearly  amorphous  background,  which  contains 
greatly  altered  plagioclases,  epidote,  kaolin,  and  some  calcite.  Frequently  the  epidote 
is  contained  in  the  plagioclase  and  clearly  is  secondary  to  it.  A  large  amygdule  con 
tains  a  core  of  epidote,  which  is  surrounded  by  an  aggregate  of  jiale  green  chlorite. 


THE  ^;ASTE1{^•  AKEA.  387 

ETposurcs  nvar  conur  of  Sees.  11,  ]'>,  ^^,  So,  T.  17  h'.,  R.  tS  TV'.,  Miehigan. 

liO.  CiiTciistiiiie-conaloim'iiitcs.  8pccimc'ii  {yj,21  (slide  :iy44),  1985  N.,  170  W.; 
7377  (slide  L'OW),  IIIIK)  N.,  ISO  W.,  See.  lili,  T.  47  N.,  R.43  W.,  Mieliigaii. 

The  matrix  of  the  rock  is  dull  jjiay,  in  ])laces  heavily  iron  stained,  aphanitic, 
aiid  sometimes  slaty.  The  apparent  i)ebbles  resemble  very  closely  the  nuitrix  in 
whieli  they  are  contained. 

The  thin  sections  are  very  fine  grained  and  obscure.  They" appear,  however,  to 
contain  largo  illy  deliued  fragments,  derived  from  a  line  grained  jrorphyrite,  M'hich  are 
mostly  composed  of  gray  material  containing  minute  tabular  plagioclases.  These 
fragments  are  set  in  a  matrix,  the  chief  constituent  of  which  is  quartz,  but  which 
also  contains  abundant  material  like  that  composing  the  tragments. 

21.  Greenstone-conglomerate.  Specimens  9222  (slide  2942),  9223  (slide  3002), 
9225  (slide  2943),  7375  (slide  1843),  1970  N.,  1640  W.,  Sec.  23,  T.  47  N.,  R.  43  W., 
Michigan. 

The  matrix  of  the  rocks  is  dark  green  and  thinly  foliated.  The  i)ebbles  are  in 
part  of  the  pale  green  sort  found  in  the  ])reviously  described  conglomerates,  while 
some  of  them  are  coarser  grained  than  usual  and  have  the  characteiistic  ajipearance 
of  massive  basic  eruptives. 

The  matrix  consists,  as  in  several  cases  before,  of  finely  crystalline  quartz, 
mingled  with  the  debris  derived  from  the  basic  fragments.  The  fragments  vary  in 
size  fi'om  this  tine  material  to  large  bowlders.  Some  of  the  smaller  fragmental 
areas  are  almost  completely  altered  to  chlorite  and  gray  material,  but  the  roundish 
forms  which  they  still  retain  probably  represent  original  fragments.  The  larger 
pebbles  are  plainly  from  basic  eruptives.  The  coarser  ones  consist  of  greatly  altered 
plagioclase,  gray  leucoxene,  chlorite,  and  blades  of  actinolite  or  hornblende,  the 
latter  being  plainly  of  secondary  origin.  At  times  these  hornblende  blades  are  so 
large  as  to  include  many  particles  of  the  other  minerals  of  the  pebbles,  even  con- 
taining so  much  foreign  material  at  times  as  to  make  a  single  individual  of  horn- 
blende appear  in  section  as  detached  areas. 

SECTION  IV.— FRAGMENTAL  AND  FERRUGINOUS  ROCKS  NORTH  AND  EAST  OF 
THE  GREENSTONE-CONGLOMERATES. 

Geographical  distribution. — North  and  east  of  the  greenstone-conglom- 
erates occurs  a  continuous  wide  belt  of  fragmental  rocks,  which  extend 
from  near  the  north  quarter  post  of  Sec.  14,  T.  47  N.,  R.  44  W.,  Michigan, 
to  the  center  of  Sec.  28,  T.  47  N.,  R.  42  W.,  Michigan,  a  distance  of  about 
11  miles.  Through  T.  47  N.,  R.  43  W.,  Michigan,  this  belt  runs  in  a  nearly 
east  and  west  du-ection,  and' probably  has  a  surface  width  through  this  town- 
ship of  about  1^  miles.     West  of  T.  47  N.,  R.  43  W.,  Michigan,  the  few 


388  THE  PENOKEE  lEON-BEAEING  SERIES. 

exposures  found  indicate  a  northern  trend  to  the  belt.  In  the  other  direction 
— that  is,  east  of  T.  47  N.,  R.  43  W.,  Michigan,  the  belt  bends  southward, 
entering  T.  47  N.,  R.  42  W.,  Michigan,  in  Sees.  18  and  19,  and  from 
this  place  describes  approximately  the  arc  of  a  circle,  the  easternmost  point 
of  which  is  in  the  center  of  Sec.  28. 

Surrounding  r.ocks. — The  rocks  south  of  the  belt  vary  in  their  charac- 
ter. In  Sec.  14,  T.  47  N.,  R.  44  W.,  Michigan,  this  rock  is  a  porphyrite,  of 
such  a  nature  as  to  indicate  that  it  is  a  surface  flow.  Through  the  greater 
part  of  the  distance,  from  the  west  line  of  Sec.  13,  T.  47  N.,  R.  44  W., 
Michigan,  to  the  northwest  part  of  Sec.  23,  T.  47  N.,  R.  43  W.,  Michigan, 
the  greenstone-conglomerate  is  the  nearest  known  rock.  It  is  to  be  observed 
that  here  is  a  strip  of  country,  in  some  places  as  much  as  half  a  mile  wide, 
in  which  no  exposures  are  known  between  the  fragmental rocks  and  the  green- 
stone conglomerates  ;  therefore  there  may  be  between  these  two  formations 
another  belt.  From  the  north  quarter  post  of  Sec.  23,  T.  47  N.,  R.  43  W., 
Michigan,  to  the  eastern  end  of  the  belt,  its  southern  boundary  is  the  under- 
lying complex  of  hornblende-schists  and  mica-schists,  gneisses  and  granites. 
Here  there  is  little  doubt  that  these  rocks  are  the  immediately  underlying 
ones  ;  for  in  several  places  in  the  fragmental  series  are  basal  conglomerates 
and  recomposed  granites,  which  are  chiefly  composed  of  debris  derived 
from  the  crystalline  rocks  immediately  to  the  south,  while  in  one  sec- 
tion the  actual  contacts  between  the  fragmental  and  nonfragmental  rocks 
are  seen. 

The  rocks  north  of  the  fragmental  belt,  from  its  western  end  to  near 
the  center  of  Sec.  18,  T.  47  N.,.  R.  42  W.,  Michigan,  are  the  greenstones  of 
the  overlying  Keweenaw  series.  In  two  places  the  fragmental  rocks  are 
found  very  close  to  the  greenstones,  and  it  is  therefore  probable  that  these 
rocks  lie  immediately  to  the  north  of  the  fragmental  belt.  East  of  the  center 
of  Sec.  18,  T.  47  N.,  R.  42  W.,  Michigan,  no  exposures  are  found  northeast 
of  the  belt  except  in  Sec.  28,  where  the  Eastern  sandstone  is  found  in  hori- 
zontal position,  unconformably  overlying  the  fragmental  belt  of  rocks 
under  discussion. 

Continuation  of  the  helt  east  and  west. — Whether  this  belt  continues  east 
and  west  of  the  area  outlined  for  it  upon  PI.  xiii  is  an  open  question.    West 


.TIIK   HAHTICUN   AltlOA.  389 

of  the  westernmost  exposure  of  the  belt,  in  tlie  nortli  part  of  Sec.  14,  T.  47 
N.,  R.  44  \V.,  Michigan,  is  tlie  valley  of  the  Little  Presque  Isle,  in  which 
there  are  no  exposures.  About  a  mile  west  of  the  Presque  Isle,  and  1.^ 
miles  west  of  the  exposure  referred  to,  is  developed,  b}-  a  test  pit,  rocks 
which  are  almost  purely  nonfraymental  sediments,  but  which  have  mingled 
with  them  some  fragmental  sedimentation.  It  is  doubtful  whether 
the  belt  under  discussion  ought  not  to  be  carried  west  to  this  pit,  and  from 
here  be  continued  westward  until  it  merges  into  the  iron-bearing  belt  of  the 
main  area.  The  eastern  «nd  of  the  belt,  in  Sec.  28,  T.  47  N.,  R.  42  W.,  Mich- 
igan, is  very  narrow.  Between  the  gneisses  and  granites  and  the  horizontal 
Eastern  sandstone  are  but  a  few  score  of  feet,  or  at  most  one  or  two  hundred 
feet.  There  is,  then,  but  little  room  at  surface  for  the  belt  under  considera- 
tion. Whether  southeast  of  the  center  of  section  28  it  is  entirely  covered 
by  the  Eastern  sandstone  or  not  we  have  no  means  of  knowing'.  In 
case  the  Eastern  sandstone  does  thus  overlap,  we  have  here  the  eastern 
end  of  the  Penokee  series. 

Struchire  of  the  belt. — The  exposures  in  T.  47  N.,  R.  44  W.,  Michigan, 
and  most  of  those  in  Sec.  28,  T.  47  N.,  R.  42  W.,  Michigan,  are  without 
structure;  but  the  most  of  the  exposures  in  T.  47  N.,  R.  43  W.  and  42  W., 
Michigan,  are  clay-slates  or  graywacke-slates,  which  have  well  defined 
strikes  and  dips.  In  the  few  outcrops  in  T.  47  N.,  R.  42  W.,  Michigan,  in 
which  this  dip  is  known,  it  is  northeast.  These  dips  are  not  indicated 
upon  the  map  because  they  are  not  so  accurately  known  as  could  be 
desired.  However,  at  one  exposure  in  the  southeast  part  of  Sec.  20,  the 
dip  is  clearly  northeast,  while  the  statement  of  the  explorers  who 
have  done  the  test  pitting  generally  agree  with  the  above  statement  as  to 
a  northeasterly  dip.  In  T.  47  N.,  R.  43  W.,  Michigan,  however,  where  the 
exposures  are  the  most  numerous  of  anywhere  in  the  belt,  a  portion  of 
them  dip  north  and  a  portion  south,  the  greater  number  of  exposures  hav- 
ing the  latter  inclination.  These  apparent  southern  dips  are  of  great  impor- 
tance, because  these  are  the  only  known  ledges  with  a  southern  dip  which 
unquestionabl}^  belong  to  the  Penokee  series. 

The  more  ferruginous  of  these  rocks — that  is,  the  few  exposures  which 
approach  a  jasper — have  a  northern  dip.     The  rocks  which  show  a  southern 


390  THE  PENOKEE  lEOlSr-BEARING  SERIES. 

inclination  are  rather  laard,  green  and  black  slates.  All  ledges  in  wliich  a 
southern  dip  was  found  were  closely  examined,  in  order  to  ascertain 
whether  this  apparent  southern  dip  is  true  bedding,  or  is  due  to  slaty  cleav- 
age. It  was  found  impossible  to  determine  this  point  in  most  cases. 
In  one  or  two  ledges  there  is  a  decided  banding  with  northern  dips  trans- 
verse to  the  southern  cleavage.  In  these  cases,  at  least,  it  seems  probable 
that  the  southern  dips  are  cleavage  and  not  bedding.  Taking  all  the  facts 
into  consideration,  it  is  probable  that  all  of  these  apparent  southern  inclina- 
tions are  due  to  cleavage  and  the  true  dip  of  the  whole  belt  is  north- 
ward, as  a  part  of  it  certainly  is,  and  as  are  all  of  the  remaining  rocks 
of  the  whole  area  belonging  to  the  Penokee  series.  This  probability  is 
rendered  greater  by  the  fact  that  all  rocks  which  do  not  readily  take  on  a 
slaty  cleavage,  like  the  jaspers,  have  a  northern  dip ;  while  the  clayey 
rocks,  kinds  which  are  known  to  most  readily  take  a  slaty  cleavage,  are 
the  only  ones  which  exhibit  a  southern  inclination. 

General  petrographical  character. — The  rocks  of  the  belt,  as  before  stated, 
are  essentially  fragmental,  although  the  amount  of  nonfragmental  sedi- 
ments is  not  inconsiderable.  Many  of  the  exposures  are  simple  frag- 
mental rocks,  but  in  numerous  places,  mingled  with  the  fragmental,  is  a 
greater  or  less  quantity  of  nonfragmental  material ;  either  a  carbonate  and 
the  products  of  its  alteration,  or  chert,  or  both.  Grenerally  this  nonfrag- 
mental material  is  subordinate  in  quantity  to  the  fragmental,  but  in  a  num- 
ber of  places,  in  narrow  belts,  nonfragmental  sedimentation  has  built  up 
the  larger  part  of  the  rock,  while  a  considerable  thickness  in  several  places 
is  formed  by  nonfragmental  and  fragmental  sediments  in  about  equal 
proportions.  The  pure  fragmental  kinds,  those  which  are  both  fragmental 
and  nonfragmental,  and  those  which  are  purely  nonfragmental,  can  not  be 
separated  from  one  another  in  any  stratigraphical  order.  In  some  parts  of 
the  belt  its  whole  section,  so  far  as  known,  is  partly  nonfragmental ;  while 
in  others  fragmental  sediments  exclude  altogether  nonfragmental  sediments; 
and  in  yet  other  sections  both  classes  of  rocks  are  found.  The  only 
possible  classification  of  the  rocks  of  the  belt  is  then  a  lithological  one,  and 
as  their  phases  are  exceedingly  numerous,  and  the  various  phases  merge 
into  one  another,  any  classification  would  be  to  a  large  extent  arbitrary. 


THE  EASTERN  AREA. 


391 


Below  is  •^■ivcn  ii  list  of  kinds,  luit  even  this  doos  not  give  a  full  idea  of  the 
iiuuiy  pliiiscs  of  i-(ifk  whicii  iin-  nliiutst  iis  luiiiieroiis  as  the  ledges  found,  and 
whieh  if  the  wliole  tiMitli  were  knowii,  would  doubtless  comprise  all  possible 
pradations  between   tlic  \arities  mentioned: 


Fragmental 


Mingled  fragmental  and  nonfragmental . . 


Quartzitos. 

Foldspatliic  (piartzites. 
Recoiiiiioscd  fjiaiiitc — often  (onglomeratio. 
Graywacke. 
Graywacke-slate. 
Clay-slato. 
Sericite-slate. 
Chlorite-slate. 

Hematitic  and  magnetitic  quartzite. 
Henuititic  and  magnetitic  graywacke. 
Hematite-schist. 

Hematitic  and  magnetitic  graywacke-slate, 
Ferro-doloniitic  slate  and  graywacke. 
Chert-breccia. 
L   Cherty  quartzite. 

f   Chert. 

Nonfragmental  sediments ^    Cherty  ferro-dolomite. 

[   Ferro-dolomitic  chert. 

The  rocks  above  included  among  the  fragmental  sediments  are  given 
names  which  have  been  before  used  in  the  descriptions  of  the  Quartz-slate 
and.  Upper  slate  members  in  the  main  Penokee  area.  These  names  are 
here  used  with  the  same  significance  as  before  and  no  characterization  need 
be  given  of  them  as  a  whole.  In  the  tabulations  following  the  rocks 
are  described  in  detail.  The  points  of  general  interest  shown  by  the  belt 
are  the  mingled  fragmental  and  nonfragmental  sediments ;  the  change  in 
the  nature  of  the  nonfragmental  sediments  in  following  the  belt  from  west 
to  east;  and  the  presence  of  chert-conglomerates  and  basal  conglomerates 
at  the  east  end  of  the  belt. 

Mingled  fragmental  and  nonfragmental  sediments. — The  westernmost 
exposure  of  the  belt  is  a  hematitic  schist  and  quartzite  in  the  north  part  of 
Sec.  14,  T.  47  N.,  R.  44  W.,  Michigan.  The  proportion  of  iron  oxide  is  so 
great  as  to  constitute  a  continuous  ramifying  background  in  which  the  frag- 
mental quartz  and  feldspar  is  stuccoed.  In  no  other  ledges  of  as  large  size 
in  the  belt  is  the  proportion  of  iron  oxide  so  great.  ■  Rocks  almost  identical 


392  THE  PENOKEE  IROi^-BEAEING  HERIES. 

iu  character  with  these  are  found  in  the  fragmental  belt  south  of  the  green- 
stone-conglomerate, in  the  southwest  part  of  Sec.  19,  and  the  northwest  part 
of  Sec.  29,  T.  47  N.,  R.  43  W.,  Michigan.  So  like  are  the  exposures  from' 
these  different  localities  that  one  is  inclined  to  explain  their  similarity  by 
supposing  the  area  to  have  a  structure  which  makes  them  contempo- 
raneous in  formation.  It  can  as  well  be  explained,  however,  by  consider- 
ing one  of  these  belts  as  older  than  the  other,  the  similarit}'  being  due  to 
the  recurrence  of  like,  or  nearly  like  conditions  at  two  difiPerent  times. 
In  passing  to  the  eastward,  all  the  exposures  in  Sec.  13,  T.  47  N.,  R.  44  W., 
Michigan,  and  in  Sees.  18  and  19,  T.  47  N.,  R.  43  W.,  Michigan,  are  ferru- 
ginous— most  of  them  heavily.  The  larger  part  of  the  iron  contained  in  the 
rocks  is  in  the  form  of  hematite,  although  in  some  places  magnetite  is  found. 
In  the  northeast  part  of  Sec.  19,  T.  47  N.,  R.  43  W.,  Michigan,  layers  from 
mere  films  to  several  feet  thick  are  so  largely  composed  of  hematite  as  to 
resemble  somewhat  an  iron  ore.  This  amount  of  iron  oxide  is  so  great  as 
to  have  encouraged  explorers  to  follow  these  beds  to  some  depth  in  the  hope 
of  obtaining  merchantable  iron  ore.  In  Sees.  17  and  20,  and  16  and  21,  T 
47  N.,  R.  43  W.,  Michigan,  the  exposures  are  much  less  heavily  ferruginous 
than  farther  to  the  westward.  The  amount  of  hematite  and  limonite 
remains  considerable,  and  this  is  particularly  true  iu  the  south  half  of  the 
belt.  The  most  of  the  iron  is  in  the  form  of  oxide  as  before,  but  with  this 
iron  oxide  a  considerable  quantity  of  ferriferous  carbonate  is  associated.  The 
ferro-dolomite  or  siderite  is  seen  in  all  stages  of  alteration  to  iron  oxide, 
the  areas  of  the  latter  at  times  being  beautiful  pseudomorps.  This  car- 
bonate is  also  accompanied  by  a  good  deal  of  finely  crystalline  or  cherty 
quartz.  In  the  cases  of  some  exposures  in  Sees.  1  6  and  20,  T.  47  N.,  R. 
43  W.,  Michigan,  the  amount  of  nonfragmental  material  is  fully  as  great  as 
the  fragmental  quartz  and  feldspar  mingled  with  it.  From  the  east  side 
of  Sec.  15  to  the  east  end  of  the  area  in  Sec.  28,  T.  47  N.,  R.  42  W.,  Mich- 
igan, the  quantity  of  iron  oxide  contained  in  the  belt  is  small.  Upon  the 
other  hand,  the  amount  of  ferriferous  carbonate  and  of  cherty  silica  is  far 
greater  than  to  the  westward.  The  exposure  in  the  eastern  part  of  Sec. 
15,  T.  47  N.,  R.  43  W.,  Michigan,  is  in  almost  equal  quantit}^  fragmental 
and  nonfragmental  material.     In  places,  in  narrow  bands,  cherty  silica  and 


THE  EASTERN  AKEA.  393 

carbonate  are  almost  free  from  any  fragmentxil  material,  but  the  rock  com- 
jjosinginost  of  the  exposure  has  a  background  of  chert  and  carbonate  which 
contains  abundnnt  fragniental  material.  Almost  in  contact  with  the  crys- 
talline rocks  to  the  soutli,  in  the  north  part  of  Sec.  23,  is  a  band  a  few  feet 
thick  of  nearly  pure  cherty  ferro-dolomite,  and  the  fragmental  rocks  to  the 
north  contain  quite  a  quantity  of  chert  and  carbonate.  In  the  exposures 
along  the  east  side  of  Sec.  14,  T.  47  N.,  R.  43  W.,  Michigan,  feiTO-dolomite 
is  a  chief  constituent.  From  the  west  side  of  Sec.  13,  T.  47  N.,  R.  43  W., 
Michigan,  to  the  center  of  Sec.  28,  T.  47  N.,  R.  42  W.,  Miclijgan,  chert  and 
feiTO-dolomite  (chiefly  the  former,  except  locally)  constitute  the  background 
in  which  the  abundant  fragmental  material  i's  contained;  while  in  some  of 
the  quartzites  and  conglomerates  the  n( mfragmental  material  sinks  to  an 
insignificant  quantity. 

It  will  thus  be  seen  that  there  is  a  marked  change  in  the  nonfrag- 
mental  sediments,  in  passing  from  west  to  east.  Throughout  its  eastern 
portion  this  matei'ial  is  cherty  silica  and  a  ferriferous  carbonate;  in  its 
central  part  it  is  cherty  carbonate  and  iron  oxides,  and  here  in  most  of  the 
sections  the  iron  oxides  are  seen  in  actual  process  of  formation  from  the 
carbonate;  in  the  western  part  of  the  belt  the  iron  present  is  almost 
wholly  in  the  form  of  oxides.  Considering  these  facts  by  themselves,  it 
seems  a  natural  conclusion  that  it  is  probable  that  all  of  the  iron  was 
originally  present  as  a  carbonate<and  that  the  iron  oxides  now  found  have 
been  formed  from  the  alteration  of  such  material.  This  probability  is 
made  almost  a  certainty  when  the  facts  are  taken  in  connection  with  what 
has  gone  before  in  reference  to  the  iron  oxides  of  the  Iron-bearing  member 
in  the  main  area  to  the  westward.  We,  then,  have  here  another  belt  in 
which  the  widespread  occurrence  of  iron  oxide  is  due  to  the  presence  of  an 
©riginal  feniferous  carbonate. 

From  the  foregoing  it  is  evident  that  this  belt  of  rocks  is  different 
from  any  considerable  area  found  in  the  main  western  area.  The  series 
there  consists  of  four  great  formations,  which  are  alternately  nonfragmental 
and  fragmental  sediments,  and  which  are  separated  from  each  other  with 
surprising  sharpness.  In  only  a  few  localities  are  found  transition  belts 
between  the  two  classes  of  sediments,  and  these  are  mostly  narrow;  but 


394  THE  PENOKEE  lEON-BEARING  SEEIES. 

tlie  belt  under  discussion — probably  as  much  as  2,000  feet  thick — is  in 
part  a  fragmental  and  in  part  a  nonfragmental  sediment.  Such  deposits 
are  rather  unusual  in  the  Iron-bearing  series  in  the  Northwestern  states. 
Layers  of  mingled  fragmental  and  nonfragmental  material  are  present 
in  certain  localities  in  the  Animikie  and  other  series.  According  to 
the  ideas  generally  accepted  among  geologists  nonfragmental  sediments 
originate  under  water  of  considerable  depth  which  is  comparativel}^  quiet. 
How,  then,  does  it  occur  that  a  thick  belt  of  sediment  contains  both  frag- 
mental and  nonfragmental  material!  This  singular  occurrence  can  be 
explamed  by  supposing  imusual  conditions  to  have  prevailed  at  the  time 
of  the  formation  of  the  belt.  *In  the  present  place  it  will  only  be  remarked 
that  it  is  deemed  probable  that  the  exceptional  conditions  were  due  to 
eruptive  activity  in  the  immediate  neighborhood.  This  probable  connec- 
tion between  the  mingled  fragmental  and  nonfragmental  sediments  of  the 
belt  under  consideration  and  the  eruptives  of  the  greenstone  and  greenstone- 
conglomerate  areas  will  be  more  fully  considered  in  another  place. 

Coarsely  fragmental  rocks. — In  the  north  part  of  Sec.  23  and  the  north- 
east part  of  Sec.  24,  T.  47  N.,  R.  43  W.,  Michigan,  and  in  Sec.  28,  T.  47 
N.,  R.  42  W.,  Michigan,  occur  exceptional  phases  of  rock,  recomposed 
granites,  conglomerates,  and  chert-breccias,  mingled  with  which  are  cherty 
and  ferro-dolomitic  quartzites.  Recomposed  granites  occur  at  each  of  the 
places  mentioned.  In  the  field  these  rocks  so  closely  resemble  the  orig'inal 
granite  that  they  are  with  difficulty  separated  from  ii.  Only  in  thin  section 
is  the  fundamental  difference  appreciated.  The  granites  to  the  south  have 
a  structure  characteristic  of  a  thoroughly  crystalline  rock.  Thin  sections 
of  the  recomposed  granite,  upon  the  other  hand,  show  it  to  consist  of 
fragments  of  various  sizes,  which  are  packed  very  closely  together. 
Most  of  these  fragments  are  complex,  and  if  a  section  chances  to  be  cut 
from  a  single  one  of  them  it  is,  of  course,  precisely  like  a  section  from  an 
ordinary  granite.  A  set  of  sections,  however,  is  sure  to  disclose  the  fact 
that  these  rocks  are  clastic,  there  being  always  discoverable  thin  films  and 
small  areas  of  finer  material  filling  the  spaces  between  the  larger  fragments 
and  cementing  them  together.  This  cementing  material  is  generally  finely 
crystalline   silica  and  a  carbonate  whicli  is   usually  ferriferous.      Tliese 


THE  EASTERN  AREA.  395 

peculiar  rocks  iirc  of  j^rcjit  interest  liecause  tlle^■  sliow  how  closely  a 
completely  crystalliiu'  and  a  traji'iiieutal  rock  iiia\'  reseiulilc  each  other. 
The  resemblance  in  the  ledge  is  at  times  so  chise  as  to  make  it  iiupossihle  to 
determine  where  the  massive  g-ranite  ends  and  the  f'ragmental  rock  Ix^gins. 

The  conglomerates  differ  from  the  recomposed  granites  in  that  a-mong 
the  fragments  of  which  they  are  contained  are  other  materials  with  the 
granite  debris.  They  are  generally  closely  associated  with  the  recomposed 
granites,  both  often  occurring  within  a  short  distance  of  each  other  and 
practically  in  the  same  exposm-e.  As  would  naturally  be  expected,  sim[)le 
and  complex  granite  fragments  are  very  abundant  in  the  associated  con- 
glomerates, but  mingled  with  these  granite  fragments  are  white  quartz, 
green  schist,  and  less  frequently  other  varieties  of  pebbles.  The  matrix 
of  the  conglomerates  is  essentially  like  that  of  the  recomposed  granites. 
In  general  the  matrix  is  more  abundant,  the  simple  grains  of  quartz  buried 
in  the  matrices  are  often  enlai'ged,  and  in  a  few  cases  the  feldspar  fragments 
also  appear  to  have  imdergone  a  second  growth; 

Closely  associated  with  the  recomposed  granites  and  the  conglomerates 
are  cherty  quartzites  and  chert-breccias.  The  chert-breccias  are  a  variety 
of  rock  in  which  a  background  of  almost  pure  chert  detritus  contains 
rounded  and  angular  fragments  of  the  same  material.  In  its  purest  phase 
finely  crystalline  and  amorphous  silica  only  is  present.  In  the  cherty 
quartzites  a  background  of  chert  and  ferriferous  carbonate  contains  much 
fragmental  material.  By  short  steps  the  chert-breccias  and  cherty  quart- 
zites grade  into  each  other. 

The  close  association  of  the  recomposed  granites,  the  conglomerates, 
and  the  chert-breccias  and  quartzites  coiTesponds  to  their  lithological 
chai-acter,  their  matrices  being  essentially  the  same  in  each  case.  In  the 
carbonated  cherts  nonfragmental  sedimentation  has  prevailed  almost  to  the 
exclusion  of  mechanical  sedimentation.  In  the  ferro-dolomitic  and  cherty 
quartzites,  while  nonfragmental  sedimentation  was  going  on,  a  large  amount 
of  fragmental  material  was  brought  in  and  intermingled.  In  the  con- 
glomerates and  recomposed  granites  mechanical  sedimentation  was  prepon- 
derant, but  a  sufficient  amount  of  a  nonfragmental  sedimentation  occiu-red 
sinmltaneously  to  firmly  cement  the  clastic  material. 


396  '  TEE  PENOKEE  IRON-BBAEIN^G  SERIES. 

In  tlie  ordinary  quartzites  which  have  been  indurated  by  the  enlarge- 
ment of  .quartz-grains  it  is  always  taken  for  granted  that  the  induration  is 
subsequent  to  the  deposition  of  the  fragmental  quartz.  Doubtless  when 
the  grains  of  quartz  lie  close  to  one  another  and  the  enlargements  have  con- 
tinued until  they  completely  interlock  this  is  true,  but  in  the  cases  of  the 
■above  quartzites,  in  which  a  considerable  quantity  of  cherty  silica  is 
present,  it  is  possible  that  the  silica  which  now  forms  the  matrix,  and  to 
wliich  the  induration  is  due,  was  deposited  at  least  in  part  simultaneously 
with  the  fragmental  material,  perhaps  by  organic  agencies.  If  so,  it  has 
subsequently  been  rearranged. 

TABULATION   OF   PETEOGEAPHICAL   OBSEKVATIONS.' 

Sections  from  the  east  part  of  T.  47  N.,  B.  44  Tf.,  and  the  W.  2  miles  of  T.  47  If.,  R.  43 

W.i  Michigan. 

1.  Ferruginous  quartzite.  Specimens  9318  (slide  3018),  9319  (slide  2972),  9320 
(slide  2973),  1825  N.,  975  W.,  Sec.  14,  T.  47  N.,  R.  44  W.,  Miclii,^an. 

The  rocks  are  gray,  dark  brown  or  black,  medium  grained,  and  vary  from  mass- 
ive to  schistose. 

In  thin  sections  a  continuous  ramifying  mixture  of  hematite  and  brown  opaque 
oxide  of  iron  contains  numerous  rather  small  grains  of  quartz  and  feldspar.  The 
simple  particles  of  quartz  are  often  enlarged.  The  feldspar,  which  is  much  less 
abundant  than  the  quartz,  is  mostly  fresh,  although  a  few  grains  are  kaoliuized.  The 
sections  are  almost  exactly  like  2962  and  1891  (No.  2  of  fragmental  rocks  south  of 
greenstone  conglomerates,  p.  371). 

2.  Ferruginous  and  chloritic  quartzites.  Specimens  7445  (slide  1898),  900  N., 
0  W. ;  9308  (slide  3111),  925  N.,  40  W.,  Sec.  13,  T.  47  N.,  R.  44  W.,  Michigan. 

The  rocks  are  alternately  light  and  dark  greenish  gray,  fine  grained,  and  com- 
pact. 

The  thin  sections  are  mostly  composed  of  small  well  rounded  grains'of  quartz, 

with  also  some  of  feldspar.  The  induration  of  the  rocks  is  due  to  the  enlargement 
of  the  quartz  grains,  and  to  a  rather  plentifal  interstitial  carbonate,  which  is  probably 
ferriferous.    Chlorite  and  iron  oxide  are  also  plentiful. 

3.  Magnetitic  graywacke.  Specimens  7433  (slide  1888),  9309  (slide  29G9),  9310 
(sbde  3016),  450  N.,  1350  W.,  Sec.  18,  T.  47  K,  R.  43  W.,  Michigan. 

The  rocks  are  dark  green,  fine  grained,  schistose.  The  bedding  is  almost  per- 
pendicular to  the  plane  of  readiest  cleavage. 

'The  immbeis  of  specimens  and  slides  are  those  of  the  collection  of  the  Lake  Suiierior  Division. 
Locations  are  given  from  the  sontheast  corners  of  the  sections,  in  Hteps  of  2,000  per  mile. 


THE  EASTERN  AREA.  397 

The  tliiii  sections  show  (li;i I  llic  rocks  in  tlicir  ()iij;iiiiil  cDiiditioii  were  mostly 
composed  of  rather  small  fnijiiiieiital  i>arti(^les  of  (iiiartz  anil  fcldspiii-.  The-graiiis  of 
qnaitz  have  received  secondary  enlar},a'nient.  Those  of  feldspar  liav<\  larj;ely  altered 
to  aggregates  of  green  chlorite,  this  mineral  now  heitig  one  of  the  most  i)lentifiil  in 
the  section.     Uniformly  distributed  are  luiinerons  crystals  of  magnetite. 

•4.  Ilematitic  graywackes  and  henuititic  schists.      Specimens  D.'JUliA  (slide  3015) 
140  N.,  1000  W.;  7138  (slide  1S!)3),  150  K,  1000  W.;  !);!03  (slide  tL'liO);  0301  (slide  4L'31); 
0305  (slide  3109),   175   N.,  975   W.;  7439  (slide  1S94),    ISO  N.,  1000    W.;  7440  (slide 
1895),  190  N.,  1000  W.,  Sec.  18,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rocks  vary  from  rather  fine  grained,  banded,  dark  greenish  gray  gray  wackes 
to  a  dark  brown  hematitic  schist.  The  belts  of  heavily  ferruginous  material  vary  in 
thickness  from  mere  iilms  to  layers  several  feet  across. 

In  thin  sections,  the  least  ferrnginous  phases  are  mostly  composed  of  small  frag- 
mental  particles  of  quartz  and  feldspar.  The  enlargements  of  the  qnartz  are  often 
relatively  wide.  The  feldspar  is  altered  to  a  considerable  extent  to  kaolin  and  chlorite. 
In  the  interstices  are  found,  in  varying  proportions,  tinely  crystalline  quartz,  chlorite, 
hematite,  crystals  of  magnetite  and  liakes  of  white  mica.  In  the  more  ferruginous 
phases,  the  hematite,  with  some  magnetite,  constitutes  a  continuous  ramifying  sheet 
which  contains  fragmental  material. 

5.  Perrugiuuous  graywacke.  Specimen  9302  (slide  3014),  1850  IsT.,  325  W.,  Sec. 
19,  T.  47  K,  E.  43  W.,  Michigan. 

The  rock  is  dark  green,  fine  grained,  massive,  and  includes  a  few  crystals  of  pyrite. 
The  thin  section  ditfers  from  the  least  ferruginous  phases  of  4  only  in  that  in 
the  interstices  a  considerable  quantity  of  ferro-dolomite  is  present. 

6.  Chloritic  graywacke-slate.  Specimen  9238  (slide  4478),  1750  N.,  1250  W., 
Sec.  20,  T.  47  N.,  43  W.,  Michigan. 

The  rock  is  greenish  gray,  of  a  medium  uniform  grain  and  feebly  schistose. 

Fragmental  quartz  and  feldspar  in  about  equal  quantity,  the  latter  being  a 
mixtirre  of  orthoclase,  microcline,  and  plagioclase,  comj)ose  nine-tenths  of  the  thin 
section.  The  grains  of  quartz  are  enlarged  and  the  interstices  are  filled  with  finely 
crystalline  quartz  and  chlorite.  Iron  oxide  is  an  accessory.  The  rock  -is  a  typical 
graywacke-slate. 

7.  Chloritic  and  magnetitic  graywacke-slates,  from  south  part  of  belt.  Specimens 
9237  (slide  4477),  1625  N.,  750  W.;  9236  (slide  4476),  1750  N.,  750  W.,  Sec.  20,  T.  47 
N;,  E,  43  W.,  Michigan. 

The  rocks  are  like  6. 

The  thin  sections  differ  from  that  of  6  in  that  the  feldspars  have  altered  more 
extensively  to  chlorite  and  kaolin ;  in  that  there  was  originally  present  a  considerable 
amount  of  clayey  material,  and  in  that  they  contain  some  quantity  of  magnetite 
which  is  mostly  in  crystals.     Sericite  is  an  accessory. 


398  THE  pen6kee  ieon  bearing  series. 

8.  Ohloritic  graywacke-slate,  uortU  of  7.  Specimen  93'55A  (slide,  4475),  300  iST., 
375  W.,  Sec.  17,  T.  47  K,  R.  43  W.,  Michigan. 

The  rock  is  dark  green  and  fine  grained,  but  contains  numerous  small  grains  of 
quartz  and  altered  fi'agments  of  feldspar  of  sufficient  size  to  be  perceptible  to  the 
naked  eye. 

The  thin  section  shows  a  rather  abundant  grouudmass,  consisting  of  finely 
crystalline  quartz,  chlorite,  kaolin,  and  iron  oxide.  Contained  in  this  matrix  are 
abundant  grains  of  both  quartz  and  feldspar  of  widely  varying  sizes,  the  fornu^r 
often  being  slightly  enlarged,  and  in  such  cases  merging  gradually  into  the  clayey 

matrix. 

9.  Clay-slate,  north  of  8,  at  top  of  belt.     Specimen  9235  (slide  3003),  400  N., 

800  W.,  Sec.  17,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rock  is  dark  gray  to  black,  aphanitic,  and   has  a  well  developed  slaty 

cleavage. 

The  thin  section  is  exceedingly  fine  grained,  but  appears  to  consist  of  an  inter- 
mingled mass  of  chlorite,  quartz,  kaolin,  and  ferrite,  with  a  little  ferro-dolomite. 

Section  mostly  in  the  west  half  of  Sees.  16  and  21,  T.  47  N.,  B.  43  W.,  Michigan. 

10.  Chloritic  graywacke-slate,  from  south  part  of  belt.  Specimen  7416  (slide 
1875),  1830  N.,  100  W.,  Sec.  20,  T.  47  K.,  E.  43  W.,  Michigan. 

The  rock  closely  resembles  7. 

The  thin  section  differs  from  those  of  7  only  in  containing  in  the  interstices 
much  more  chlorite  and  apparently  a  little  magnetite. 

11.  Perro-dolomitic  graywackes,  north  of  10,  at  south  part  of  belt.  Specimens 
7397  (slide  1860),  9234  (slide  2947),  1900  N.,  210  W.,  Sec.  20,  T.  47  K,  R.  43  W.,  Mich- 
igan. 

The  rocks  are  dark  gray,  rather  fine  gTained,  and  contain  very  numerous  dark 
brown  areas  of  mingled  ferro-dolomite  and  iron  oxide,  and  white  areas  of  ferro- 
doloniite. 

The  sections  have  an  exceedingly  ffne  grained  matrix,  consisting  of  crystalline 
and  amorphous  silica,  of  chlorite,  and  of  black  opaque  iron  oxide,  a  little  of  which  is 
magnetite.  '  Scattered  plentifully  through  this  matrix  are  fragmental  particles  of 
quartz-  and  feldspar.  The  areas  which  give  the  rocks  their  mottled  appearance  are 
the  ferro-dolomite  or  limonite  or  the  two  combined.  All  of  these  areas  were  plainly 
once  ferro-dolomite,  and  the  alteration  of  this  mineral  has  produced  limonite.  All 
stages  of  the  change  may  be  seen,  from  those  areas  which  are  pure  ferro-dolomite  to 
those  which  are  wholly  limonite. 

12.  Chloritic  graywacke-slate,  from  lower  middle  part  of  the  belt.  Specimen 
7387  (slide  1852),  230  N.,  1850  W.,  Sec.  16,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rock  is  dark  gray,  fine  grained,  schistose,  but  weathers  to  a  dull  pale  yellow. 
The  thin  section  differs  in  no  important  respect  from  that  of  8. 


'I'lll';  i:  ASTERN  ARK  A.  399 

13.  Cliiy-slato,  from  iiiiddli^  dC  hell.  Spcciiiicii  T.'WS  (slide  1853),  380  N.,  1!)80  \V., 
Sec.  1(5,  T.  47  N.,  K  43  W.,  Micliif;iUi. 

The  rock  is  jji'ayish  green,  apliuiiilie,  and  iinely  sidiiptose. 

The  thill  section  has  a  Iinely  crystalline  iinitrix  consisting  of  (iiiartz,  chlorite, 
kaolin,  and  iron  oxide,  and  contains  many  small  Iraginental  particles  of  (jnartz  ajul 
feklispar. 

14.  Clay-slate,  from  upper  i)art  of  belt.  Specimen  73S0  (slide  1854),  650  N.,  50 
W.,  Sec.  17,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  black,  aplianitie,  and  easily  clcavable. 
The  thin  section  is  like  that  of  0. 

15.  Ferro-dolouiitic  slate,  from  lower  middle  part  of  belt.  Specimen  9233  (slide 
2946),  175  N.,  1610  W.,  Sec.  16,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rock  is  dark  grecnisli  gray,  fine  grained,  scliistose.  Weathering  gives  a 
thi<!k  outer  layer  of  dark  brown  material. 

The  thin  section  shows  a  gronndmass  consisting  of  ferro-doloniite,  cherty  silica, 
and  chlorite,  the  first  being  predominant.  Contained  in  this  gronndmass,  and  com- 
posing perhaps  one-fourth  of  the  rock,  are  plentiful  fragments  of  quartz  and  few  of 
feldspar.     Here  and  there  minute  blades  of  sericite  or  muscovite  are  seen. 

16.  Chloritic  graywacke-slate,  from  middle  part  of  belt.  Specimen  7398  (slides 
1861  and  2044),  500  N.,  1000  W.,  Sec.  16,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rock  is  hke  12. 

The  thin  section  is  like  that  of  12. 

Section  in  and  near  the  east  half  of  Sec.  15,  T.  47  N.,  B.  43  W.,  Michigan. 

17.  Quartzose  ferro-dolomites.  Specimens  7368  (slide  1836),  7369  (slide  1837), 
9213  (slide  3000),  9214  (slide  2937),  and  9215  (slide  2938),  40  N.,  1986  W.,  Sec.  14,  T. 
47  N.,  E.-  43  W.,  Michigan. 

The  rocks  vary  in  color  from  gray  to  bright  brick  red,  are  of  a  medium  uniform 
grain,  massive,  and  break  with  conchoidal  fracture. 

A  matrix  of  ferro-dolomite  comiioses  as  much  as  one-half  of  the  mass  of  the  thin, 
sections.  In  this  are  set  well  rounded  particles  of  quartz  and  feldspar,  which,  on 
acjcount  of  the  abundance  of  the  ferro-dolomite,  come  rarely  in  contact  with  one 
another.  The  grains  of  quartz  are  as  often  finely  comi)lex  as  simple.  As  accessories 
occur  chlorite,  sericite,  and  iron  oxide,  the  latter  being  so  abundant  in  shde  2938  as 
to  heavily  stain  the  section. 

18.  Chert,  interstratifled  with  17.  Specimens  7370  (slide  1838),  7371  (slide  1839), 
9216  (slide  2939),  9217  (slide  2940),  from  58  N.,  1984  W.,  to  98  N.,  1940  W.,  Sec.  14,  T. 
47  K,  E.  43  W.,  Michigan. 

The  rocks  are  light  to  dark  gray,  aphanitic,  have  a  conchoidal  fracture,  and  some 
of  them  contain  galena  and  chalcoiiyrite. 


400  THE  PENOKEB  IRON-BEAEING  SEEIES. 

The  sections  are  composed  almost  wholly  of  exceediugly  linely  crystalline  inter- 
locking quartz,  with  perhaps  some  amorphous  silica.  Perro-dolomite,  sericite,  and 
ferrite  are  accessories,  in  places,  the  sections  are  cut  by  veins  of  coarser  crystalline 
quartz. 

19.  Ferruginous  clay-slate.  Specimen  7373  (slide  1841),  400  N.,  30  W.,  Sec.  15, 
T.  47  N.^ll.  43  W;,  Michigan. 

The  rock  is  dark  greenish  gray  and  has  an  aphanitic  texture. 

The  thin  section  consists  of  an  exceedingly  flue  mixture  of  quartz,  cherty  silica, 
kaolin,  chlorite,  and  dark  brown  and  black  iron  oxide.-  The  section  resembles  9  but 
is  somewhat  coarser. 

20.  Ohloritic  graywacke-slates,  from  middle  part  of  belt,  north  of  16.  Specimens 
7396  (slide  1859),  500  N.,  1090  W.;  9270  (slide  4927);  9271  (slide  4926),  575  N.,  1145 
W.,  Sec.  15,  T.  47  K,  E.  43  W.,  Michigan. 

The  rocks  are  grayish  gxeen,  fine  grained,  and  have  a  feebly  developed  cleavage 
The  thin  sections  show  a  clayey  groundmass,  consisting  apparently  of  kaolin, 
quartz,  chlorite,  and  ferrite,  which  contain  many  fragments  of  both  quartz  and  feld- 
spar. The  sections  differ  from  one  another  only  in  the  relative  abundance  of  these 
larger  fragments.  In  slide  4926  the  large  fragments  are  nearly  wanting  and  the 
rock  approaches  a  clay-slate. 

Section  in  east  half  of  Sees.  14  and  23,  T.  47  N.,  B.  43  W.,  Michigan. 

21.  Cherty  ferro-dolomite,  from  base  of  belt.  Specimen  9276  (slide  3006),  1840 
N.,  1000  W.,  Sec.  23,  T.  47  N.,  K.  43  W.,  Michigan. 

The  rock  is  light  gray,  weathering  to  dark  brown,  fine  grained,  massive,  and  has 
a  conchoidal  fractui'e. 

The  section  consists  of  intimately  mingled  iiarticles  of  brilliantly  i^ol  arizing 
ferro-dolomite  and  minutely  crystalline  interlocking  quartz. 

22.  Quartzites,  from  near  base  of  belt.  Specimens  7362  (slide  1831),  1965  jST.,  580 
W.;  9274  (slide  4493),  1950  K,  590  W.,  Sec.  23,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rocks  are  gray,  fine  grained,  vitreous,  and  break  with  conchoidal  fracture. 

The  thiu  sections  have  a  fine  grained  interlocking  groundmass  composed  of 
cherty  silica,  which  contain  numerous  rounded  grains  of  quartz,  with  fewer  of  ortho- 
clase,  microcline,  and  oligoclase.  There  occur  quite  plentiful  areas  of  chlorite,  which 
in  most  cases  are  heavily  stained  with  oxide  of  iron. 

23.  Conglomerates  interstratifled  with  22.  Specimens  7364  (slide  1833),  1955  N., 
590  W.;  9273  (slide  4492),  1965  N.,  520  W.,  Sec.  23,  T.  47  N.,  E.  43  W.,  Michigan." 

The  matrix  of  the  rocks  is  mottled  pink  and  gray,  medium  grained,  and  massive. 
In  the  matrix  are  very  numerous  granite  pebbles,  which  vary  from  large  size  to  those 
so  minute  as  to  be  lost  in  the  matrix. 


TilE  KA8TEliJ^I  AltEA.  401 

In  I  lie  I  liiii  sections,  uixin  a  close  uxiiiiiin;il  ion,  one  plainly  sees  that  the  rock  is  ii 
recotnposcd  one.  l!nl  tho,  <^v;i.\uU'  fi-.v^uwuts  wliicli  luaivc  up  the  rock  arc  so  large  aud 
so  (do.sciy  packed  togetiuT  that  oiu'.  at  lirst  sigiit  niigiit  mistake  tiie  rock  for  a  massive 
granite.  However,  the  rounded  ilpp(^aranc,e  of  tlie  complex  (luartz-feklspar-cldorite 
fragments,  tlie  presence  of  a  few  rounded  grains  of  quartz  aud  feldspar  and  of  a  sparse 
matrix,  show  the  clastic  origin  of  the  rock.  Many  of  the  feldspar  grains  are  brokeu 
or  distorted,  the  hitter  phenomenon  being  beautifully  shown.  Tliis  is  probably  caused 
by  ])ressure  applied  after  the  grains  were  deposited  in  tho  recomposed  rock.  The 
matrix  consists  of  ttnely  interlocking  quartz,  kaolin,  ferrite,  some  carbonate  and 
chlorite.  Slide  1833  contains  in  the  groundmass  a  larger  portion  of  carbonate  than 
does  4492. 

24.  (Jhloritic  slate,  from  lower  part  of  belt.  Specimen  12590  (slide  5345),  155  N., 
225  W  ,  Sec.  14,  T.  47  N.,  K.  43  W.,  Michigan. 

The  rock  is  dark  gray,  iine  grained,  aud  finely  laminated.  The  thin  section  con- 
sists of  a  contlnuons  mass  of  chlorite,  ferrite,  feldspar,  quartz,  sericite,  or  kaolin,  all  of 
which  are  arranged  in  wavy  lines,  the  general  direction  of  which  is  parallel  to  the  lam- 
ination of  the  rocks.  Some  of  the  larger  particles  of  quartz  and  feldspar  have  a  deci- 
ded clastic  appearance,  so  there  is  little  doubt  that  the  rock  is  a  fragmental  slate. 

25.  Chloritic  graywacke-slate,  from  lower  middle  part  of  belt.  Specimen  10397 
(slide  4006),  207  N.,  500  W.,  Sec.  14,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  light  greenish  gray,  line  grained,  almost  aijhauitic,  and  is  readily 
cleavable. 

The  thin  section  bae  a  schistose  matrix,  consisting  mainly  of  chlorite  and  kaolin, 
or  liydromica,  with  some  chlorite  and  gray  amorphous  material,  in  which  are  set  small 
fragmental  particles  of  quartz  aud  feldspar,  the  latter  being  more  abundant. 

26.  Graywacke-slate,  iroma  lower  middle  horizon.     Specimen  12837  (slide  5496), 
■  370  N.,  0  W.,  Sec.  14,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rock  is  dark  gray,  medium  grained,  and  laminated. 

Fragmental  quartz  and  feldspar,  in  grains  varying  from  minute  to  those  of 
medium  size,  compose  about  one-half  of  the  section.  These  minerals  are  buried  in  a 
very  tine  grained  clay  background,  which  appears  to  consist  of  finely  crystalline  quartz, 
chlorite,  sericite,  aud  iron  oxide.  Many  of  the  fragments  of  feldspar  are  more  or  less 
altered,  the  resultant  i^roducts  being  two  or  more  of  the  minerals  which  compose  the 
background.     Calcite  or  other  carbonate  occurs  as  an  accessory. 

27.  Chloritic  graywacke.  Specimen  12589  (sUde  5344),  500  N.,  925  W.,  Sec.  14, 
T.  47  ]Sr.,  E.  43  W.,  Michigan. 

The  rock  is  greenish  gray,  fine  grained,  and  massive. 

Small  angular  particles  of  quartz  and  feldspar  compose  two-thirds  of  the  section. 
The  angularity  of  many  of  the  particles  appears  to  be  original,  and  it  is  probably  due 
to  the  fact  that  their  small  size  has  prevented  rounding.  A  portion  of  them  are  some- 
MON  XIX ^26 


402  THE  PENOKBE  lEON-BEARING  SERIES. 

what  rounded;  these  are  enlarged.  Apparently  also  a  few  of  the  orthoclase  particles 
are  enlarged.  The  rather  abundant  interstitial  material  is  chlorite,  finely  crystalline 
quartz,  flakes  of  kaolin  or  sericite,  and  particles  of  iron  oxide.  A  few  small  particles 
occur  which  api^ear  to  be  tourmaline. 

•28.  Perro-dolomitic  slate,  from  top  of  belt.  Specimen  12844  (slide  5499),  1000  N., 
80  W.,  Sec.  14,  T.  47  N,,  E.  43  W.,  Michigan. 

The  rock  is  light  greenish  gray,  fine  grained,  finely  laminated,  and  has  the 
appearance  of  an  impure  clayey  limestone. 

Eather  small  particles  of  quartz,  many  of  which  are  slightly  enlarged,  with  feld- 
sjiar,  both  of  quite  uniform  size,  compose  about  one-half  of  the  thin  section .  This  mate- 
rial is  set  in  a  grouudmass  which  consists  chiefly  of  ferro-dolomite.  The  ferro-dolo- 
mite  is  mostly  in  irregular  reticulating  areas,  filling  the  interstices,  but  many  detached 
particles  have  distinct  rhombohedral  outlines.  Mingled  with  the  ferro-dolomite  are 
some  finely  crystalline  quartz,  chlorite,  ferrite,  sericite.  A  few  small  crystals  of  zircon 
are  seen,  one  of  which  shows  finely  a  zonal  structure. 

29.  Ferro-dolomitic  slate,  interstratified  with  28.  Specimen  12843  (slide  5627), 
1000  K,  80  W.,  Sec.  14,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  diflers  from  28  in  being  composed  of  alternate  layers  of  gray  and  green 
material. 

The  thill  section  is  not  essentially  different  from  that  of  28,  except  that  it  contains 
less  fragmental  material.  Also  a  good  many  small  crystals  of  pyrite,  and  numerous 
roundish  areas  of  some  mineral  which  has  a  radial  polarization  (perhaps  chalcedony), 
are  found. 

Section  near  the  centers  of  Sees.  13  and  34,  T.  47  N.,  B.  43  W.,  Michigan. 

30.  Chloritic  slate,  from  near  base  of  belt.  Specimen  12591  (slide  5346),  1986  N., 
1077  W.,  Sec.  24,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  like  24,  except  that  it  is  coarser  grained. 

In  thin  section  the  minerals  and  their  arrangement  in  wavy  parallel  lines  are  the 
same  as  in  24.  The  constituents  are  in  coarser  particles,  and  a  portion  of  the  quartz 
and  feldspar  are  plainly  fragmental. 

"     31.  Chloritic  gray wacke- slate,  from  a  lower  middle  horizon.     Specimen   10396 
(slide  4005),  280  N.,  1500  W.,  Sec,  13,  T.  47  N.,  E.  43  W.,  Michigan. 

The' rock  is  like  25,  except  that  it  is  coarser  grained. 

The  thin  section  differs  ft'om  that  of  25,  in  being  coarser  grained,  and  in  contain- 
ing a  little  calcite  or  some  other  carbonate. 

32.  Gray  wacke- slate,  from  a  middle  horizon.  Specimen  7358  (slide  1827),  370 
N.,  1000  W.,  Sec.  13,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  black,  fine  grained,  almost  massive,  and  has  a  subconchoidal  fracture. 


TUE  EASTI^iN  AltEA.  403 

lu  tliiii  section,  a  fliio  gniiiuMl  grouiidinass  of  (luaitz,  cliloritc,  kaoliu,  pyrite,  con- 
tains uiiincrous  I'ragmeiits  of  ([uartz  and  Icldsirar,  tin-  latter  being  more  i)lcntiful. 

Section  near  range  Unebetween  B.  42  W.,  andli.  13  W.,  Michigan. 

33.  Kcconiposed  granite,  from  a  low  liorizon.  Si)C(!imen  12592  (slide  5347),  1700 
N.,  1332  W.,  Sec.  24,  T.  47  N.,  R.  43  W.,  Michigan. 

The  rock  is  i)iiik,  white  and  black  mottled,  coarse  grained,  massive.  In  nearly- 
all  respects  this  rock  is  in  appearance  like  a  granite.  It  weathers  to  some  depth  to  a 
dark  brown  color. 

The  section,  if  not  examined  closely,  would  be  taken  for  a  coarsely  crystalline 
granite.  The  minerals  abundantly  present  are  quartz  and  feldspar,  the  latter  being 
orthoclase,  uucrocliue,  and  plagioclase.  In  subordinate  quantity  are  chlorite,  pyrite, 
and  ferro-dolomite.  In  the  majjor  portions  of  these  sections  the  minerals  are  in  large 
complex  granite  fragments,  which  are  often  crowded  so  close  together  as  to  make  it 
ditihcult  to  follow  their  outlines.  The  interstitial  material  is  chiefly  linely  crystalline 
quartz,  but  mingled  with  this  is  a  little  ferro-dolomite.  In  some  places  this  intersti- 
tial material  is  in  quite  large  areas  and  contains  distinct  fragments  of  quartz  aiul  feld- 
spar. Many  of  the  striated  feldspars  show  finely  the  effects  of  bending  and  fracture- 
some  grains  are  bent  but  not  broken,  while  others  are  fractured,  each  of  the  frag- 
ments in  such  cases  showing  bending  due  to  the  strain  before  fracture  occurred. 

34.  Ferro-dolomitic  chert,  interstratified  with  33.     Specimen  12593  (slide  5348) 
1700  N.,  232  W.,  Sec.  23,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  greenish  gray,  medium  grained,  and  massive. 

A  large  part  of  the  section,  constituting  a  background,  is  composed  of  finely 
crystalline  cherty  silica  and  somewhat  coarsely  crystalline  ferro-dolomite.  Where  the 
silica  is  in  excess,  the  carbonate  occurs  largely  in  well  outlined  rhombohedra.  Con- 
tained in  the  background  are  angular  fragments  of  a  much  altered,  fine  grained  por- 
phyrite. 

35.  Chert-breccia,  from  a  low  horizon,  north  of  34.  Specimen  12594  (slide  5349), 
1748  N.,  232  W.,  Sec.  24,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  a  gray  and  green  chert-breccia,  which  contains  between  the  chert 
fragments  particles  of  coarsely  crystalline  quartz. 

Eounded  and  angular  fragments  of  chert  of  greatly  varying  sizes  compose  a  large 
part  of  the  section.  Mingled  with  these  chert  fragments  are  many  simple  grains  of 
quartz,  which  are  widely  enlarged  and  the  enlargements  of  which  merge  into  the 
matrix.  Both  classes  of  fragments  are  cemented  with  finely  crystalline  cherty  silica, 
which  is  almost  exactly  like  that  in  the  chert  fragments,  and  were  it  not  for  the  iron 
stained  outlines  of  the  latter  it  would  be  impossible  to  separate  them  from  the  matrix 
in  which  they  are  contained. 


404  THE  PENOKEE  lEON-BEARING  SERIES. 

36.  CUoritic  and  graphitic  schist,  from  a  low  horizou,  north  of  35.  Specimen 
12595  (slide  5350),  1770  K,  232  W.,  Sec;  24,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  strongly  schistose,  and  shows  alternate  bands  of  gray  qaartzose  and 
dark  green  to  black  material,  the  latter  being  the  more  plentiful.  The  cleaved  sur- 
face shows  a  black  lustrous  appearance  due  to  graphite. 

The  thin  section  has  a  finely  crystalline  background,  consisting  of  quartz  and 
ferro-dolomite  in  about  equal  quantity.  This  background  contains  much  chlorite,  vseri- 
cite,  aud  black  graphitic  material,  arranged  in  parallel,  wavy  lines.  A  portion  of  the 
quartz  is  plain'y  fragmental,  and  a  part  of  the  ferro-dolomite  is  in  small  perfect  rliom- 
bohedra. 

37.  Sericitic  slate,  from  a  low  horizou,  north  of  36.  Specimen  12596  (slide  5351), 
1824  N.,  232  W.,  Sec.  24,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  dark  green,  of  a  uniform  texture  and  finely  foliated. 

The  section  has  a  background  consisting  of  finely  crystalline  quartz,  chlorite, 
sericite,  and  pyrite.  Set  in  this  background,  and  making  up  about  one-half  of  the  sec- 
tion, are  rather  small  fragments  of  quartz  and  feldspar.  These  fragments  are  with 
great  uniformity  scattered  through  the  rock  with  their  longer  axes  in  a  common  direc- 
tion. The  appearance  which  has  resulted  from  this  arrangement  of  fine  grained 
material  is  that  of  a  net  wliicli  has  been  drawn  out  diagonally,  the  groundmass  repre- 
senting the  meshes,  and  the  fragments  of  quartz  and  feldspar  the  open  spaces. 

38.  Graywacke-slate,  from  a  low  hox'izon,  north  of  37.  Specimen  12597  (slide 
5352),  1852  N.,  232  W.,  Sec.  24,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  dark  green,  foliated,  of  a  uniform  grain,  coarse  enough  to  show  par- 
ticles of  quartz,  feldspar,  and  pyrite.     It  closely  resembles  30. 

The  section  differs  but  little  from  that  of  30,  except  that  it  is  coarser  grained. 

39.  Sericite-slate,  from  a  low  horizou,  north  of  38.  Specimen  12598  (slide  5353), 
1876  N.,  232  W.,  Sec.  24,  T.  47  N.,  E.  43  W.,  Michigan. 

The  rock  is  somewhat  finer  grained  than,  but  otherwise  precisely  like  37. 

The  thin  section  differs  from  that  of  37,  in  that  the  background  is  more  abun- 
dant, and  in  that  the  sericite  is  very  x)lentiful,  while  chlorite  is  subordinate  in 
quantity.  This  supposed  sericite  is  in  minute  i)articles,  with  the  fibers  in  a  common 
direction,  and  often  so  concentrated  as  to  make  up  solid  sheets,  all  of  the  constituent 
fibers  of  which  extinguish  simultaneously. 

40.  Quartzite,  from  a  middle  horizon.  Specimen  12599  (slide  5354),  0  N.,  1973 
W.,  Sec.  18,  T.  47  N.,  E.  42  W.,  Michigan. 

The  rock  is  a  coarse  grained,  vitreous  quartzite.  The  contained  fragments  of 
quartz  have  a  somewhat  different  color  from  the  matrix  in  which  they  are  contained, 
so  that  their  rouuded  outlines  are  distinctly  seen.  They  vary  greatly  in  magnitude, 
the  largest  ones  being  oue-fourth  of  an  iuch  in  diameter,  while  from  this  they  run  to 
those  so  small  that  they  can  not  be  distinguished  from  the  matrix  iu  which  they  are 


THE  EASTERN  AREA.  405 

coTitaiuod.    Tlio  matrix  of  tin'  rock  is  so  stroiit;'  tliii(  wiicii  hrolvcii  tlic  fracture  passes 
through  the  hirge  partich's  ol'ciuait/.  instc^ad  of  tearing  them  from  their  sockets. 

The  ai)pcarain'i'  in  hand  specimen  is  hoine  ont  in  tliin  section.  Nearly  all  of 
tlie  ronnded  tVagments  are  (piartz,  altliough  ndngled  with  them  area  few  small  ones 
offeUlspar.  Most  of  these  grains  are  simple,  but  sonn^  of  them  are  very  eomphix. 
But  few  of  them  are  euhirged,  and  tliese  but  slightly.  The  plentiful  matrix  in  which 
they  are  contained  consists  of  linely  crystalline  (juartz  and  sericite  or  niuscovite, 
mingled  with  a  little  pyrite.  In  this  rock  the  induration  is  as  great  as  in  any  vitreous 
(piartzite,  yet  it  is  not  caused  by  interlocking  enlargements  so  common  in  quartzites, 
but  by  the  deposition  of  independent  interstitial  quartz. 

41.  Feldspathio  quartzite,  from  a  middle  horizon,  north  of  40.  Specimen  12G0O 
slide  3355),  62  N.,  1973  W.,  Sec.  IS,  T.  47  N.,  R.  42  W.,  Michigan. 

The  rock  is  a  gray,  coarse  grained,  vitreous  quartzite,  which  breaks  with  con- 
choidal  fracture. 

The  thin  section  differs  fi-om  that  of  40  chiefly  in  that  mingled  with  the  large 
quartz  fragments  are  many  of  orthoclase,  microcline,  and  plagioclase. 

Uxposure  in  the  NE.  ^  of  Sec.  19,  T.  47  JSf.,  R.  42  W.,  Michigan. 

42.  Chloritio  and  sideritic  slate.  Specimen  12841  (slide  5498),  1500  N.,  250  W., 
Sec.  19,  T.  47  N.,  R.  42  W.,  Michigan.' 

The  rock  is  dark  green,  very  fine  grained,  and  finely  foliated.  The  specimen  is 
cut  by  veins  of  quartz. 

The  mass  of  the  section  consists  of  exceedingly  fine  material,  which  appears  to 
be  quartz,  chlorite,  kaolin  or  sericite  (or  both),  and  oxide  of  iron.  Mingled  with  this 
matinx  are  small  grains  of  quartz  and  feldspar,  which  have  a  strong  fragmeutal 
appearance.    The  rock  is  taken  to  be  a  fine  grained  squeezed  clayey  one. 

Section  in  the  east  part  of  Sec.  20,  T.  47  N.,B.42  W.,  Michigan. 

43.  Clay-slate,  from  a  low  horizon.  Specimens  9206  (slide  2935),  9207  (slide 
3305),  9208  (slide  2998),  9209  (slide  2936),  7356  (slide  1826),  425  N.,  50  W.,  Sec.  20,  T. 
47  K,  E.  42  W.,  Michigan. 

The  rocks  are  from  light  grayish  to  very  dark  green,  aphanitic,  and  have  a  ready 
cleavage. 

The  sections  are  all  exceedingly  fine  grained.  They  consists  of  finely  crystalline 
and  eherty  silica,  abundant  chlorite,  kaolin,  iron  oxide,  and  perhaps  other  constituents. 
In  one  section  many  of  the  quartz  grains  are  of  suflicient  size  to  make  it  plain  that  they 
are  fragmental. 

44.  Chloritic  sericite-slate,  from  a  low  horizon,  north  of  43.     Specimen  12602 
(slide  5356),  550  IST.,  300  W.,  Sec.  20,  T.  47  N.,  R.  42  W.,  Michigan. 

The  rock  is  greenish  gray,  aphanitic,  and  finely  foliated. 


406  THE  PENOKEB  lEON-BEAEING  SERIES. 

The  tWn  section  contains  an  almost  solid  mass  of  minute  fibers  of  clilorite  and  a 
mineral  whicli  is  taken  to  be  sericite,  mingled  with  which  are  finely  crystalline  quartz 
and  ferrite.  Contained  in  this  background  are  numerous  small  particles  of  quartz 
and  feldspar,  which  are  ijlainly  fragmental.  The  sericite  is  the  most  abundant  min- 
eral of  the  slide.  It  has  strong  absorptive  power  and  gives  brilliant  polarization  colors. 
The  fibers  lie  with  their  longer  axes  in  a  common  direction,  and  so  extinguish  simul- 
taneously. The  laminse  are  in  wavy  lines,  often  broken  asunder  or  sharply  foliated  as 
if  they  had  been  subject  to  intense  pressure,  and  thus  correspond  in  their  appearance 
to  the  foliated  character  of  the  hand  specimen.  % 

45.  Chloritic  and  sericitic  slates,  from  a  middle  horizon,  north  of  44.  Specimens 
12603  (slide  5357),  427  K,  356  W.;  12605  (slide  5359),  1000  N.,  340  W.,  Sec.  20,  T.  47 
N.,  E.  42  W.,  Michigan. 

The  rocks  are  dark  green,  fine  grained,  and  finely  laminated.  12G05  is  some- 
what coarser  grained  than  12603, 

The  thin  sections  of  these  rocks  are  closely  allied  to  that  of  44,  the  chief  differ- 
ence being  that  they  contain  more  numerous  and  larger  fragments  of  feldspar. 

Exposures  in  NW.  i  of  Sec.  38,  T.  47  N.,  B.  42  W.,  Michigan.. 

46.  Sericite-schist.  Specimens  12816  (slide  5490),  12911  (slide  5515),  1472  N., 
1499  W.,  Sec.  28,  T.  47  IST.,  E.-  42  W.,  Michigan. 

The  rocks  are  gray,  aphanitic,  finely  foliated,  are  hard  and  felsitic  appearing. 

The  tliin  sections  have  a  finely  crystalline  background  composing  two-thirds  or 
more  of  the  sections,  which  contains  roundish  and  angular  grains  of  quartz  of  varying 
sizes.  The  background  appears  to  be  chiefly  made  up  of  quartz  and  sericite,  mingled 
with  which  is  some  oxide  of  iron.  The  quartz  grains,  set  in  a  matrix  of  finely  crys- 
talline quartz  and  sericite,  at  once  suggests  a  coarse  fragmental  material  in  a  non- 
fragmental  sediment;  but  the  excessive  angularity  of  some  of  the  quartz  grains,  and 
the  lack  of  any  enlargements,  makes  the  fragmental  character  of  the  quartz  somewhat 
doubtful,  and  thus  it  is  difflcult  to  determine  whether  this  rock  is  a  fragmental 
one  or  a  felsitic  schist. 

47.  Chert-breccia.     Specimen  12625  (slide  5374),  1730  N.,  1480  W.,  Sec.  28,  T.  47     ' 
N.,  E.  42  W.,  Michigan. 

This  rock  consists  of  angTilar  to  rounded  fragments  of  gray  and  black  chert,  which 
vary  in  size  from  minute  particles  to  pebbles  several  inches  in  diameter.  These  cherty 
fragments  are  set  in  a  matrix,  which  aj)pears  to  consist  mostly  of  the  same  material, 
in  which  occur  large  simple  grains  of  vitreous  quartz  and  cleavage  areas  of  feldspar. 

In  general  the  section  as  seen  under  the  microscope  corresponds  in  appear- 
ance in  hand  specimen.  The  large  fragments  of  the  section  are  all  finely  crystalline 
cherty  silica,  the  outlines  of  which  are  only  discoverable  by  using  a  low  power. 
Two  of  these  fragments  are  somewhat  peculiar.     One  contains  besides  the    finely 


THE  EASTEKX  ARKA.  407 

crystalline  silica  n  large  amoiiul  df  luilliaiitly  iMtlari/iiiji-  miueral  in  minute  scales 
and  many  hrown  i)articlos  of  liydiatcd  iron  oxide.  Whetlicr  this  fragment  is  impure 
()l)al  or  is  from  an  altered  Iclsite  it  is  impossiijle  to  say.  The  otlier  is  im  almost  pure 
specimen  <>!'  chert,  exce|)t  that  it  coutaius  many  small  fragmental  particles  of  (piartz 
as  though  it  were  a  non  fragmental  sediment  wliicii  had  received  a  considerable  (luau- 
tity  of  mecluuiically  deposited  (piart/,.  The  interstitial  material  of  the  section  consists 
of  cherty  silica,  which  contains  large  brilliantly  polarizing  Hakes  of  scricite  and  also 
areas  of  brown  hydrated  iron  oxide.  This  matrix  includes  many  large,  well  rounded, 
simple  grains  of  quartz  and  few  of  feldspar. 

48.  Eecomposed  granite.  Specimen  12626  (slide  5375),  1730  N.,  1480  W.,  Sec; 
28,  T.  47  N.,  E.  42  W.,  Michigan. 

The  rock  is  chiefly  composed  of  the  coarse  granite  detritus,  but  contains  some 
seams  of  fine  quartzite-like  material  and  few  white  quartz  pebbles.  The  granite 
detritus  is  often  in  large  complex  fragments,  but  also  there  are  present  numerous  large, 
apparently  simple  grains  of  quartz  and  feldspar. 

The  section  is  cut  from  the  fine  grained  quartzite-like  part.  It  consists  of  a 
matrix  and  contained  fragments  in  about  equal  quantity.  These  fragments  are 
almost  wholly  quartz,  rather  small,  mostly  well  rounded,  and  often  enlarged,  the 
enlargements  fading  into  the  fine  grained  matrix.  Flakes  of  muscovite  of  some  size 
appear,  which  are  taken  to  be  fragmental.  The  matrix  is  mostly  cherty  quartz,  but 
mingled  with  it  is  a  good  deal  of  sericite,  hematite,  and  limonite. 

49.  Sericite-schist.  Specimen  12629  (slide  5378),  1748  N.,  1462  W.,  Sec.  28,  T.  47 
N.,  R.  42  W.^  Michigan. 

The  rock  varies  from  dirty  yellow  to  brown,  is  finely  foliated,  and  upon  its 
cleavage  surface  has  a  micaceous  sheen. 

The  thin  section  is  essentially  like  that  of  44,  the  only  difference  being  that  the 
fragmental  particles  are  larger,  the  folia  of  the  sericite  is  not  in  short  wavy  folds, 
and  the  section  is  heavily  stained  with  iron  oxide. 

50.  Recomposed  granite.  Specimen  12818  (slide  5492),  1,507  N.,  1404  W.,  Sec. 
28,  T.  47  N.,  E.  42  W.,  Michigan. 

In  hand  specimens  this  rock  would  be  taken  foran  ordinary  granite  unless  it 
were  closely  inspected.  The  eye  recognizes  coarse  pink  feldspar,  translucent  quartz, 
and  a  green  material,  all  being  arranged  so  as  to  show  a  somewhat  banded  appear- 
ance like  a  coarse  gneiss.  The  only  indications  that  it  is  different  from  an  ordi- 
nary crystalline  rock  are  its  somewhat  nodular  weathering,  as  though  comjiosed 
in  part  of  i^ebbles,  and  a  roundish  appearance  of  the  larger  white  quartz  areas. 

A  large  part  of  the  section  consists  of  complex  granite  fragments,  composed  of 
quartz,  microcline,  orthoclase,  chlorite,  iron  oxide,  and  other  accessoi-ies.  In  the  inter- 
stices are  finely  crystalline  quarts,  (ihlorite,  and  some  ferrite,  included  in  which 
are  large  simple  particles  of  quartz  and  feldspar. 


408  THE  PENOKEE  lEON-BEARING  SERIES. 

51.  Cherty  qnartzites.  Specimens  12620  (slide  5371) ;  12820  (slide  5493),  IGOO  K, 
13C0W.;  12621  (slide  5372),  1600  N.,  1300  W.;  12912  (slide  5516),  1615  K,  1360W.; 
12917  (slide  5519),  1628  N.,  1360  W.;  12627  (slide  5376),  1730  K,  1480  W.;  12812  (slide 
5488),  1712  ISr.,  1462  W.,  Sec  28,  T.  47  N".,  E.  42  W.,  Micliigan. 

Tliese  quartzites  are  tolerably  coarse  grained  and  massive.  Some  of  them  con- 
tain rather  large  pebbles  of  white  and  cherty  quartz.  They  all  have  a  rough  fracture, 
due  to  the  fact  that  when  broken  the  large  quartz  fragments  are  torn  from  their 
sockets.  In  color  they  vary  from  greenish  gray  to  grayish  brown  or  red.  Specimens 
12620  and  12621,  besides  being  reddened  are  somewhat  honeycombed,  as  though  some 
constituent,  possibly  a  carbonate,  had  been  leached  out. 

The  sections  of  these  rocks  consist  of  two  parts,  a  matrix  and  coarse  material. 
The  latter  is  chiefly  quartz  in  large,  simple,  or  coarsely  complex  grains,  which  are  often 
well  rounded,  but  also  quite  as  often  more  or  less  angular,  while  part  of  them  are 
very  angular.  There  are  a  few  fragments  of  very  finely  crystalline  and  i^erhaps  partly 
amorphous  cherty  silica.  Many  of  the  simple  quartz  grains  are  slightly  enlarged,  the 
enlargements  merging  gradually  into  the  matrix.  Fraginental  feldspar  is  quite  plen- 
tiful in  one  of  the  sections,  and  sparse  in  the  others.  Also  in  one  section  are  nu- 
merous large  altered  leaflets  of  biotite.  The  predominant  constituent  of  the  matrix  is 
cherty  silica,  mingled  with  which  is  much  sericite  or  kaolin,  in  small  brilliantly  jjolar- 
izing  flakes.  In  one  or  two  sections  this  sericite  is  so  plentiful  as  to  be  comparable 
with  the  silica  in  abundance.  In  such  sections  some  of  the  complex  seri(Mte-quartz 
areas  have  a  roundish  appearance  as  though  they  were  complex  fragments.  They 
perhaps  represent  altered  feldspar  detritus,  as  indicated  by  the  fact  tJiat  in  one  of 
the  sections  many  undoubted  feldspars  have  undergone  alteration  to  quartz  and  seri- 
cite. The  cavities  seen  in  hand  specimen  are  found  in  thin  section  to  be  mostly  bor- 
dered by  iron  oxide.  They  are  of  very  irregular-  form,  resembling  in  this  respect 
areas  of  iron  oxides  found  in  the  matrix;  and  it  seems  probable  that  the  cavities  were 
either  entirely  filled  with  iron  oxide,  or  more  jirobably  ferriferous  carbonate,  which 
largely  went  into  solution,  but  from  which  the  iron  oxides  now  remaining  were  pro- 
duced. This  probability  is  further  strengthened  by  the  fact  that  in  other  rocks  pre- 
cisely similar  to  these  and  associated  with  them  is  found  a  good  deal  of  carbonate. 
The  sections  differ  from  those  of  the  slates  and  graywackes  in  one  important  partic- 
ular. The  fragmental  particles  are  all  large,  none  nearly  aj)proaching  in  minuteiiess 
the  particles  of  the  matrix,  while  in  the  graywackes  there  is  every  gradation  from 
the  coarsely  fragmental  material  to  the  fine  matrix.  The  minutely  crystalline 
matrix  in  these  rocks  is  taken  to  be  a  nonfragmental  or  recomj)osed  sediment.  The 
sericite  in  the  matrix  is  largely  arranged  with  its  blades  in  a  common  direction. 

52.  Cherty  and  ferro-dolomitic  quartzites.  Specimens  12915  (slide  5517);  12916 
(slide  5518),  1628  N.,  1360  W.;  12628  (slide  5377),'  1730  N.,  1480  W.;  12922  (slide  5520), 
1744  N.,  1466  W.,  Sec.  28,  T.  47  N.,  R.  42  W.,  Michigan. 


THE  KASTEim  AREA.  409 

TliC!  rocks  vary  troiii  Hiccnisli  gray  to  inotllcd  fiicenisli  gray  or  red,  aro  inetlixiin 
{jraiiu'il,  massive,  and  break  witli  a  soiiiewlial  rouKli  fVaeture,  resemliliii};-  in  tliis 
res])wt  51. 

ill  must  respects  liie  tliiii  sections  are  similar  to  those  of  "d,  tlie  essential  (lif- 
ferencii  between  the  two  beinj;'  tliat  the  matrices  <H)ntain  a  lar^e  amount  of  fen'o-dolo- 
mite.  Tlie  rehitions  of  the  liematite  and  limonite  to  the  l'erro-(h)loiiiite  are  often  snch 
as  to  clearly  indicate  that  these  oxides  arc  the  ri^snlt  of  decomiiosition  of  that  mineral. 

").■?.  Oonglomerate.  Specimens  12G11  (slide  5364),  12G12  (slide  5305),  1280!)  (slide 
5481),  1050  K,  1000  W.,  Sec.  28,  T.  47  N.,  R.  42  W..  Michigan. 

The  conglomerate  contains  numerous  white  quartz,  green  scihist,  and  worn 
granite  ])cbbles.  No  unmistakable  greenstone  pebbles  are  seen,  although  some  speci- 
mens of  the  conglomerate  appear  to  contain  small  fragments  of  basic  detritus.  At 
times  the  conglomerate  is  so  fine  grained  as  to  become  a  quartzite. 

Thin  section  5481  is  from  a  red  (juartzite  pebble.  It  is  composed  almost  wholly 
of  enlarged  grains  of  cprartz  of  nearly  uniform  size.  Mingled  with  this  quartz  is  a 
little  feldspar.  Between  these  grains  are  also  films  of  limonite.  In  slide  5364  the 
pebbles  are  from  green  schist  fragments.  The  green  schist  is  very  mxrch  altered,  and 
consists  mainly  of  confusedly  mingled  pale  green  chlorite,  much  altered  feldspar,  cal- 
cite,  or  other  carbonate,  sericite  or  muscovite,  and  leucoxene.  Whether  it  is  an 
altered  eruptive  it  is  impossible  certainly  to  say.  The  matrix  of  the  conglomerate  is 
of  the  most  complex  nature,  being  comiiosed  of  finely  crystalline  and  coarse  frag- 
mental  portions.  The  coarse  grains  are  quartz  and  feldspar.  They  have  been  in 
part  somewhat  rounded,  but  are  mostly  angular.  The  (juartz  grains  are  often  quite 
widely  enlarged,  and  frequently  the  enlargements  gradually  merge  into  the  matrix. 
The  fragments  of  feldsj)ar  are  as  numerous  as  the  quartz,  and  include  orthoclase, 
microcline,  and  plagioclase.  Some  of  the  latter  particles  have  excessively  fine  twin- 
ning. Many  of  the  particles  of  feldspar  are  plainly  enlarged.  In  some  cases,  particu- 
larly with  orthoclase,  tlie  added  portions  extinguish  simultaneously  with  the  cores 
but  in  the  striated  feldspars  the  added  portions,  which  entirely  encircle  the  cores, 
extinguish  wholly  with  one  set  of  the  bands.  This  may  mean  that  the  polysynthetic 
twinning  of  the  original  feldspar  has  been  due  to  some  cause  which  acted  upon  the 
kernels  of  feldsx^ar  before  they  were  deposited  in  the  i^resent  rocks.  A  few  frag- 
ments of  mica  are  also  found.  These  large  areas  are  contained  in  an  excessively  fine 
grained  groundmass,  the  chief  constituent  or  which  is  crystalline  and  half  amorphous 
silica,  mingled  with  chlorite,  sericite,  or  kaolin,  and  many  minute  rectangulai  and  hex- 
agonal ciystals  of  hematite.  The  groundmass  contains  also  a  few  large  areas  of  ferro- 
dolomite. 


410  THE  PEISTOKEE  lEON-BEARING  SERIES. 

SECTION  v.— THE  GREENSTONES. 

The  eruptives  belonging  to  the  Eastern  area  may  be  considered  in 
three  divisions,  based  upon  apparent  geographic  continuity  or  lithologic 
Hkeness,  or  both.  The  first  division  includes  the  very  numerous  exposures 
of  greenstone  which  outcrop  in  Sees.  15,  16,  23,  25,  26,  and  27  (except 
the  northern  exposure  in  Sec.  15),  T.  47  N.,  R.  44  .W.,  Michigan.  The 
second  area  is  in  Sees.  20,  29,  and  30,  T.  47  N.,  R.  43  W.,  Michigan,  and 
the  third  is  in  Sees.  13,  14,  15,  and  24,  T.  47  N.,  R.  44  W.,  Michigan. 

The  main  area. — The  exposures  in  Sees.  15  and  16,  and  those  in 
Sees.  26  and  27,  belonging  to  this  area,  are  of  large  size.  The  greenstone 
in  Sees.  15  and  16  forms  a  continuous  high  range,  running  east  and  west, 
and  apparently  joins  the  great  greenstone  ridge  to  the  north  and  west 
belonging  to  the  Copper-bearing  series.  In  traversing  the  ridge  in  a 
north  and  south  direction  one  has  to  ascend  a  height  of  some  500  feet  above 
the  valleys  to  the  north  and  south.  Also  the  large  exposure  near  the  center 
of  Sec.  26  is  another  notable  landmark,  rising  to  about  the  same  elevation 
above  the  level  country  surrounding. 

Lithologically,  the  exposures  in  this  area  are  essentially  the  same 
rock.  The  greater  number  of  them  are  diabases,  but  they  grade  into 
gabbros.  The  gabbros  have  a  granitic  structure,  and  the  pyroxene  shows 
the  diallagic  cleavage.  The  contrast  between  the  structure  of  these  gabbros 
and  the  diabases  with  perfect  ophitic  structure,  both  belonging  to  the  same 
area  and  probably  to  the  same  rock  mass,  is  very  great.  One  of  the 
exposures  in  Sec.  25  has  very  large  augites,  each  of  which  includes  scores, 
if  not  hundreds,  of  small  lath-shaped  plagioclases.  The  gabbros  are  found 
in  the  central  and  western  parts  of  the  areas,  while  it  is  further  notable 
that  the  rocks  in  the  west  part  of  Sec.  23  and  nBar  the  center  of  Sec.  15, 
that  is,  those  which  are  nearest  to  the  greenstone-conglomerate  area,  are 
the  ones  which  become  fine  grained,  and  the  typical  ophitic  structure 
appears.  And  in  the  exposure  near  the  center  of  Sec.  15  an  amygdaloidal 
or  pseudo-amygdaloidal  appearance  is  presented.  It  does  not,  then,  appear 
improbable  that  these  greenstone  masses  are  or  were  really  connected  with 
the  greenstone-conglomerates,  the  latter  perhaps  being  the  surface  material, 
while  the  diabases  and  gabbros  were  deep  seated.  ■   As  these  diabases  are 


THE  EASTERN  AREA.  411 

in  most  respects  precisely  like  the  diabases  in  the  inniii  area  t(t  the  west- 
ward, described  on  pp.  350-354,  a  description  of  them  will  not  lie  given  in 
detail,  only  p( tints  of  ditference  being  mentioned. 

The  augite  of  this  area  frequently  sliows  twinning.  The  twinning  is 
of  the  form  most  common,  the  twins  each  being  in  two  parts  and  the  i-.om- 
position  plane  the  orthopinacoid. 

The  enlargement  of  augite  and  hornblende  alluded  to,  pp.  353,354,  as 
occm-ring  in  some  of  the  dialiases  at  the  extreme  west  end  of  the  western 
area,  here  most  beautifully  occurs.  The  cases  of  enlargement  of  both  the 
augite  and  hornblende  are  finer  than  any  known  to  the  westward.  Such  a 
renewed  growth  of  hornblende  in  a  kersautite  has  been  described  liy  Fried- 
rich  Becke.^ 

The  following  quotation  is  from  his  description: 

Die  primtiren  compacten  Hornbleude-Krystalle  sind  selu-  hiiufig  von  einer  Riude 
iimgebeii.,  welclie  aus  stiiugeliger  griiiier  Hornblende  bestelit,  die  ganz  mit  der 
Hornblende  des  Uralit  iibereinstiinnit.  In  Qnersclmitten  beobaclitet  man  die  dnrcli 
Kern  and  Hiille  gleiclimjissig  fortsetzendeu  Spaltrisse.  Mitunter  liat  der  .so  weiter- 
gewachsene  Krystall  andere  Krystallflachen  in  der  Prismenzoiie,  als  der  Kern.  In 
Langsclinitten  sieht  man  den  compacten  Kern  mit  Biindeln  parallel  angewaclisener 
Hornblendenadeln  versehen,  welclie  sicli  weiterhin  zu  divergireuden  Biiscbeln  auf- 
loseu.  *  *  *  Die  Art  des  Auftretens  dieser  Portwacbsungen  lasst  keinen  Zweifel, 
dass  wahrend  der  Ausbildung  dieser  Portwacbsungen  mecbaniscbe  Bewegungon 
innerhalb  dieses  Gesteins  nicbt  mebr  stattgefunden  baben.     (P.  158.) 

It  thus  appears  that  Becke  discovered  in  1883  the  enlargement  by 
secondary  growth  of  individuals  of  hornblende  in  an  eruptive  rock,  this 
change  having  taken  place  siibsequent  to  its  consolidation.  I  have  described 
the  enlargement  of  hornblende  by  second  growth  in  certain  fragmental 
rocks  in  northeastern  Minnesota.^  Here  the  change  is  of  greater  impor 
tance  than  in  eruptive  rocks,  for  it  explains  in  part  at  least  the  induration 
of  such  clastic  sediments.  However,  in  the  eruptives  iinder  consideration 
I  have  met  with  cases  of  new  growths  occurring  upon  augite  and  horn- 
blende, which  are  corroborative  of  and  add  something  to  the  observations 

'  Tsclierraak's  Min.  unci  Petr.  Mitth.,  vol.  v,  pai't  ii,  1883,  in  a  paper  entitled  "  Ernptivgestelne 
ans  tier  Gueissforraatiou  des  niederosterreichisclien  Waldviertels." 

'^  C.  E.  Van  Hise :  Enlargements  of  Hornblende  Fragments.  Am.  Jonr.  Sci.,  3d  series,  1885,  vol. 
XXX,  pp.  231-235. 


412  THE  PENOKEE  lEON-BEAEING  SERIES. 

made  by  Becke.  In  many  of  the  sections  a  new  amphibole  has  attached 
itself  to  secondary  hornblende.^  There  are  here,  then,  two  hornblendes, 
one  of  which  is  pararaorphic,  while  the  other  has  grown  after  the  rocks 
reached  a  solid  state.  The  added  amphibole  has  found  room  for  itself,  as 
in  the  case  described  by  Becke,  by  penetrating  the  surrounding  feldspars. 
The  new  amphibole  is  also  found,  as  has  been  noted,  included  in  the  partly 
decomposed  feldspar  in  numerous  small  independent  individuals.  This 
new  amphibole  is  fibrous,  of  a  pale  green  color,  is  not  strongly  pleochroic, 
and  often  shows  distinctly  its  intersecting  prismatic  cleavage.  It  has  all 
the  characteristics  of  the  variety  described  as  smaragdite.  The  crystallo- 
graphic  continuity  of  the  paramorphic  and  new  amphibole  when  the  two 
are  contiguous  is  as  plain  as  in  the  case  described  and  figured  by  Becke, 
where,  however,  the  hornblende  cores  seem  to  have  been  original. 


Fig.  10. — Hornblende  enlargement  of  augite  in  diabase. 

In  other  diabases  from  the  same  locality  the  greater  part  of  the  augite  is 
unaltered.  In  these  cases  nearly  every  individual  of  this  mineral  is  surrounded 
by  a  sheath  of  smaragdite.  This  smaragdite  has  clearly  formed  subsequently 
to  the  consolidation  of  the  rock,  as  is  shown  by  the  following  facts:  It  cuts 
into  the  surrounding  feldspar  in  the  most  irregular  manner.  The  augite 
cores  have  the  forms  common  in  many  diabases,  being  bovmded  by  well 

'  E.  D.  Irving  and  C.  R.  Van  Hise:  Geol.  of  Wis.,  1882,  vol.  iv,  p.  663.  R.  D.  Irving:  On  the 
Paramorphic  Origin  of  the  Hornblende  of  the  Crystalline  Rocks  of  the  Northwestern  States;  Am. 
Jour.  Sci.,  3d  series,  188.5,  vol.  xxvi,  p.  27.  R.  D.  Irving:  Supplement  to  paper  on  the  Paramorphic 
Origin  of  the  Hornblende  of  the  Crystalline  Rocks  of  the  Northwestern  States;  Am.  .Joiir.  Sci.,  3d 
series,  1884,  vol.  xxvii,  p.  130.  M.  E.  Wadsworth :  Notes  upon  the  Geology  of  the  Iron  and  Copper 
Districts  of  Lake  Superior,  1880.  G.  H.  Williams:  On  the  Paramorphosis  of  Pyroxene  to  Hornblende 
iu  Rocks;  Am.  Jour.  Sci.,  3d  series,  1884,  vol.  xxviii,  p.  259.  G.  H  Williams:  Bull.  U.  S.  Geol.  Sur- 
vey, Nos.  28  and  62. 


THE  EASTEKN  AllEA.  413 

* 

defined,  l)rokcu  right  lines,  or  lines  sonievvliiit  eurved,  ;is  the  spaces  left  by 
the  feldspars  allowed.  Often  the  new  growth  has  continued  farther  in  i)laces 
than  in  adjoining  ones,  and,  as  it  went  on,  it  has  sometimes  widened  out, 
forming  club-shapetl'  protuberances  of  hornblendes  within  the  feldspar.  In 
longitudinal  sections  (Fig.  10)  the  smaragdite  is  inore  plentiful  at  the 
extremities  of  the  individuals  than  at  the  sides.  The  terminations  of  these 
enlargements  are  sharply  serrate.  The  constituent  fibers  at  times  are 
slightly  divergent  and  usually  cut  deeply  into 
the  feldspar.  Ordinarily  the  fine  fibrous  cleav- 
age of  the  smaragdite  is  coincident  with  the 
cleavage  of  the  augite.  However,  the  angles 
C  :  C  in  the  augite  and  in  the  enveloping  smarag- 
dite show  their  characteristic  relations.  In  trans- 
verse sections  (Fig.  11),  where  the  intersecting 
prismatic  cleavages  of  the  augite  and  smaragdite     .^         „         ,     , 

i^  o  o  o  Fig.  11.— Hornblende  enlargemeDt  of 

are  seen,  their  relations  are  such  as  to  indicate  ''"»"" '"  '^'^i^^se. 

that  the  ortho-  and  clino-pinacoids  in  the  two  minerals  are  parallel.  The 
crystallographic  relations  of  the  two  minerals  are,  then,  in  both  longitudi- 
nal and  transverse  sections,  precisely  like  those  well  known  to  occur  be- 
tween augite  and  amphibole  paramorphic  after  it.-' 

Another  difference  betweenthe  diabases  of  the  Western  and  Eastern 
areas  is  the  relative  freshness  of  the  pyroxene  as  compared  with  the  plagio- 
clase  in  the  latter.  Generally  pyroxene  decomposes  more  rapidly  than 
plagioclase.  The  reverse  has  been  the  case  here.  In  nearly  every  section 
the  plagioclases  have  been  found  to  be  quite  extensively  altered.  The 
double  angles  in  the  zone  001  :  100  do  not  exceed  62°,  which  would  indi- 
cate that  the  most  basic  feldspar  is  labradorite.  The  most  frequent  altera- 
tion products  of  the  feldspar  are  small  brilliantly  polai'izing  flakes,  taken  to 
be  kaolinite,  green  smaragdite,  chlorite,  epidote,  biotite,  and  quartz.     In  the 

1  Teall,  J.  J.  H. :  Quart.  Jour.  Geol.  Soc,  London,  vol.  40,  p.  353,  PI.  29.  Fig.  3.  Rohrbacli,  Carl 
E.  M. :  Ueber  die  Eruptivgesteiue  iiu  Gebiete  der  schlesisch-milhrischeu  Kreideformatiou;  Miii.  n. 
petrog.  Mittb.,  vol.  vil,  -p.  24,  PI.  1,  Figs.  1-7.  Van  Hise,  C.  E. :  Note  on  the  enlargement  of  horn- 
blendes and  augitcs  in  fragmental  and  eruptive  rocks;  Am.  Jour.  Sci.,  3d  series,  vol.  33,  1887,  p.  385. 
Hotibs,  Wm.  H. :  On  the  petrographical  characters  of  a  dike  of  diabase  in  the  Boston  basin ;  Bull.  Mus. 
Comp.  Zool.  Harvard  Coll,,  vol.  IC,  1887,  p.  10,  PL  1,  Fig.  2. 


414  THE  PENOKEE  IKON-BEARING  SEEIES. 

abuudauce  of  secondary  kaolin  and  smaragdite  these  rocks  are  exactly  like 
tlie  diabases  to  the  westward;  but  in  the  large  development  of  chlorite  and 
epidote  there  is  a  difference.  In  nearly  every  section  minerals  are  found 
included  in  the  feldspars.  Frequently  within  the  chlorite  has  developed  the 
epidote,  but  at  times  the  alteration  seems  to  have  been  directly  from  feldspar 
to  epidote,  in  which  case  the  epidote  is  in  "small  granules.  Biotite  and 
quartz  are  infrequently  present  as  secondary  products  in  the  feldspar. 

The  minerals  secondary  to  the  augite  are  the  same  as  those  included 
by  the  feldspar,  with  the  exception  that  quartz  is  absent;  but  as  the  augite 
is  much  fresher  than  the  feldspar,  the  quantity  of  secondary  material  from 
this  source  is  much  less.  Chlorite  and  amphibole  are  at  times  both  abun- 
dant. Next  in  importance  to  these  is  biotite,  which  in  one  or  two  cases  is 
largely  found  in  both  the  augite  and  feldspar. 

The  iron  oxide  is  always  in  the  form  of  menaccanite,  which  in  every 
case  is  altered  to  a  large  degree  to  leucoxene,  some  specimens  showing 
beautiful  alternating  bars  of  menaccanite  and  leucoxene. 

The  area  in  Sees.  20,  29,  and  30,  T.  47  N.,  B.  43  W.,  Michigan.— 
Between  Sees.  20  and  29,  but  mostly  in  Sec.  29,  is  a  continuous  range  of 
greenstone  which  extends  from  the  valley  of  the  Presque  isle  for  more  than 
a  mile  westward.  Here  there  is  a  break,  and  again  in  Sec.  30  this  green- 
stone ridge  appears.  As  examined  on  the  ground  it  has  the  appearance  of 
an  interbedded  flow.  That  it  was  really  contemporaneous  with  the  forma- 
tion of  the  series  is  further  indicated  by  the  fact  that  at  its  northern  base, 
in  Sec.  20,  a  jasper-conglomerate  is  found  which  is  largely  composed  of 
debris,  like  the  material  of  the  greenstone.  (See  pp.  369-370.)  The  iron- 
bearing  formation  in  Sec.  30  is  found  upon  the  south  side  of  the  greenstone 
ridge,  and  upon  the  north  side  there  is  also  a  heavily  ferruginous  rock.  In 
Sees.  20  and  29  the  iron  belt  is  on  the  north  side,  but  whether  a  similar  belt 
is  on  the  south  side  has  not  been  proved  by  exploration,  but  it  probably 
exists  there. 

Petrographically,  this  rock  also  has  all  the  characteristics  of  a  surface 
flow.  It  is  a  much  altered  augite-porphyrite.  It  varies  a  good  deal  in 
coarseness  of  grain  and  in  degree  of  alteration,  but  the  original  minerals 
are  found  with  sufficient  frequency,  and  the  secondary  minerals  are  so  uni- 


THE  EASTERN  AKKA.  415 

forinl}-  tla;  sainc  tliat  tliero  is  every  retisou  to  believe  that  the   detached 
exposures  are  parts  of  a  continuous  belt. 

The  alterations  have  extended  so  far  in  places  as  to  make  the  rock 
a  poiphyrite.  If  any  of  the  rocks  have  liad  a  glassy  background,  it  has 
now  wholly  devitrified,  and  while  it  is  at  times  very  fine  grained,  the  fresher 
parts  are  always  holocrystalline.  The  structure  of  tliis  background,  in 
rocks  fresh  enough  to  determine  this  point,  is  hypidiomorphically  granular. 
The  original  minerals  were  menaccanite,  plagioclase,  and  augite,  this  being 
the  order  of  crystallization.  The  secondary  ones  are  kaolinite,  chlorite,  epi- 
dote,  leucoxene,  and  smaragdite.  The  oidy  mineral  which  appears  in  two 
generations  is  the  plagioclase.  It  is  always  greatly  altered  and  no  separa- 
tion of  it  was  attempted,  but  as  indicated  by  its  double  angle,  according  to 
Pumpelly's  method,  it  is  rather  acidic,  the  angles  determined  being  so  low 
as  to  indicate  a  feldspar  not  more  basic  in  any  case  than  labradoi'ite.  The 
augite  is  occasionally  found  in  unaltered,  or  little  altered  individuals,  but  in 
general  it  has  very  largely  decomposed,  while  in  several  sections  its  decom- 
position is  complete.  The  change  in  the  feldspars  and  augite  has  resulted 
in  the  formation  of  the  same  set  of  minerals.  One  of  the  most  abundant  of 
these,  if  not  the  most  abundant,  is  smaragdite.  It  is  found  in  small  particles 
and  well  defined  blades,  which  penetrate  the  rock  in  every  direction  and 
thus  cut  the  feldspar  thi-ough  and  through.  Chlorite  is  also  an  abundant 
secondary  product,  and  in  it  has  developed  a  large  quantity  of  epidote. 
As  a  result  of  these  changes  in  the  augite  and  feldspai-,  the  smaragdite,  chlo- 
rite, and  epidote  are  the  three  most  plentiful  minerals  of  the  rock.  The 
menaccanite  is  always  altered  to  a  greater  or  less  degree  to  leucoxene,  and 
frequently  gives  the  characteristic  gi'ate-like  appearance. 

In  original  minerals  contained  and  in  secondary  products,  these  augite- 
porphyrites  are  almost  precisely  like  the  diabases  of  the  first  division.  The 
similarity  is  so  remarkable  as  to  make  it  probable  that  the  rocks  of  the  two 
divisions  are  or  once  were  connected.  This  likeness  is  still  further  reenforced 
by  the  correspondence  of  the  double  angles  of  the  feldspars,  as  measured 
by  Pumpelly's  method. 

The  area  in  Sees.  13,  34,  14,  and  15,  T.  47  N.,  E.  44  W.,  Michigan. — 
The  greenstones  here  included,  unlike  those  of  the  two  previous  areas,  are 


416  THE  PENOKEE  IKOIJJ^-BEAEING  SERIES. 

in  several  detaclied  areas.  They  are  all  closely  associated  with  greenstone- 
conglomerates.  That  in  the  northeast  corner  of  Sec.  24  is  apparently  in  the 
inidst  of  a  greenstone-conglomerate  area;  that  in  the  north  part  of  Sec.  15  is 
about  half  way  between  two  known  greenstone-conglomerate  exposures;  while 
the  exposures  m  the  west  part  of  Sec.  13,  and  in  Sec.  14,  are  bounded  upon  the 
south  and  west  by  greenstone-conglomerates.  In  fact,  the  exposures  here 
included  are  so  closely  connected  with  the  greenstone-conglomerates  that 
the  lines  which  separate  them  are  to  some  extent  arbitrary.  Where  the 
pebbles  disappear  from  a  conglomeratic  area,  and  the  rock  at  thQ  same  time 
has  the  character  of  a  basic  eruptive,  it  is  mapped  as  a  greenstone;  but  it 
is  probable  that  the  greenstone-conglomerates  in  the  north  part  of  Sec.  14 
are  parts  of  the  same  rock  mass  as  the  greenstones  to  the  west  and  east.  In 
macroscopic  appearance  these  eruptives  are  very  like  the  greenstone- 
conglomerates.  The  finer  grained  of  them  sometimes  take  on  a  schistose 
structure ;  they  have  a  light  green  or  grayish  color,  and,  in  short,  so  closely 
resemble  the  pebbles  and  some  of  the  matrices  of  the  greenstone-conglom- 
erates as  to  be  indistinguishable  from  them. 

The  close  field  association  of  the  eruptives  with  the  greenstone-con- 
glomerates is  still  further  emphasized  by  a  study  of  the  thin  sections.  Tliey 
are  all  porphyiites,  or  augite-porphyrites — many  of  them  being  amygda- 
loidal.  The  background  varies  from  not  very  fine  grained  holocrystalline, 
to  a  devitrified  glass,  the  various  phases  being  exact  repetitious  of  the  peb- 
bles and  matrices  of  the  greenstone-conglomerates.  Their  macroscopic  and 
microscopic  characters  are  then  those  of  surface  flows,  and  they  were  doubt- 
less contemporaneous  with  the  greenstone-conglomerates  which  have  been 
seen  in  large  part,  at  least  (pp.  377-381),  to  be  volcanic  products. 

The  exposures  in  the  west  part  of  Sec.  13  have  a  not  very  fine  grained 
holocrystalline  background.  They  are  augite-porphyrites  and,  as  their 
stracture  is  a  true  diabasic  one,  they  would  be  classed  by  Rosenbusch  as  the 
variety  diabase-porphyrite.  The  menaccauite  is  often  in  small  rod-like 
areas,  but  also  has  in  many  cases  well  defined  crystal  outlines.  As  in  all  the 
other  rocks  of  the  eastern  area,  it  is  altered  to  a  greater  or  less  extent  to 
leucoxene.  The  plagioclase  occurs  to  quite  an  extent  in  idiomorphic  forms. 
While  much  altered,  it  is  fresh  enough,  so  that  by  Pumpelly's  method 


THE  EASTERN  AREA.  417 

it  is  determined  to  he  probahly  not  so  hiisic  as  labradorite.  Its  alteration 
products  are  kaolin,  sniarao-ditc,  cliloritc,  and  (■[)i(lotc'.  Aug'ite,  the  last  min- 
eral to  crystallize,  maintains  n-ulations  with  plao-ioclase  and  magnetite  such 
as  are  usual  in  diahasus.  It  has  generally  altered  to  n  larger  extent  than 
the  feldspar,  the  resultant  products  being  usually  biotite  and  chlorite, 
although  smaragdite  is  found. 

The  exposures  in'the  northeast  part  of  Sec.  24  are  not  greatly  dif- 
ferent from  the  above,  except  that  they  contain  no  augite,  and  tlierefore 
must  be  classed  as  ])ori)hyrites  rather  than  augite-porphyrites.  In  all 
probability  the}'  were  originally  nearly  alike.  These  rocks  arc  liolo- 
crystalline.  In  some  cases  the  background  is  composed  very  largely  of 
small,  somewhat  altered  plagioclases,  but  in  other  cases  the  smaragdite  is  as 
plentiful  as  the  feldspar.  Aside  from  these  two  minerals  secondary  quartz, 
chlorite,  epidote,  and  gray,  nonpolarizing  material  are  found.  The  minor 
minerals  are  somewhat  confusedly  mingled  with  the  predominant  ones. 
The  constituents  completely  interlock,  and  there  is  no  doubt  that  the  rock  is 
a  crystalline  eruptive.  The  exposures  in  the  north  part  of  Sec.  15  resem- 
ble closely  the  last  described,  except  that  they  are  more  finely  crystalline, 
the  backgrounds  of  the  finer  grained  ones  being  very  minutely  crystalline. 
The  additional  diff'erences  are,  that  the  plagioclases  lie  with  their  longer  axes 
in  a  common  direction  (which  probably  indicates  a  flowage  structure),  that 
menaccanite  is  abundant  in  small  crystals  and  minute  grains,  and  that  its 
alteration  has  resulted  in  leucoxene,  which  appears  to  be  of  the  nature  of 
titanite.  The  secondary  minerals  are  the  same  as  in  the  previously  described 
exposures. 

The  exposures  in  Sec.  14  contain  rocks  in  which  the  groundmass  is 
of  agray,  a  morphous  or  feebly  polarizing  material,  which  is  taken  to  be 
devitrified  glass,  and  also  holocrystalline  kinds.  The  glassy  backgrounds 
include  nnimte,  much  altered  tabular  plagioclases  and  particles  of  material 
which  may  be  leucoxene.  Less  abundantly  are  found  other  minerals  which 
have  been  mentioned  as  occurring  in  the  previously  described  exi^osures. 
From  this  glassy  variety  the  background  varies  to  one  which  resembles 
closely  the  exposures  in  the  north  part  of  Sec.  15,  except  that  alteration  has 
extended  farther.     In  some  cases  the  feldspar  has  very  largely  decomposed 

MON  XIX 27 


418  THE  TENOKEE  lEON-B EARING  SEEIES. 

and  the  sections  are  then  made  up  mostly  of  chlorite  and  calcite,  with  spme 
quartz,  in  which  the  particles  of  plagioclase  are  set. 

In  nearly  all  of  the  rocks  of  the  several  areas  of  this  division  are  por- 
phyritic  crystals  of  feldspar,  which  is  the  only  mineral  that  occurs  in  two 
generations.  They  are  ^'ery  numerous  in  the  coarser  grained  varieties  of 
the  area,  but  are  altogether  absent  in  the  glassy  varieties.  This  porphy- 
ritic  feldspar  is  generally  greatly  altered  and  its  exact  nature  can  not  be  deter- 
mined, but  the  double  angles  measured  by  Pumpelly's  method  indicate  that 
it  is  of  the  same  character  as  the  feldspar  of  the  matrix.  Its  decomposition 
products  are  tlie  same  hs  the  feldspar  of  the  background;  that  is,  smarag- 
dite,  chlorite,  epidote,  and  kaolinite.  The  rocks  of  the  division  are  always 
amygdaloidal,  with  the  exception  of  the  exposures  in  the  north  part  of  Sec. 
16.  The  amygdules  are  at  times  very  numerous  and  frequently  of  quite 
large  size.  They  are  generally  of  an  elongated  oval  shape,  the  longer  axes 
of  which  are  parallel  and  probably  indicate  flowage  in  this  direction.  The 
filling  of  the  amygdules  is  one  or  more  of  the  minerals  chlorite,  calcite, 
epidote,  or  quartz,  the  latter  usually  being,  when  present,  chalcedony. 
Chlorite  and  calcite  are  the  most  plentiful  minerals,  the  two  frequently 
occurring  together  in  the  same  amygdule  in  concentric  belts.  The  chlorite 
is  always  of  a  green  nonpolarizing  variety,  or  that  giving  steel-blue  interfer- 
ence colors.  Often  within  the  chlorite  have  developed  large  areas  of  epidote. 
By  comparing  the  above  descriptions  with  those  of  the  pebbles  and  certain 
of  the  matrices  of  the  greenstone-conglomerates,  it  will  be  seen  that  there 
is  a  remarkable  likeness  between  the  various  phases  of  these  eruptives  and 
the  conglomerates,  with  the  exception  that  in  the  conglomerates  no  diabase- 
porphyry  phase  is  found.  The  resemblance  between  these  fine  grained 
eruptives  and  the  greenstone-conglomerates  is  still  further  emphasized  by 
veins  which  cut  them,  these  being  composed  at  times  of  intricately  inter- 
locking quartz,  chlorite,  and  epidote;  that  is,  these  sparse  veins  are  almost 
precisely  like  the  background  which  contains  the  apparent  fragments  of 
some  of  the  rocks  which  have  been  classified  with  the  greenstone-conglom- 
erates. Whether  these  rocks  really  belong  there,  that  is,  whether  these 
apparent  fragments  are  really  genuine  or  not,  is  uncertain.     If  the  more 


TIIH  EASTERN  AREA.'  411) 

mmierous  r;unU'yiii<r  veins  ol'  tlicst^  siij)[)().sc(l  coiii'-loiiKinites  are  all  secondary 
tliis  phase  of  the  ('oiiyU)nierate  ou<^-lit  to  Ik-  phiced  with  the  basii;  eriiptiveis 
of  this  area. 

SECTION  VI  —STRATIGRAPHY. 

Lithological  evidence  (t.s  to  equivalence  with  the  main  Penokee  area. — The 
litholoi^'ical  character  of  the  rocks  described  as  belonging-  to  the  Iron-bearing- 
member,  the  fragnieutal  and  fei-riiginous  rocks  south  of  the  greenstone- 
conglomerate,  and  the  fragp'ental  and  ferruginous  i-ocks  north  and  east  <if 
the  greenstoue-cong-lonier->.te,  closely  resemble  belts  in  the  Penokee  series 
to  the  westward.  So  far  as  the  Iron-bearing-  member  is  concerned  there  is 
identity.  The  ferruginous  and  fragmental  rocks  north  and  south  of  the 
g-reen.stone-conglonierates  differ  from  the  fragmental  rocks  to  the- westward, 
in  that  nonclastic  is  mingled  with  the  clastic  material.  Aside  from  this  dif- 
ference, the  above  belts  of  the  eastern  area  are  the  exact  parallels  of  belts 
in  the  main  area.  The  lithological  evidence,  then,  indicates  that  the  eastern 
area  belongs  to  the  same  great  period  of  time  as  the  Penokee  series  to  the 
Avest. 

The  only  question,  then,  which  needs  here  to  be  discussed  is  whether 
the  greenstone-conglomerates  belong-  to  this  same  series.  It  appears  to 
me  that  all  the  facts  indicate  that  they  do.  The  data  upon  wl'ich  this 
opinion  is  based  are  as  follows  :  The  conglomerates  are  inextricably  mingled 
and  interlaminated  with  black,  cleaved,  unmistakably  fragmental  slates. 
That  these  slates  and  greenstone-conglomerates  have  a  common  age  can 
hardly  be  doubted,  for  there  appears  to  be  every  gradation  between  the  two 
classes  of  rocks.  This  fact  is  strong  lithological  evidence  for  placing  these 
rocks  in  the  Penokee  succession,  where  similar  slates  are  so  largely  developed, 
and  not  in  the  Southern  Complex  or  in  the  Keeweenaw  sei-ies  to  the  north, 
where  no  such  rocks  are  known,  nor  in  the  cherty  limestone  series,  with 
which  they  have  no  apparent  coiuiection. 

It  may  be  asserted  that  the  greenstone-conglomerates  are  more  altered 
and  obscure  than  any  other  i-ock  in  the  Penokee  series.  This  argument 
loses  its  force  when  the  unusual  nature  of  the  materials  of  which  they  are 
composed  is  considered.     The  great  mass  of  the  fragmental  rocks  of  the 


420  THE  PENOKEE  IRON-BEARING  SERIES. 

series  is  derived  from  granites,  gneisses,  mica-schists,  hornblende-schists, 
etc.;  in  other  words,  from  rocks  which  are  always  strongly  quartzose  and 
which  contain  a  large  qu.antity.  of  acid  feldspar.  From  materials  of  this 
sort  quartzites,  graywackes,  and  slates  are  naturally  formed.  That  the 
quartz  and  acid  feldspars  are  often  in  a  very  fresh  condition  is  not 
strange,  but  it  has  been  seen  that  even  these  acid  feldspars  have  exten- 
sively, in  the  Upper  slate  member,  altered  to  the  more  basic  minerals, 
mica  and  chlorite,  with  tlie  simultaneous  separation  of  quartz  and  the 
resultant  formation  of  a  mica-schist  or  chlorite-schist.  These  mica-sclnsts 
are  unmistakably  parts  of  ■  the  Penokee  series,  and  are  as  much  changed 
from  their  original  condition  as  is  the  basic  eruptive  material  of  the 
greenstone-conglomerates,  notwithstanding  the  fact  that  the  latter  ax'e 
composed  of  materials  very  readily  alterable.  Basic  eruptive  rocks  are 
well  known  to  have  undergone  exteirsive  changes,  even  when  occurring  in 
solid  masses  which  belong  to  geologic  periods  long  subsequent  to  that  of 
the  Penokee  series.  It  thus  appears  that  the  widespread  alteration  of  the 
basic  detritus  in  these  greenstone-conglomerates  is  no  proof  that  they  are 
not  of  the  same  age  as  the  rocks  of  the  typical  succession."  Also  in  the 
greenstone-conglomerates  themselves  is  found  a  small  quantity  of  well  worn 
fragmeiital  quartz  and  feldspar,  which  in  every  respect  resembles  like 
material  in  the  graywackes  and  quartzites.  It  is  thus  plain  that  the  litho- 
logical  evidence  points  toward  a  classification  of  the  greenstone-conglomer- 
ate with  a  series  subsequent  to  the  Southern  Complex,  a  series  of  clastic 
rather  than  crystalline  rocks. 

Finally,  the  association  and  relations  of  the  greenstone-conglomerates 
to  the  other  classes  of  rock  adjoining,  which  unquestionably  are  the  equiv- 
alent of  the  main  Penokee  succession,  are  such  as  to  make  it  scarcely 
conceivable  that  they  could  be  a  part  of  the  eruptives  of  the  overlying 
Keweenaw  series. 

StratifjrapMcal  evidence  as  to  equivalence  with  the  main  Penokee  area. — 
The  stratigraphical  evidence  that  the  eastern  area  rocks  belong  to  a  series 
separated  both  from  the  schists,  gneisses,  and  granites  to  the  south  of  them 
and  from  the  great  range  of  greenstones  to  the  north  is  precisely  like 
that  which  proves  that  the  clastic  formations  to  the  west  belong  to  a  dis- 


THE  KASTEUN  AIJEA.  421 

tinct  series.  Without  reiH'iitin<>-  tlu^  Mr^-uinciit  in  dctnil  wliich  is  given  in 
anotlier  place  (Chap,  ix,  8ec.  ii),  it  is  only  uecussary  to  iiiciition  the  salient 
tacts  which  show  that  tlie  same  relations  ))revail  l)etween  the  three  great 
series  of  rocks  east  of  the  Little  Prescpie  Isle  that  arcf  fonnd  to  the  w<ist. 

The  schists,  gneisses,  and  granites,  mingled  with  occasional  eruptive 
greenstones,  constitute  a  complex  of  rocks  in  the  southern  part  of  the  area 
mapped  on  PI.  xiu.  There  is  no  uniformit}-  in  the  strike  and  dip  of  these 
schists  and  gneisses.  Within  a  few  rods  the  strike  of  the  rock  may  change 
90°.  The  dip  is  also  in  all  directions,  the  only  constant  thing  being  tliat 
tke  angles  of  inclination  are  generally  high,  indicating  that  the  rocks 
have  probably  passed  through  a  period  of  extreme  folding.  Taken  as  a 
whole,  the  schists  and  the  granites,  the  latter  including  gneissoid  granites, 
are  in  tolerably  distinct  large  areas ;  but  along  the  lines  of  contact  the  two 
classes  of  rocks  are  mingled  in  inextricable  confusion,  and  included  in  the 
large  granite  asea  west  of  Gogebic  lake,  in  several  places,  exposures  of 
hornblende-schists  and  chlorite-schists  are  foimd.  Lithologically,  all  the 
rocks  belonghig  in  this  complex  are  either  massive  eruptives  or  crystallhie 
schists,  in  which  the  extreme  of  foliation  and  crystalline  character  is  found. 
Nowhere  is  there  any  plain  evidence,  unless  foliated  and  schistose  structure 
is  so  considered,  that  any  of  them  are  of  fragmental  origin.  Thus  in  all 
respects  this  comiplex  series  is  seen  to  be  like  the  basement  rocks  upon 
which  the  Penokee  series  to  the  west  rests,  and  it  is  therefore  a  continu- 
ation of  them. 

In  strong  contrast  to  this  complex-  are  the  rocks  under  discussion. 
While  their  strikes  vary  somewhat  as  the  belts  bow  north  oi-  south,  their 
general  course  is  east  and  west.  Their  dips  are  generally  to  the  north,  but 
in  some  cases  are  to  the  south.  This  variation  of  dip  has  to  do  with  the 
relation  of  the  belts  of  rocks  included  in  the  series  among  themselves,  but 
does  not  affect  their  relations  to  the  surrounding  rocks.  We  have  here,  then, 
as  in  the  western  area,  a  series  which  Is  north  of  and  separable  from  the 
complex  to  the  south  by  a  fundamental  difference  of  strike.  Also  the 
series  under  discussion  is  seen  to  be  essentially  a  mingled  nonfragmental 
and  fragmental  set,  which  is  for  the  most  part  so  little  changed  that  its 
origin  is  certainly  determinable. 


422  THE  PENOKEB  IRON-BEAEING  SERIES. 

Furtlier,  as  fun}^  indicated  subsequent! 3^,  in  Sec.  23,  T.  47,  N.,  R.  43 
W.,  Michigan,  and  in  Sec.  28,  T.  47  N.,  R.  42  W.,  Michigan,  are  basal  con- 
glomerates, and  in  the  latter  place  the  conglomerates  rest  directly  upon  the 
complex  below.  These  unconformable  relations  between  the  two  sets  of 
rocks  are  here  the  same  as  between  the  Southerii  Complex  and  the  Avestward 
Penokee  succession.  The  series,  then,  is  an  independent  conformable  one, 
resting  upon  an  older  complex,  which  served  as  the  basement  upon  which 
the  newer  rocks  were  formed  as  fragmental  and  nonfragmental  water- 
deposited  sediments. 

It  is  thus  seen  that  in  every  particular  the  facts  and  the  conclusions 
as  to  the  relations  of  these  rocks  and  the  series  below  them  are  the  same 
as  in  the  rocks  to  the  west.  This  complex  to  the  south  and  the  series 
under  discussion  are  but  eastern  continuations  of  the  same  two  sets  to  the 
west  already  described. 

West  of  Sec.  18.  T.  47  N.,  R.  42  W.,  Michigan,  the  rocks  of  the  Eastern 
area  are  bounded  upon  the  north  by  a  continuous  prominent  range  of 
greenstone,  whicli  rises  abruptly  from  the  lower  ground  to  the  south. 
Through  T.  47  N.,  R.  43  W.,  Michigan,  this  range  runs  almost  due  east  and 
west.  In  the  next  two  miles  westward  it  makes  north  about  a  half  a 
mile  and  then  again  continues  westward.  It  is  plain  that  this  greenstone 
overlies  the  rocks  of  the  Eastern  area.  That  there  is  any  great  or  certain 
discordance  between  the .  two  series  can  not  be  proven  from  the  Eastern 
area.  However,  this  greenstone  range,  through  the  east  part  of  T.  47  N., 
R.  44  W.,  Michigan,  and  through  T.  47  N.,  R.  43  W.,  Michigan,  appears  to 
overlap  to  some  extent  the  rocks  of  the  Eastern  area,  which  to  this  degree 
would  indicate  an  unconformity,  but  would  not  be  certain  proof  of  one. 
However  this  may  be,  this  range  is  an  eastward  continuation  of  the  green- 
stone which  overlies  the  Penokee  series  to  the  westward,  just  as  the  rocks 
under  discussion  are  an  eastern  continuation  of  that  series  itself  There 
is,  then,  no  doubt  that  the  same  relations  obtain  between  them  that  pre- 
vail to  the  west. 

The  only  further  point  in  this  connection  is  whether  the  formations  of 
the  Eastern  area  are  to  be  placed  with  the  Penokee  series  proper  or  with 
the  series  to  which  the  Cherty  limestone  belongs.     The  lower  formation  of 


TIIH  KASTKKN  AHKA.  423 

the  Eastern  nrca  hciirs  rr;iL;incnts  of  I'cd  j;is|icr  ;it  (iiic  pljicc,  ;iiiil  llic  upper 
formation  coiitiiins  nuicli  cIutI  <l('tritus,  w  lilcli  in  iill  probability  was  derived 
from  tlie  (Jlierty  limestone  i'orniation  itself.  ConseqiU'iitl}-  the  rocks  of  the 
Eastern  area  bear  the  same  relation  to  the  Cherty  hme.stone  as  do  the 
Penokee  series  proper.  Finally,  the  outcrops  of  the  iron  formation  of  the 
Peuokee  series  j)roper  an<l  tluxse  of  this  formation  phaced  with  the  Eastern 
area  have  such  relations  as  to  leave  little  doubt  that  the  two  are  continuous. 
It  is  therefore  conclntled  that  the  rocks  of  the  Eastern  area  are  the  continu- 
ation of  the  Penokee  series  proper. 

Relnt'tons  of  the  helfs  of  the  Eastern  area  to  one  another. — It  is  evident  that 
the  rocks  of  the  Eastern  area  have  formed  under  different  conditions  from 
those  of  the  western  extension.  This  is  indicated,  as  has  been  seen,  by  the 
presence,  first,  of  a  great  thickness  of  rocks,  which  are  mingled  clastic  and 
nonclastic  sediments  ;  second,  by  the  presence  between  the  nortliern  and 
southern  boundary  of  the  normal  rocks  of  the  area  of  a  great  thickness  of 
greenstone-conglomerates;  and,  thii'd,  by  numerous  southern  dips  found  in 
the  slates  and  conglomerates.  These  diiferences  do  not  seem  to  be  so  great, 
however,  as  necessarily  to  preclude  a  belief  in  a  simple  conformable 
succession  like  that  to  the  west.  At  any  rate,  the  expectation  of  such 
a  succession  here  is  a  natural  one,  and,  if  this  explanation  fairly  represents 
the  known  facts,  should  be  preferred  to  any  which  implies  either  folding  or 
a  reference  of  a  part  of  the  rocks  contained  within  the  outer  limits  of  the 
area  to  other  than  the  Penokee  series. 

The  two  evident  obstacles  to  considering  the  Eastern  area  as  a  simple 
succession  are,  first,  the  great  width  of  the  series  in  the  eastern  part  of  T. 
47  N.,  R.  44  W.,  Michigan,  as  compared  with  its  width  a  few  miles  east  or 
west;  and,  second,  the  southern  dips. 

Great  width  of  parts  of  the  Eastern  area. — The  unusual  width  of  the 
Eastern  area  in  T.  47  N.,  R.  44  W.,  and  T.  47  N.,  R  43  W.,  Michigan,  is 
almost  wholly  due  to  the  greenstone-conglomerates  and  greenstones.  It 
has  been  seen  that  the  greater  part  of  this  area  is  composed  of  volcanic 
products.  A  considerable  thickness  is  composed  of  direct  surface  flows. 
Another  large  part  is  volcanic  products,  which  in  places  have  been  mingled 
to  some  extent  with  water  deposited  sediments.     Now,  the  accumulation  of 


424  THE  PENOKEE  lEON-BEAEING  SEMES. 

volcanic  material  may  go  on  with  rapidity  much  greater  than  that  of  ordi- 
nary sedimentation.  For  such  material  to  accumulate  to  a  depth  of  7,000  or 
8,000  feet  while  a  few  hundred  feet  of  the  Iron-bearing  member  to  the  west- 
ward were  forming  is  not  contrary  to  known  geological  facts.  The  northern 
boundary  of  the  Eastern  area,  throughout  the  district  is  approximately  east 
and  west,  with  no  greater  variation  than  in  the  main  area.  The  increased 
thickness  of  the  series  is  given  space  by  rapid  bowing  to  the  southward  of 
the  underlying  complex.  This  southern  bowing  is  explicable  as  resulting 
from  a  continuous  sinking,  Avhich  was  contemporaneous  with  and  caused 
by  the  accumulation  of  great  quantities  of  ejected  material.  This  bowing 
is  simply  the  well  known  geological  phenomenon  of  the  inward  sinking 
of  the  strata  about  a,  crater  when  burdened  with  a  mountain  mass  of  vol- 
canic products. 

The  sudden  bowing  of  the  iron-bearing  belt  of  the  series  to  the  south 
may  be  due  in  part  to  a  fault.  Upon  the  whole,  it  seems  not  unlikely 
that  such  a  fault  has  combined  with  the  extravasation  of  lava  and  tuff 
to  produce  the  present  condition  of  affairs.  The  strike  of  the  various 
exposures  found  in  T.  47  N.,  R.  44  W.,  and  R.  43  W.,  Michigan,  shows 
distinctly  that  a  bowing  to  the  south  has  occurred.  This  is  evidenced 
by  the  strike  of  the  easternmost  exposure  of  the  continuous  Iron-bearing 
member  in  Sees.  IG  and  17,  T.  47  N.,  R.  44  W.,  Michigan,,  as  well  as  the 
strike  of  the  exposures  in  T.  47  N.,  R.  43  W.,  Michigan.  It  is,  however, 
noticeable  tha,t  the  iron  formation  rocks  in  the  NW.  ^  of  Sec.  30  strike  about 
E.  20°  S.  The  cause  of  this  anomalj^  is  not  known.  The  greatest  appar- 
ent discordance  between  the  various  exposures  of  the  iron  formation 
is  between  that  in  the  west  part  of  Sec.  23,  T.  47  N.,  R.  44  W.,  and 
that  in  the  southwest  part  of  Sec.  25.  It  is  also  to  be  observed  that 
the  base  of  the  Keweenaw  series,  which  in  T.  47  N.,  R.  42  W.  and  43 
"W".,  Michigan,  has  a  nearly  east  and  west  trend,  and  which  west  of  the 
S.  ^  post  of  Sec.  11,  T.  47  N.,  R.  44  W.,  Michigan,  also  has  a  very  uniform 
east  and  west  trend,  between  these  two  points  differs  a  half  mile  in  position. 
Thus  it  is  probable  that  if  a  fault  exists  it  runs  from  near  the  southwest 
corner  of  Sec.  25  to  a  short  distance  west  of  the  E.  -J  post  of  Sec.  13,  T.  47 
N.,  R.  44  W.,  Michigan.     The  existence  of  such  a  fault  is  stiU  further  sug- 


Tin:  EASTKliN  AREA.  425 

gested  by  tlie  position  of  the  great  mass  of  basic  ci-uptives  in  Sees.  26  and  27, 
T.  47  N.,  R.  44  W.,  Micliio-an.  'Tbeso  ex'posures  are  all  essentially  the  same 
kind  ttf  rocks,  coarse  grained  diabases,  ap])roacliing-  gabliro.  Such  rocks 
woidd  commonly  b(^  regarded  ])y  petrogra})liers  as  deep  seated  ones,  which 
have  now  reached  the  surface  by  erosion.  I'hey  are  reinarkably  similar  to 
the  massive  eruptives  in  Sees  15,  16,  and  23,  T.  47  N.,  R.  44  W.,  Michigan, 
and  they  are  probably  contemporaneous  in  time,  if  not  actually  connected 
with  these  rocks.  They  now  lie  at  the  same  apparent  horizon  as  the  large 
exposures  of  typical  greenstone-conglomerates  just  east  of  them,  which  are 
surface  accumulations.  If  the  discrepancy  between  the  locations  of  the  iron 
formation  in  vSec.  25,  T.  47  N.,  R.  44  W.,  Michigan,  and  those  in  Sec.  23 
can  be  taken  a*  a  guide  to  the  amount  of  throw  of  the  supposed  fault,  it 
would  reach  several  thousand  feet.  If  such  a  fault  occurs,  it  would,  to  the 
extent  of  its  throw,  lessen  the  thickness  of  the  formations  belonging  to  the 
eastern  area,  as  will  readilj'  be  seen  by  examining  PI.  xui.  Supposing  no 
fault  to  exist,  the  maximum  thicjiness  of  the  series,  if  the  succession  is  a 
simple  one,  nuist  be  about  11,000  feet  in  the  extreme  eastern  part  of  T.  47  N., 
R.  44  W.,  Michigan.  If  we  suppose  a  fault  to  exist  with  a  throw  of  3,000 
feet,  this  would  reduce  the  maximum  width  to  8,000  feet.  East  and  west 
of  this  locality  the  thickness  rapidly  lessens.  At  the  center  of  T.  47  N., 
R.  43  W.,  Michigan,  3  miles  east,  the  surface  width  of  the  series  is  about 
1  mile,  which  represents  a  thickness  of  about  4,500  feet.  In  passing  far- 
ther east,  the  series  continues  to  narrow  until  it  is  almost  entirely  over- 
lapped by  the  Eastern  sandstone  in  Sec.  28,  T.  47  N.,  R.  42  W.,  Michigan. 
To  the  west  the  series  also  rapidly  narrows  until  it  joins  the  normal  Peno- 
kee  area. 

The  rapid  variation  in  thickness  of  the  Eastern  area  may  be  urged  in 
opposition  to  the  above  explanation  of  its  great  thickness  and  of  the  bow- 
ing of  the  iron-bearing  formation  to  the  south.  In  answer  it  may  be 
said  that  the  distance  from  the  easternmost  to  the  westernmost  exposure 
of  the  greenstone-conglomerate  is  fully  7  miles,  and  the  evidence  of  dis- 
turbance, as  shown  by  the  mingling  of  fragmental  and  nonfragmental 
material,  extends  to  twice  this  distance.  Also,  when  it  is  considered  that 
the  greater  part  of  this  thickness  is  due  to  greenstone-conglomerates,  which 


426  THE  PENOKEE  IRON-BEARING  SERIES. 

are  largely  volcanic  tuffs  and  lavas,  the  objection  loses  its  force,  for  it  is 
well  known  that  this  class  of  material  varies  greatly  in  thickness  within  a 
short  distance. 

The  southern  clips. — The  slaty  phases  of  the  greenstone -conglomerate 
and  the  clay-slates  north  of  the  greenstone-conglomerate  in  the  W.  J  of  T. 
47  N.,  R.  43  W.,  Michigan,  so  far  as  observed,  appear  to  dip  south,  at 
least  they  possess  a  parting  which  dips  in  this  direction.  This  apparent 
southern  dip  varies  from  55°  to  80°.  But  the  southern  dips  are  not 
universal;  for  instance,  in  one  exposure  in  the  SE.  \  Sec.  17,  T.  47  N.,  R. 
43  W.,  Michigan,  an  exposure  has  a  northern  dip,  although  the  s  ates  a 
short  distance  southward  have  the  ordinary  dip  of  the  locality.  Of  far 
greater  importance,  the  jaspery  rocks  in  the  south  part  of  Sec.  18,  T.  47 
N.,  R.  43  W.,  Michigan,  have  an  unmistakable  northern  dip.  The  other 
extremely  ferruginous  rocks  in  the  northern  fragmental  belts,  in  Sees.  13 
and  14,  T.  47  N.,  R.  44  W.,  Michigan,  are  too  massive  to  furnish  dips. 
Whether  the  apparent  southern  dips  represent  true  bedding  or  cleavage  it 
seems  impossible  to  certainly  determine  from  the  exposure  or  from  the 
examinations  of  thin  section.  In  the  cases  of  one  or  two  ledges  showing 
southern  dips,  an  apparent  bedding  transverse  to  this  parting  was  found, 
but  this  was  exceptional. 

All  the  rocks  of  the  iron-bearing  belt  situated  in  the  southern  part  of 
the  Eastern  area,  from  the  west  part  of  Sec.  23,  T.  47  N.,  R.  44  W.,  Michi- 
gan, to  the  northwest  part  of  Sec.  23,  T.  47  N.,  R.  43  W.,  Michigan,  have, 
so  far  as  observed,  a  northern  dip.  The  slates  prominently  exposed  in  two 
places  in  Sec.  21,  T.  47  N.,  R.  43  W.,  Michigan,  below  the  iron-bearing 
belt,  have  unmistakable  northern  dips,  as  have  also  the  half  fragmental 
slates  and  schists  interlaminated  with  the  rocks  of  the  iron-bearing  belt 
just  above.  If  we  had  but  the  E.  ^  of  T.  47  N.,  R.  44  W.,  Michigan,  and 
the.W.  I  of  T.  47  N.,  R.  43  W.,  Michigan,  to  deal  with  we  would  feel 
inclined  to  explain  the  structure  by  considering  the  rocks  as  a  synclinal 
trough,  in  which  the  greenstone-conglomerate  overlies  the  fragmental 
i-ocks  to  the  north  and  south  of  them.  This  would  necessitate  tliat  the 
north  arm  of  the  trough,  rising  into  an  anticlinal,  had  again  descended, 
dipping  to  the  north,  and  that  this  part  of  the  fold  had  been  removed  by 


Till':   i; ASTERN  AHKA.  427 

an  erosion  beforo  tlic  l)('^iniiin<;-  ot"  Kowecnnwan  tiinc  We  would  tlius 
luive  tlie  siin])lc  nKUKicliiic  ul'  tlic  I'ciKikcc  siicccssidii  to  the  west  piissiu}^ 
into  a  folded  scries  in  tlie  Kasteni  area.  Tliis  Inpotliesis  lias,  however, 
some  serious  ditliculties  to  contend  with:  (1)  The  rocks  on  the  north  and 
the  south  sides  of  tlu'  synclinal  are  (juite  different  in  character.  (2)  Some 
explanation  nmst  l)e  furnislusd  for  the  northern  dips  refei-red  to  'in  the 
north  frag-mental  belt.  (3)  'I'he  higli  and  variabk'  soutliei-n  dips  of  the 
greenstone-conglomerates  throughout  their  whoh^  thickness  and  close  to 
the  northern  dips  of  the  rocks  of  the  inin-bearing  "iind  fragmental  belts  to 
the  south  must  be  explained.  (4)  It  would  seem  that  if  the  Penokee 
monocline  passes  into  a  folded  series  moi'e  definite  evidence  would  be 
found  of  the  change  as  the  supposed  folded  part  of  the  area  Avas  approached. 

If  the  position  is  taken  that  the  apparent  southern  dips  represent 
cleavage  rather  than  bedding  the  difficulties  largely  disappear.  (1)  The 
rocks  in  wdiich  the  inclinations  have  most  value — that  is,  al'  the  rocks 
which  belong  to  the  Iron-bearing  member  or  are  very  ferruginous  and 
banded  and  consequent]}'  take  on  a  slaty  cleavage  with  g-reater  diffi- 
culty— have  northern  dips  whether  they  occur  north  or  south  of  the 
greenstone-conglomerates,  and  these  northern  dips  vary  within  narrow 
limits.  (2)  All  tlie  rocks  south  of  the  Iron-bearing  member,  like  the 
quartz-slate  of  the  main  Penokee  area,  have  northern  dips,  and  these  dips 
correspond  to  the  inclination  of  the  overlying  iron-bearing  belt.  (3)  The 
considerable  variation  in  strike  and  the  great  variation  in  the  apparent 
southern  dips  of  the  clay-slates  aiad  greenstone-conglomerates  at  once  lose 
significance  if  regarded  as  cleavage.  (4)  By  regarding  the  part  of  the 
Eastern  area  which  has  the  southei'n  dips  as  a  simple  series  it  becomes  the 
natural  link  between  the  rocks  of  the  Penokee  series  to  the  Avest  and  those 
to  the  east  which  have  only  northern  dips.  (5)  By  regarding  this  Eastern 
area  as  a  simple  succession  we  have  the  same  orderly  arrangement  between 
the  three  ffreat  series  of  rocks  which  obtains  to  the  w^estward. 

That  slat}^  cleavage  should  be  found  in  the  Eastern  area  and  nowhere 
else  in  the  Penokee  succession  is  not  at  all  strange,  for  the  slates  here  are 
much  finer  grained  and  more  clayey  than  those  elsewhere  found.  Sorby 
has  shown  that  such  material  is  particular]}'  likely  to  take  on  a  slatj' 


428  THE  PENOKEE  lEON-BEARlNG  SEEIES. 

cleavage.  Further,  the  great  increase  in  thickness  in  the  central  part  of 
the  Eastern  area  might  readily  cause  at  the  time  of  the  upturning  of  the 
series  an  increased  pressure.  That  from  this  or  some  other  cause  these 
rocks  have  been  subjected  to  unusual  pressure  is  shown  by  the  foliated 
character  of  some  of  the  sericite-schists  and  the  arrangement  of  the  par- 
tides  with  then-  longer  axes  in  a  common  direction  in  some  of  the  quartzose 
slates  (pp.  406-408).  To  the  east  and  west  of  the^central  portion  of  the 
area,  where  the  pressure  has  been  less,  no  southern  dips  are  found. 

There  is  one  other  possible  explanation  of  the  southern  dips.  They 
may  be  regarded  as  overturns.  The  dip  of  the  Penokee  series  is  usually 
high,  averaging  from  60°  to  10°.  In  the  neighborhood  of  Tylei's  fork  it 
becomes  very  high,  reaching  from  70°  to  80°,  while  at  one  place  the  slates 
of  the  upper  seiies  are  vertical,  if  not  slightly  inclined  to  the  south.  It 
might  readily  be  the  case  that  the  rapid  widening  of  the  Eastern  area 
and  the  exceptional  nature  of  the  material,  caused  it  to  be  overtunied  during 
the  time  of  its  iiplifting.  Upon  the  whole,  however,  it  is  not  so  jDrobable 
that  there  has  been  an  overturn  as  it  is  that  the  southern  dips  are  due  to 
slaty  cleavage.  Tlie  variations  in  dip  are  too  wide  and  the  change  from 
southern  dips  to  northern  dips  too  abrupt  to  be  satisfactorily  explained  by 
an  overturn. 

Our  conclusion  is,  then,  tliat  the  rocks  of  the  Eastern  area  form,  in  all 
probability,  a  continuous  simple  conformable  succession  from  south  to  north, 
which  are  the  eastern  extension  of  the  main  Penokee  series. 

Sequence  of  events. — We  are  now  prepared  to  give  a  brief  history  of  the 
geology  of  the  area  and  to  coi-relate  the  belts  here  found  with  those  to 
the  west.  As  in  the  main  area,  the  complex  of  schists,  gneisses,  and 
granites  constitiite  a  basement  upon  Avhich  fhe  sedimentary  rocks  were 
deposited.  This  basement  was  probably  uneven,  showing  variations  in 
topography,  as  a  consequence  of  which  deposition  began  earlier  in  some 
places  than  in  others.  The  present  occurrence  of  the  belts  is  such  as  to 
suggest  that  the  eastern  end  of  the  area  was  higher  than  the  Avestern  part 
and  did  not  receive  any  deposits  belonging  to  the  lower  formations. 

The  exposures  of  the  Quartz-slate  member  west  of  the  Presque  Isle  are 
so  numerous  as  to  leave  no  doubt  as  to  the  continuity  of  this  belt.     In  the 


THE  EASTERN  AREA.  429 

Eastern  area  its  equivalent  is  tnuml  in  oiil\  two  places,  within  a  short  dis- 
tance from  ciuli  ntlit  r,  ill  Si'c.  21,  T.  47  N.,  H.  43  W.,  Michigan.  So  far  as 
present  iiilunuation  yocs,  ihc  (^hiartz-slatc  is  now  prol)al)ly  the  oldest  con- 
tinuous sedimentary  formation  of  the  area,  since  nowliere  are  exposures 
found  which  can  l)e  referred  to  the  Clierty  limestone,  unless  the  ferruginous 
limestone  in  the  north  part  of  Sec.  23,  T.  47  N.,  11.  43  W.,  just  north  of 
the  large  exposures  of  «»rystalliue  schists  is  to  be  here  placed.  The  thickness 
of  the  quartz-slate  in  Sec.  21  is  considerable,  and,  as  has  been  seen  by  the 
description  of  the  exposures  (pp.  368-3(i9),  they  are  typical  in  every  way  of 
the  like  I'ocks  from  this  member  to  the  west,  and  can  without  hesitation 
be  correlated  with  them. 

Following  above  the  quartz-slate  are  the  nonclastic  sediments  of  the 
Iron-bearing  member.  The  exposures  are  sufficiently  numerous  to  shoAv 
that  there  is  but  little  doubt  of  its  continuity.  It  outcrops  as  far  east  as 
Sec.  23,  T.  47  N.,  R.  43  W.,  Michigan,  are  pure  nonfragmental  sedi- 
ments. There  is  no  evidence  that  the  belt  has  any  great  thickness  in  Sec. 
25,  T.  47  N.,  R.  44  W.,  and  it  is  certainly  not  more  than  one-half  the  thick- 
ness of  the  iron-bearing  belt  to  the  west,  even  if  the  entire  space  be- 
tween the  hornblende-schists  of  the  underlying  complex  and  the  known 
•  exposures  of  the  greenstone-conglomerate  is  entirely  occupied  by  this  mem- 
ber. If  a  part  of  the  unexposed  space  is  occupied  by  the  quartz-slates,  the 
thickness  of  the  iron-bearing  formation  would  here  be  no  more  than  a  frac- 
tion of  its  thickness  in  the  western  area.  East  of  Sec.  30,  T.  47  N.,  R.  43 
W.,  in  the  south  part  of  Sec.  20,  and  in  Sees.  21,  22,  and  23,  the  iron-bear- 
ing sediments  are  spread  over  a  considerable  distance  north  and  south,  but 
here  they  are  mingled  with  mechanical  sediments.  A  comparatively  great 
thickness  of  such  material  might  be  deposited  in  the  time  taken  for  the  for- 
mation of  a  thin  layer  of  pure  nonclastic  sediments.  If  the  iron-bearing  belt 
extends  farther  than  the  NW.  ^  Sec.  23,  T.  47  N.,  R.  43  W.,  it  must  be  exceed- 
ingly narrow,  for  there  is  but  a  small  space  between  the  exposure  of  green- 
stone-conglomerate and  the  crystalline  schists  of  the  Southern  Complex. 

The  deposition  of  this  thin  layer  of  nonfragmental  sediment  thus 
traced  out  did  not  go  on  undisturbed,  for  the  flow  of  porphyrite  in  Sees.  29 
and  30   is  plainly  interleaved.     This  interbedded  flow  of  basic  eruptive 


430  THE  PBNOKEE  IRON-BEARING  SERIES. 

rock  is  doubtless  the  cause  of  the  minghng  of  clastic  and  nonclastic  sedi- 
ments which  is  so  characteristic  of  the  exposures  just  to  the  north  and  less 
marked  in  the  exposures  in  Sees.  21,  22,  23,  T.  47  N.,  R.  43  W.,  Michigan. 

After  the  deposition  of  a  thin  layer  of  nonclastic  sediments  with  the, 
modifications  indicated,  but  long  before  the  cessation  of  the  nonfragmental 
sedimentation  of  the  Iron-bearing  member  of  the  western  area,  came  the 
great  volcanic  outbreak  which  resulted  in  piling  up  the  series  of  eruptive 
flows,  tuffs  and  interstratified  sediments  of  the  greenstone-conglomerate  area. 

The  truly  aqueous  interstratified  rocks  doubtless  received  much  of 
their  material  from  the  contemporaneoiis  volcanics.  This  group  of  forma- 
tions is  then  very  similar  to  the  central  portion  of  the  Keweenaw  series, 
consisting  of  basic  lava  flows,  with  interbedded  sedimentary  rocks.-' 

Evidently  the  center  of  the  volcanic  activity  was  the  E.  J  of  T.  47  N., 
E,.  44  W.,  Michigan.  To  the  east  this  material  reaches  no  farther,  or  little 
farther  than  the  NW.  {  of  Sec.  23,  T.  47  N.,  R.  43  W.,  Michigan.  This  com- 
paratively short  eastern  extent  of  disturbaiUce  may  have  been  due  to  the 
fact  that  the  higher  lands  to  the  east  prevented  farther  extension ;  for,  as  has 
been  seen,  all  of  the  Eastern  area  rocks  east  of  this  point  very  probably  are 
at  a  higher  lioriEon  than  wehave  yet  reached.  To  the  west,  the  influence  of 
the  volcanic  activity  extended  farther.  The  accumulation  of  the  volcanic 
outflows  and  of  the  greenstone-conglomerates  went  on  with  great  rapidity 
as  compared  with  the  exceedingly  slow  process  of  nonclastic  sedimentation 
which  was  continuing  to  the  west  in  the  iron-bearing  area.  As  explained, 
this  accumulation  was  accompanied  by  simultaneous  sinking  of  the  area 
thus  burdened. 

If  the  above  conclusions  are  correct,  a  shore  line  existed  not  far  to  the 
east  during  the  time  of  the  deposition  of  the  Quartz-slate,  Iron-bearing  and 
Greenstone- conglomerate  belts.  It  follows  that  the  volcanic  center  was  prob- 
ably submarine  at  the  inauguration  of  the  igneous  action,  and  was  near 
a  shore  line,  as  is  usually  the  case  with  existing  volcanoes.^ 

'  K.  D.  Irving:  Copper-boariug  rocks  of  Lake  Superior.     Mon.  V,  U.  S.  Geol.  Survey. 

'^cjince  the  above  Avas  written,  further  field  work  north  of  Crystal  Falls,  Miehigan,  has  showu 
the  existence  of  an  extensive  area,  identical  in  most  respects  with  the  grceustone-iougloiLierate  group 
of  the  Eastern  area,  i.  c,  it  consists  of  diabase,  jiorphyrites,  auiygdaloids,  greeustone-conglOnierates 
and  contemporaneous  sandstones  and  conglomerates  the  material  of  which  was  mainly  derived  ftoffl 
the  contemporaneous  volcanics. 


THE  i:astekn  area.  431 

At  or  near  tlir  ci-ssariMii  ot'  the  |K'rioi|  of  \iilc;mic  activity,  the  clastic 
and  iiouclastic  scdiinciits  wiiicli  uvcrlic  tliciii  lic^an  to  Inrin.  How  far 
upward  in  tlu'  Pciiukei'  .suci-e.ssioii  tlic  (Icpusitioii  of  scdimciits  to  the 
west  had  extended  is  not  certain,  hut  apparentl}'  tiie  period  of  iionchistic 
sedinientation  of  the  iron-l)eanii<j;-  helt  had  not  ceased,  as  is  indicated 
by  the  nature  of  the  fragniental  rttcks  nortli  and  east  of  the  greenstone- 
conglonierates.  In  the  northwest  part  of  Sec.  15,  T.  47  N.,  R.  43  W., 
Michigan,  are  unmistakable  typical  nonfragniental  sediments,  separable 
from  similar  sediments  in  Sec.  16  by  an  almost  mountain  mass  of  diabase. 
Exposures  in  Sees.  13  and  14,  T.  47  N.,  R.  44  W.,  Michigan,  and  in  Sec.  18, 
T.  47  N.,  R.  43  W.,  Michigan,  might  tully  as  well  be  classified  with  the 
nonfragmental  sediments  of  the  iron-bearing  belt  as  with  any  other  forma- 
tion. From  Sec.  18,  T.  47  N.,  R.  43  W.,  Michigan,  to  the  easternmost 
exposure  known  in  Sec.  28,  T.  47  N.,  R.  42  W.,  Michigan,  the  rocks  are 
quite  largely  nonclastic  ones,  which  contain  a  considerable  portion  of  iron, 
either  as  a  carbonate  or  oxide.  This  is  particularly  true  of  the  more  southern 
exposures  of  the  belt,  but  such  materials  are  also  found  in  the  most  northern 
exposiu-es  known,  neai-  the  east  quarter  post  of  Sec.  14,  T.  47  N.,  R.  43  W., 
Michigan. 

Above  the  ferruginous  and  fragmental  slates  north  and  east  of  the 
greenstone-conglomerates,  "followed  probably  fragmental  rocks  which  were 
the  equivalent  of  the  rocks  of  the  tipper  slate  belt  of  the  Penokee  succes- 
sion ;  but  if  this  was  the  case  such  material  was  removed  by  erosion,  and 
then  followed  the  unconformably  overlying  Keweenaw  series. 

Before  the  Eastern  sandstone  at  the  east  end  of  the  area,  subsequently 
described,  was  deposited,  the  three  series  of  the  district  were  tilted  into 
their  present  inclined  position,  and  during  this  time  or  later  underwent 
great  erosion.  That  the  Eastern  sandstone  is  in  direct  contact  with  the 
Eastern  area  rocks  near  their  base  at  Gogebic  lake,  is  simply  due  to  the 
fact  that  these  rocks  were  more  resistant  than  the  Keweenaw  and  upper 
Eastern  area  rocks,  and  have  been  left  as  a  cliff"  against  which  the 
Eastern  sandstone  was  deposited.  How  much  more  widespread  than 
at  present  the  sandstone  has  been,  we  have  no  means  of  knowing, 


432  THE  PElSrOKEE  lEOF  BEAEING  SERIES. 

Mingled  fragmental  and  nonfragmental  sediments. — If  the  above  account 
represents  the  facts,  it  explains  how  it  is  that  the  sharply  separated  frag- 
mental and  nonfragmental  sediments  of  the  Western  area  pass  into  the 
mingled  sediments  so  characteristic  of  the  Eastern  area.  The  time  was 
one  in  which  the  conditions  throughout  the  great  part  of  the  Penokee  basin 
were  favorable  to  nonfragmental  sedimentation.  When  the  sea  was  undis- 
turbed, pure  sediments  of  this  class  were  formed  ;  but  in  the  Eastern  area, 
almost  as  soon  as  this  period  was  inaugurated,  came  a  time  of  intei-mittent 
eruptive  activity,  which  formed  the  masses  of  porphyrite  and  greenstone- 
conglomerate  there  found.  Even  when  these  materials  were  not  being 
ejected  with  such  rapidity  as  to  preponderate,  over  other  varieties  of  rock, 
the  sea  was  so  disturbed  that  only  for  brief  intervals,  and  locally,  was  it 
sufficiently  clear  to  form  pure  nonfragmental  sediments.  In  this  part  of  the 
basin,  the  nonfragmental  sediments  belonging  to  the  iron-bearing  formation 
are  interbedded  with  layers  of  or  mingled  with  mechanical  sediments.  Non- 
fragmental sediments  continued  to  deposit  in  greater  or  lesser  purity,  how- 
ever, at  -every  favorable  occasion  throughout  the  whole  of  the  time  of 
deposition  of  the  eastern  rocks,  later  than  the  Quartz-slate.  The  mass  of 
rocks  above  these  slates  in  the  Eastern  area  is  several  times  thicker  than 
the  nonfragmental  sediments  of  the  iron-bearing  formation  to  the  west. 
In  estimating  time  equivalents,  however,  we  must  take  into  account  the 
fact  that  water-deposited  mechanical  sediments  may  accumulate  much 
more  rapidly  than  nonfragmental  sediments,  and  volcanic  products  far  more 
rapidly  than  ordinary  mechanical  sediments.  This  great  difference  of 
thickness,  then,  is  no  obstacle  to  correlating  the  Iron-bearing  member  to 
the  west  with  the  mass  of  mingled  fragmental  and  nonfragmental  sediments, 
and  the  interbedded  eruptives  and  greenstone-conglomerates.  The  fact 
that  at  every  cessation  of  volcanic  activity  nonfragmental  sediments 
were  deposited,  which  are  precisely  like  the  sediments  of  the  iron  forma- 
tion to  the  west,  seems  decisive  evidence  in  favor  of  this  correlation. 
Further,  the  greenstone-conglomerate  itself,  as  has  been  seen,  has  in  places 
phases  which  have  many  of  the  characteristics  of  the  iron-bearing  forma- 
tion, intei'leaved  with  other  phases  which  are  not  at  all  like  the  rocks  of 
that  member.     Finally,  the  greenstone-conglomerate  is  not  always  clearly 


THE  EASTERN  AREA.  433 

separated  from  tlip  t'cnuninous  i^raywiickcs  mikI  (|iiiirtzites  to  the  houIIi  jiihI 
north. 

Summari/. — The  airii  cast  (.('  tlie  center  of  'V.  47  N.,  R.  44  W.,  Miclii- 
"•an — or  roii,ylily,  cast  of  tlic  Little  Pre.s(|ue  Isle — diHers  from  tlie  simple 
scries  described  in  tlie  previous  chapters  in  man>-  important  points.  Tliis 
area  was  the  center  of  ^reat  volcanic  activity ;  conse(piently  the  sedimen- 
tary succession  includes  large  thicknesses  of  interstratified  lava  flows  and 
volcanic  tutfs,  which  are  not  paralleled  by  any  rocks  that  ai-e  found  in  the 
western  area..  Further,  this  volcanic  material  has  greatly  disturbed  the 
water-deposited  sediments  in  the  district,  so  that  it  is  difficult  to  certahdy 
correlate  the  formations  east  of  the  Little  Presque  Isle  with  those  west  of 
it.  Another  point  in  which  this  area  differs  from  the  western  one  is  that  in 
one  place  the  relations  of  the  horizontal  Eastern  sandstone  to  the  Penokee 
series  can  be  made  out. 

No  exposures  certainly  belonging  to  the  Cherty  limestone  member  are 
found  within  the  area.  The  Quartz-slate  member  is  not  known  to  be  con- 
tinuous in  this  area.  Typical  exposures  of  the  formation,  which  have  near 
the  base  variegated  slates,  and  for  the  upper  horizon  the  vitreous  quartzite, 
are  found  at  one  or  two  localities. 

The  lowest  belt  which  certainly  is  continuous  for  some  distance  is  the 
Iron-bearing  member.  Even  in  this  wide  gaps  occur  in  which  no  exposures 
are  known.  Its  typical  rocks  are  almost  exactly  like  those  of  the  iron  for- 
mation to  the  west.  There  is  here,  however,  closer  relations  between 
the  original  siderite  and  the  actinolite  and  magnetite  than  to  the  westward. 
In  this  respect  the  rocks  of  this  belt  resemble  very  closely  those  of  the 
Animikie  iron-bearing  formation.  The  one  important  point  in  which  this 
part  of  the  Iron-bearing  member  differs  from  that  to  the  westward  is  that 
mingled  Avith  it  is  fragmental  material.  There  are  all  gradations  between  the 
pure  nonfragmental  sediments  of  the  iron  belt  and  the  fragmental  material 
of  the  slate  formations ;  also  there  are  in  places  interlaminations  of  the  two. 
It  thus  is  clear  that  during  the  time  of  the  formation  of  the  Iron-bearing 
member  there  were  interruptions,  as  a  result  of  which  the  belts  of  pure  non- 
fragmental, water-deposited  sediments  are  always  narrow.  Probably  result- 
ing from  this  is  the  fact  that  within  this  area  no  workable  ore  deposits  have 

MON  XIX 28 


434  THE  PENOKEE  lEON  BEAEmG  SERIES. 

been  found,  the  belt  always  being  narrow,  so  that  the  amount  of  iron  in 
the  carbonate  present  was  insufficient  when  concentrated  to  form  large  ore 
bodies. 

The  greenstone-conglomerate  area  varies  from  a  material  which  is  a 
basic  eruptive  amygdaloidal  flow,  through  various  volcanic  or  semivolcanic 
elastics,  to  rocks  which  are-  wholly  water-deposited. 

North  and  east  of  the  greenstone-conglomerate  is  a  continuous  wide 
belt  of  fragmental  rocks,  which  extends  nearly  to  Gogebic  lake.  The  rocks 
south  of  this  belt  vary  in  their  character,  being  in  the  western  part  a  por- 
phyrite  of  such  a  nature  as  to  indicate  that  it  is  a  surface  flow;  through  the 
central  part  of  the  area,  greenstone-coilglomei'ate,  and  in  the  eastern  part  of 
the  area,  the  gneisses  and  granites  of  the  Southern  Complex.  North  of  the 
belt  are  the  eruptives  of  the  Keweenaw  series.  At  one  place,  just  west  of 
Gogebic  lake,  the  Eastern  sandstone  is  the  overlying  rock.  It  seems  prob- 
able that  this  belt  of  fragmentals  is  the  equivalent  of  the  northern  part  of 
the  Iron-bearing  member  with  perhaps  the  lower  part  of  the  Upper  slate 
member  to  the  westward.  If  it  extends  farther  east  than  Gogebic  lake,  it 
probably  passes  under  the  Eastern  sandstone. 

The  rocks  of  this  belt  are  very  largely  clay-slates  which  have  an 
apparent  southern  dip.  In  places,  however,  they  become  strongly  ferrugi- 
nous, when  a  northern  dip  is  usually  found.  The  latter  are  believed  to 
represent  true  bedding  and  the  southern  dips  to  be  secondary  cleavage 
induced  by  pressure.  The  rocks  have  a  Avide  variation  in  lithological 
character,  running  from  those  which  are  purely  fragmental  to  those  which 
are  purely  nonfragmental  sediments.  •  The  phases  intermediate  between  these 
two  extremes  are  those  most  abundant.  The  fragmentals  are  very  nmcli 
like  those  north  of  the  iron-bearing  belt  in  the  main  area.  The  iningled 
nonfragmental  materials  are  the  various  oxides  of  iron,  siderite,  and  chert. 
The  rocks,  then,  vary  from  those  which  lithologically  belong  to  the  Iron- 
bearing  member  to  those  which  are  typical  Of  the  Upper  slate.  The  iron 
varies  in  its  combination  from  an  oxide  in  the  western  part  of  the  area  to 
a  siderite  in  the  eastern  part.  This  is  another  case  in  which  the  widespread 
occurrence  of  iron  oxide  is  due  to  the  alteration  of  a  ferriferous  carbonate. 
The  basal  portion  at  several  places  in  the  eastern  part  of  the  belt  are 


THE  EASTERN  AREA.  435 

recoraposed  <;'riiiutic  rocks,  whicli  consist  of  liiinl\'  rt'cemented  granitic 
debris  and  arc  separable  tVoiii  tlic  orij^-iual  rocivs  only  lt\'  an  exctminatioii  of 
the  thin  sections. 

The  eruptives  of  the  Eastern  area  are  in  several  detached  exposures, 
some  of  tlieni  behiy  of  hirge  size.  A  jutrtion  of  them  are  porplnrites  con- 
temporaneous witli  the  rocks  of  the  series  in  which  they  are  fotuid;  otliers 
have  the  structure  of  a  diabase,  or  even  tliat  of  a  gabbro,  and  these  are 
taken  to  be  deep  seated  rocks.  Between  these  two  extremes  there  is  found 
in  various  parts  of  the  area  intermediate  i)hases.  The  augite  of  the 
diabases  has  often  altered  to  hornblende,  and  this  secondary  hornblende 
has  undergone  a  new  growth.  Also,  the  unaltered  augite  is  not  infre- 
quently surrounded  by  a  sheatli  of  smaragdite,  the  crystallographic  rela- 
tions of  the  original  and  secondary  minerals  being  those  known  to  occur 
between  augite  and  amphibole  paramorphic  after  it. 

The  rocks  of  the  various  belts  of  the  Eastern  area,  with  the  exception 
of  the  greenstone-conglomerate,  correspond  closely  in  lithological  characters 
with  those  of  the  Penokee  series  proper  to  the  westward,  the  only  differ- 
ence of  importance  being  that  in  the  Eastern  area  there  is  more  fre- 
quently found  mingled  nonfragmental  and  fragmental  sediments.  The 
greenstone-conglomerates  bear  such  structural  relations  to  the  sediments 
whicli  certainly  belong-  to  the  Penokee  series,  are  mingled  with  them  so 
intricately,  and  are  so  certainly  surface  volcanic  accumulations,  that  there 
is  no  siifificient  reason  for  placing  them  elsewhere. 

The  stratigraphical  evidence  that  the  Eastern  area  rocks  belong  to  a 
series  separated  both  from  the  Southern  Complex  and  from  the  Keweenaw 
series  is  precisely  like  that  which  proves  the  clastic  formations  to  the  west 
to  belong  to  a  distinct  series.  Notwithstanding  the  exceptional  width  of 
the  Eastern  area  in  certain  parts  and  the  southern  dips  which  are  found 
in  the  slate  belt  north  of  the  greenstone-conglomerates,  it  is  believed  that 
the  succession  here  is  probably  a  simple  conformable  one. 

The  sequence  of  events  in  the  district  seems  to  have  been  as  follows: 
As  in  the  main  area,  the  complex  of  schists,  gneisses,  and  granites,  and 
probably  the  series  to  which  the  Clierty  limestone  belongs  constitute  a  base- 
ment upon  whicli  the  Penokee  succession  was  deposited.  In  certain  places 
rocks  which  are  the  equivalent  of  the  Quartz-slate  member  were  deposited. 


436  THE  PElSrOKEE  IRON-BEARING  SERIES. 

Above  the  Quartz-slate  begins  tlie  iionclastic  sedimentation  of  the  Iron- 
bearing  member.  The  deijosition  of  this  belt  did  not  go  on  undisturbed, 
but  alternated  with  clastic  sediments;  and  long  before  the  whole  of  t!  : 
iron-bearing  member  of  the  west  was  built  up  came  the  great  volcanic 
outbreak  which  resulted  in  piling  up  the  series  of  flows,  tuffs  and  inter- 
stratified  elastics  of  the  greenstone-conglomerate  area.  At  or  near  the  ces- 
sation of  volcanic  activity  the  mingled  clastic  and  nonclastic  sediments 
which  overlie  them  began  to  form.  At  this  time  apparently  the  Iron-bearing 
member  of  the  western  area  had  not  yet  wholly  formed.  Above  the  latter 
followed  probably  fragmentals  which  were  the  equivalent  of  the  Upper 
slate  member;  but  if  this  were  the  case  they  were  wholly  or  nearly  wholly 
removed  by  erosion.  After  a  long  period  of  degradation  began  Keweenawan 
time.  Before  the  Eastern  sandstone  at  the  east  end  of  the  area  was  depos- 
ited the  three  series  of  the  district  were  tilted  into  their  present  inclined 
position,  and  during  this  time  and  subsequently  underwent  enormous 
denudation. 


CHAPTER    IX 


By   C.  E.  Van   Hise. 


GENERAL  GEOLOGY  OF  THE  DISTRICT. 

Section  I.     Flexures  aud  faults. 

Curviug  of  the  layers.     Fault  at  Bad  river.     Fault  at  Potato  river.     Fault  in  the  Eastern  area. 
Section  II.     Structure. 

The  Southeru Complex.  The  Cherty  limestone  aud  Quartz-slate  members.  Unconformity  between 
the  Southern  Comjilex  and  the  overlying  Cherty  limestone  and  Quartz-slate.  Unconformity 
between  the  Cherty  limestone  aud  the  Peuokee  series  proper.  The  Iron-bearing  aud  Upper  slate 
members.  The  unconformity  at  thebase  of  the  Keweenaw  series.  The  Eastern  sandstone  and 
the  unconformity  at  its  base.  R^sum**  of  geological  history.  Why  the  district  is  given  a  sepa- 
rate memoir.  Dejith  aud  metamorphism. 
Section  III.     Correlation. 

Equivalency  of  Penokee  series  proper  with  Animikie  series.     Equivalency  of  Penokee  and  Mar^ 
quette  series.     Comparison  with  other  series. 

SECTION  I.— FLEXURES  AND  FAULTS. 

Curving  of  the  layers. — The  cnrving'  of  the  layers  of  the  Penokee  series 
subordinate  to  its  uplifting  as  a  whole,  has  in  the  main  been  of  so  gentle  a 
nature  that  they  have  not  been  broken.  At  the  west  side  of  R.  3  W.,  Wis- 
consin, there  are  several  very  sharp  bends  in  the  Quartz-slate  and  Iron- 
bearing  members.  There  is  a  somewhat  sudden  turn  in  the  trend  of  the 
layers  near  the  middle  of  R.  2  W.,  Wisconsin.  West  of  Sunday  lake  the 
nearest  known  rock  in  the  iron  formation  is  about  one-fourth  of  a  mile 
south  of  the  north  quarter  post  of  Sec.  17,  T.  47  N.,  R.  45  W.,  Michigan. 
East  of  the  lake  the  southern  boundary  of  the  iron  formation  is  one-fourth 
mile  north  of  the  southwest  corner  of  Sec.  10  in  the  same  township.  (PL  xit.) 
Consequently  the  iron  belt  suddenly  swings  from  an  east  and  west  course 
north  more  than  a  half  mile  in  a  distance  of  1^  miles.     In  the  northwestern 

437 


438  THE  PENOKEE  IKON  BEARING  SERIES. 

part  of  Sec.  IG  is  an  exposure  of  green  schist  belonging  to  the  basement 
complex ;  so  that  most  of  the  northern  swing  occurs  before  this  point  is 
reached.  Tlie  relations  are  such  that  it  is  possible  to  explain  this  sudden 
change  in  direction  by  a  sharp  curve,  but  it  is  possible  tliat  it  is  wholly  or 
in  part  due  to  a  fault.  Sunday  lake  and  the  surroimding  low  ground  make 
it  not  easy  to  settle  this  question.  However,  at  none  of  the  places  men- 
tioned is  it  known  that  there  has  been  any  actual  faulting.  Thi'oughout 
the  whole  length  of  the  series,  which  has  been  subjected  to  two  great  oro- 
graphic movements,  no  dislocations  great  enough  to  make  the  belt  difficult 
to  follow  have  occurred ;  but  at  three  places  there  is  pretty  decisive  proof 
that  faulting  has  taken  place  to  some  extent. 

Fault  at  Bad  rwer.— The  most  considerable  fault  is  that  at  Bad  river, 
in  R.  3  W.,  Wisconsin.  The  relations  of  the  exposiires  and  the  line  of  fault 
have  been  accurately  mapped  and  described  by  Pi'of  Irving.  ^  PL  xxxvi  is 
wholly  from  data  furnislied  by  this  map.  The  explanation  is  also  largely 
taken  from  Prof  Irving,  it  only  being  modified  to  correspond  with  the 
present  understanding  of  the  succession  of  belts.  West  of  the  line  marked 
"Supposed  position  of  fault  line  "  the  layers  of  the  Penokee  series  follow  in 
the  regular  order.  Near  the  base  of  the  left  hand  side  of  the  plate  are 
]napped  large  exposures  of  granite  and  gneiss.  North  of  these  on  Bad 
river,  and  very  close  to  the  gneiss  are  exposures  of  the  Cherty  lime- 
stone member.  Next  follows  the  Quartz-slate,  which  is  exposed  through- 
out its  whole  thickness,  having,  as  usual,  as  its  upper  layer  a  vitreous 
quartzite.  Just  above  this  quartzite  follow  the  exposures,  which  rise  in  high 
cliffs  above  the  river,  belonging  to  the  Iron-bearing  member.  They  are  so 
numerous  and  large  that  they  practically  show  the  whole  thickness  of  the 
formation.  Upon  the  north  side  of  this  bluff  is  a  thin  layer  of  garnetiferous 
magnetitic  slate,  which  passes  into  a  garnetiferous  black  slate,  plainly  con- 
stituting the  base  of  the  Upper  slate  member.  North  of  this  point  are 
a  number  of  large  exposures  belonging  to  this  belt.  On  both  banks  of  Bad 
river  is  shown  a  lai'ge  exposure  of  diabase.  East  of  the  "Supposed  line  of 
fault "  the  succession  of  belts  in  the  Penokee  series  is  found  to  be  identical 
with  that  just  given,  the  only  difference  being  that  but  a  small  part  of  the 

'Geol.  of  Wis.  vol.  iii,  pp.  150-152,  with  aoeoinpanyiiig  atlas,  PI.  xxiil. 


(IKXF.lJAl.  C.KOUKlY  OK  Till';   DISTUIOf.  43V> 

Quartz-slate  is  exposed  ;  linwever,  tlie  cliai-actefistic  (juartzite  at  its  upper 
liorizoii  is  seen.  Tlie  ii-on  I'oniiation  shows  a  pi-actically  coiitiiiuous  expos- 
ure and  at  its  uppermost  iioi-izcn  is  found  the  peculiar  o,arnetit'erous  slate, 
wiru-h  ])asses  into  the  i;arnetit'erous  hlack  slate  at  tlu^  hasi^  oC  the  Upper 
slate  member  just  as  on  the  west  side  of  the  fault.  The  occurrence  of 
these  characteristic  layers  both  north  and  south  of  the  iron  formation — east 
and  west  of  the  supposed  fault  line — is  most  foi-tunate,  enabling  one 
to  determine  the  exact  limits  of  that  formation  and  thus  to  give  data 
for  a  satisfactory  explanation  of  the  distribution.  This  order  is  in  such  per- 
fect accordance  upon  both  sides  of  this  line  that  there  could  be  little  doubt 
that  the  belts  represented  a  once  continuous  set  of  layers,  eveir  if  we  knew 
nothing  about  the  series  farther  to  the  east  and  west.  But  when  it  is  remem- 
bered that  these  several  belts  stretch  in  the  same  order  for  many  miles  in 
both  these  directions,  this  former  continuity  may  be  considered  demon- 
strated. The  distribution  of  exposures  at  Penokee  Gap  can  then  only  be 
explained  as  having  been  caused  by  a  fault  between  the  two  sets  of  expos- 
ures, for  there  is  not  room  between  them  for  accordance  to  be  produced  by 
a. bend  of  the  layers,  however  sharp.  The  space  between  the  quartz-slates 
on  the  west  side  of  the  fault  and  the  upper  part  of  the  iron  formation  on 
the  east  side  is  not  one-half  the  thickness  of  the  latter,  yet  between  these 
two  the  whole  of  it  must  pass  if  there  is  not  here  a  fault.  The  Quartz-slate 
on  the  west  is  seen  to  l)e  farther  north  than  the  northernmost  exposure  of 
the  magnetite  schists,  toward  which  it  strikes,  whereas  it  ought  to  be  con- 
tinuous with  the  belt  of  the  same  kind  on  the  east  side  of  the  line  marked 
"supposed  line  of  fault."  The  amount  of  discordance  is  even  more  strik- 
ingly shown  by  the  positions  of  the  exposures  of  the  iron  formation.  It  inay 
be  that  a  part  of  the  discordance  can  be  accounted  for  by  a  sudden  bowing 
of  the  layers,  although  the  strike  of  the  rock  gives  little  indication  that  this 
is  the  case.  If  the  entire  dislocation  is  taken  to  be  due  to  a  fault,  the  throw 
must  have  been  at  least  900  feet  if  at  right  angles  to  the  direction  of  the 
strike  of  the  rocks;  and  if  it  is  diagotial,  as  is  probably  the  case,  must  have 
been  more  than  this.  That  this  fault  extends  south  of  the  southernmost 
exposure  of  the  Penokee  series  and  north  of  the  exposures  of  the  basement 
layer  of  the   black  slate  is  plain,  but  the  layers  of  the  upper  slate  are  so 


440  THE  PENOKEE  IRON-BEARIK^G  SERIES. 

closely  like  one  another,  and  so  little  is  known  of  the  relations  of  the  rocks  of 
the  Southern  Complex  to  one  another,  that  its  direction  can  not,  beyond  these 
limits,  be  made  out.  If  the  greenstone  upon  Bad  river  is  taken  to  be  an 
interbedded  flow,  it  appears  that  the  fault  line  jjasses  between  this  and  the 
black  slates  east  of  this  place  on  the  railroad;  but  if  it  is  an  eruptive  dike 
cutting  across  the  formation,  as  is  probable,  it  gives  no  indication  of  the 
location  of  the  fault  line. 

Fault  at  Potato  river. — At  Potato  river,  near  the  east  side  of  Sec.  19,  T. 
45  N.,  R.  1  E.,  Wisconsin,  there  is  again  strong  evidence  that  a  fault  exists, 
althousrh  here  the  throw  is  not  so  great  as  at  Bad  river.  The  relations  of 
the  expostires  are  exhibited  by  Fig.  6.  Upon  the  east  side  of  the  river  is  a 
large  exposure  of  green  schist  belonging  to  the  Basement  Complex.  The 
Quartz-slate  is  in  contact  with  the  schist  and  extends  northward  in  contin- 
uous exposure,  with  only  a  very  slight  break  throughout  its  whole  thick- 
ness. North  of  and  immediately  adjacent  to  the  vitreous  quartzite,  con- 
stituting the  uppermost  member  of  this  slate,  are  exposures  of  the  Iron- 
bearing  member.  The  strike  of  the  slate  east  of  the  river  is  west  20°  south. 
On  the  west  side  of  the  I'iver  large  exposures  of  the  underlying  green  schist 
are  noted.  North  of  these  follow  smaller  exposures  of  quartz-slate,  and  still 
farther  north  the  vitreous  quartzite,  and  above  this  the  rocks  of  the  iron- 
bearing  belt;  so  that  east  and  west  of  the  river  we  have  the  exact  location 
of  the  junction  between  the  quai-tzite  and  iron-bearing  belt,  Avliile  east  of 
the  river  is  known  the  exact  contact  of  the  quartz-slates  and  the  under- 
lying schists.  Now,  it  is  manifest  that  the  large  exposures  of  green  schists 
west  of  the  river  lie  directly  athwart  the  course  of  the  quartz-slates.  The 
same  is  true  of  the  quartz-slate  and  quartzite  on  the  west  side  of  the  river 
as  compared  with  the  rocks  of  the  iron  belt  on  the  east  side.  Here,  again 
it  may  be  suggested  that  a  sharp  bowing  will  explain  the  structure,-  and 
hence  a  fault  is  not  necessary  to  explain  the  facts;  but  between  the  slates 
and  the  nearest  schistose  rock  there  are  but  200  feet  of  space,  in  which  a 
much  greater  thickness  of  slates  must  pass.  Further,  there  is  absolutely  no 
indication  by  a  change  of  direction  of  the  strike  of  the  slates  that  an}'  such 
bowing  has  here  occurred.  If  the  whole  of  the  discordance  here  is  taken  to  be 
due  to  the  fault,  the  horizontal  throw  amounts  to  280  feet,  as  determined  by 


GENKIJAL  (ii:«)LUi;v  OF  Tllh:  J)18Ti:iCT.  441 

a  transit  survcv  made  1)\-  Mr.  J.  Parke  CliaiiiiiiiL;',  takliin'  as  tlu'  points  of 
ineasun'iiu'iil  tlic  jiiiictioiis  between  tlic  vitreous  (juartzitcs  and  the  over- 
lying {'erruginous  rt»cks  on  tlie  east  and  west  sides  oftlie  rivci-. 

Fault  ill  the  Juisfmi  urea. — In  elia])ter  viii,  p.  424,  it  has  been  suggested 
that  there  is  a  fault  in  tiie  east  side  of  T.  47  N.,  R.  44  W.,  Michigan,  run- 
ning in  a  diagonal  direction  from  the  southwest  corner  of  Sec.  26  to  a  short 
distance  west  of  the  east  quarter  post  of  Sec.  13.  The  rocks  west  of  this 
line  have  probably  been  thrown  south  of  the  corresponding  layers  to  the 
east,  thus  explaining  part  of  the  discordance  in  the  rocks  of  the  iron 
formation,  as  Avell  as  the  difference  of  one-half  mile  in  the  southern  bound- 
ary of  the  Keweenaw  rocks  in  the  eastern  part  of  the  township,  and  also 
lessening  the  thickness  of  the  Penokee  rocks  in  this  locality,  but  as  the 
probability  of  this  fault  and  its  structural  relations  have  been  fully  treated 
in  the  chapter  referred  to,  the  subject  will  not  be  further  discussed  here. 

At  all  of  these  places  the  ground  between  the  exposures  is  covered  on 
the  east  and  west  sides  of  the  fault  lines,  so  that  it  is  not  possible  to  accu- 
rately locate  them.  As  is  common,  the  broken  layers  have  readily  yielded 
to  erosion  and  the  fault  lines  in  two  cases  have  been  utilized  by  the  drain- 
age system  of  the  district. 

SECTION  II.— STRUCTURE.' 

The  Southern  Complex. — The  subject  proper  of  this  memoir  is  the  con- 
formable succession  of  rocks  which  contains  the  Iron-bearing  member, 
and  has  therefore  often  been  called  the  iron-bearing  series.  It  has  often 
been  necessary-  to  allude  to  the  series  south  and  north.  The  latter  series 
has  not  been  given  a  separate  chapter,  for  the  varied  rocks  there  found 
are  parts  of  the  great  Keweenaw  series  which  has  already  been  treated  in 
a  separate  memoir.^  In  chapter  ii  the  former  rocks  have  been  described 
in  sufficient  detail  to  compare  them  in  their  structural  features  and  litho- 

'  The  separability  of  tlie  Penokee  series  from  the  Southern  Complex  and  the  Keweenaw  series 
by  means  of  vmconformities  has  been  maintained  by  Prof.  Irving  in  several  of  his  publications.  (See 
Literature,  Chap,  i.)  As  to  these  general  relations,  the  only  thing  added  is  new  evidence  supporting 
these  positions. 

'^Copper-bearing  rocks  of  Lake  Superior;  by  R.  D.  Irving,  U.  S.  Geol.  Survey,  Monograph  v. 


442  THE  PBNOKEE  lEON-BEARIlSG  SERIES. 

logical  cliaraeters  with  the  various  rock  belts  of  the  Penokee  succession. 
It  has  been  seen  that  these  rocks  are  of  two  general  types:  first,  massive 
granites  and  gneissoid  granites,  and,  second,  fine  grained  schists,  which 
technically  are  gneisses.  None  of  them  can  properly  be  said  to  have  a  sedi- 
mentary strike  and  dip.  All  but  the  granites  have  a  foliation.  In  the  case 
of  the  gneissoid  granites  this  foliation  is  of  the  coarsest  character,  but  the 
foliation  of  the  rocks  varies  from  this  to  that  so  fine  that  it  is  only  possible 
to  detect  it  in  the  hand  specimen  by  the  direction  of  readiest  cleavage.  In 
these  latter  cases,  however,  in  thin  section  the  schistose  structure  is  just  as 
distinct  as  in  the  coarser  phases  in  which  it  is  so  easily  recognizable. 
While,  then,  these  rocks  have  no  stiike  and  dip  in  a  proper  sense,  they 
have  a  foliation  to  which  the  terms  may  be  applied.  It  has  been  said  that 
more  often  than  not  this  foliation  is  approximately  east  and  west.  How- 
ever, it  has  wide  variations  within  narrow  ranges  (as  will  be  seen  by  look- 
ing at  the  detailed  maps.  Pis.  v  to  xin),  and  at  times  the  foliation  is  almost 
directly  perpendicular  to  the  strike  of  the  rocks  of  the  Penokee  series.  If 
there  are  abrupt  and  sudden  changes  in  the  direction  of  strike,  the  varia- 
tion in  dip  is  still  more  marked,  frequently  within  a  few  rods  varying  from 
a  dip  in  one  direction  to  that  in  the  reverse.  Further,  while  on  the  maps  there 
are  sharp  lines  of  separation  drawn  between  the  fine-grained  gneisses  and 
the  granites  and  granitoid  g'ueisses,  in  the  field  there  is  at  times  an  apparent 
transition  rather  than  an  abrupt  change  (pp.  123-125).  More  often,  along 
the  border  of  the  schists  are  granitic  intrusions,  and  these  become  more  and 
more  numerous  in  passing  toward  the  granite,  until  this  rock  becomes  pre- 
dominant. The  great  variation  in  the  direction  of  foliation  in  the  gneisses 
is  readily  understood  when  some  of  the  larger  exposures  are  examined, 
their  banding  being  extremely  contorted  and  consequently  varying  greatly 
in  dip  within  a  single  exposure.  From  the  foregoing  it  will  be  seen  that  if 
the  rocks  included  within  this  belt  have  a  distinct  succession  it  is  an 
extremely  complicated  one,  and  that  at  present  we  have  no  data  which  will 
enable  us  to  reach  even  an  approximate  notion  of  it.  The  only  structural 
fact  that  can  be  certainly  stated  is  that  some  of  the  massive  granites  and 
gneissoid  granites  are  intrusives  of  later  age  than  the  green  schists. 

When  the  rocks  of  the  Southern  Complex  are  examined  in  thin  section 


( !  !•:  N  !■;  It  A 1 .  ( i  !':< )  I .( x ;  v  ( » !•'  twd  i  )  i  st  ii  i  <  ;t.  443 

they  are  seen  to  be  of  completely  crystiilliiic  kinds.  Sucli  massive  rocks  as 
the  granites  and  syenites  are  now  regarded  as  ernpti\'e.  'I'lie  origin  of 
many  of  the  thinly  foliated  schists  and  gneisses  is  not  known.  It  is  enongh 
here  to  say  that,  as  indicated  (pp.  12f)-\2G),  nowhere  in  tliis  area  is  there 
any  suflicient  evidence  to  show  that  any  of  them  are  of  a  fragmental  char- 
acter. Certain  of  the  gneisses  of  tliis  area  have  such  relations  with  the 
*massive  syenites  and  granite^  that  they  may  be  considred  as  of  eruptive 
origin;  so  that  all  that  can  be  said  as  to  the  original  condition  of  these 
rocks  is  that  a  large  proportion  of  them  are  eruptive  while  the  origin 
of  others  is  unknown.  The  rocks  to  the  south  of  the  PenokQe  series  are, 
then,  a  set  of  massive  rocks  and  crystalline  schists  having  a  peculiar  com- 
plex contact  and  neither  certainly  possessing  a  sedimentary  structure  of 
any  kind.  This  Southern  Complex,  as  a  whole,  may  be  taken  as  the  base- 
ment group  of  the  district. 

,  The  Chertjj  limestone  and  Quarts-slate  members. — To  the  north  of  this 
complex  are  the  belts  of  the  Penokee  series  and  the  Cherty  limestone.  In 
chapters  in  to  viii  these  rocks  have  been  described  in  detail.  Here  it  will 
be  necessary  to  recapitulate  such  of  the  facts  as  bear  upon  their  relation  to 
another  and  their  relation  to  the  rocks  to  the  south  and  north  of  them. 
The  lowest  formation  is  a  cherty  limestone.  This  rock  is  found  in  large 
exposures  at  various  places  from  near  Atkins  lake,  T.  44  N.,  R.  5  W.,  Wis- 
consin, to  T.  47  N.,  R.  44  W.,  Michigan.  In  its  greatest  development  it  is 
about  300  feet  thick.  It  appears  to  be  a  tolerably  continuous  formation 
throughout  its  western  portion,  bur  in  the  eastern  half  of  the  district  it  is 
found  only  here  and  there.  Next  in  order  to  the  north  is  the  Quartz-slate 
member.  This  formation  is  a  fragmental  one  in  which  the  particles  are 
chiefly  quartz  and  feldspar.  In  the  vicinity  of  the  cherty  limestones  it  fre- 
quently carries  a  considerable  quantity  of  material  from  this  member,  even 
becoming  at  times  a  conglomerate.  This  cherty  limestone  and  the  slate  belt 
are,  however,  in  apparent  conformity  with  each  other,  so  that  while  there 
certainly  was  an  erosion  interval  between  the  two  there  was  found  no  evi- 
dence that  the  Cherty  limestone  was  closely  folded.  The  case  is  analogous 
to  that  of  the  relations  of  the  Penokee  and  Keweenaw  series.  It  will  be 
shown  that  such  an  erosion  interval  implies  a  considerable  physical  break. 


444  THE  PENOKEE  IRON-BEARING  SERIES. 

The  narrowness  as  well  as  the  permanence  of  the  Quartz-slate  mem- 
ber is  strongly  brought  out  by  PI.  ii.  Here  is  a  belt  of  rocks  which  in 
surface  width  is  upon  an  average  not  more  than  400  to  450  feet,  and  the 
thickness  of  which  is  riot  more  than  350  to  400  feet.  Its  maximum  thick- 
ness east  of  Sunday  lake  is  not  more  than  800  feet.  In  chapter  iv  the  re- 
markable essential  likeness  of  the  various  parts  of  this  belt  is  indicated. 
Its  southern  part  is  a  layer  of  green,  brown,  gray  and  red  quartz- slates.* 
T!ts  uppermost  layer  is  a  vitreous  quartzite.  In  whatever  part  of  the  range 
a  section  is  made  across  the  belt,  its  essentially  fragmental  character  is  at 
once  discoverable,  the  rocks  being  comparatively  little  changed  since  they 
were  originally  deposited.  Tlie  induration  of  both  the  quartzite  and  the 
slates  has  been  explained  to  be  due  to  the  enlargement  of  quartz-grains 
and  to  metasomatic  changes  of  the  feldspar. 

Unconformity  between  the  Southern  Complex  and  ilie  overlying  Clierty 
limestone  and  Quartz-slate. — There  is,  then,  between  the  Quartz-slate  and 
the  rocks  to  the  southward  the  great  fundamental  difference  between  rocks 
which  are  easily  provable  to  be  of  clastic  origin  and  those  which  are  com- 
pletely crystalline  schists  and  massive  eruptives.  There  can  be  nowhere  a 
wider  lithological  difference  in  the  characters  of  two  sets  of  rocks  tlian  this. 
But  more  remarkable  than  the  lithological  simplicity  of  the  Quartz-slate  are 
its  straightforward  field  relations.  This  slate  once  seen  is  easily  recognized. 
It  always  dips  northward,  varying  within  narrow  limits.  Take  the  strike 
of  any  ledge  and  follow  the  direction  either  to  the  east  or  to  the  west  with 
a  little  latitude  of  movement  and  almost  invariably  another  ledge  of  like 
rock  is  found  within  a  short  distance  if  exposures  are  at  all  plentiful.  The 
only  part  of  the  area  in  Avhich  natural  exposures  within  the  belt  were  not 
found  has  been  penetrated  and  exposed  by  mining  operations,  so  that  its 
continuity  can  be  with  certainty  asserted.  It  is  true  it  bows  locally  from  its 
general  course,  and  in  two  or  three  places  it  is  somewhat  faulted,  but  the 
variation  is  never  sufficient  to  lose  the  belt.  As  one  traverses  this  range 
north  and  south  at  short  intervals,  the  complexity  and  variety  of  the  litholog- 
ical characters  and  of  the  strikes  and  dips  of  the  rocks  of  the  Southern  Com- 
plex and  the  likeness,  simplicity,  directness,  and  uniformity  of  this  slate 
belt  now  just  north  of  the  granites  and  now  in  contact  with  the  crystalline 


GENERAL  C.EOUMiY  OF  THE   DISTKICT.  445 

schists  are  most  strikiiij;-.  Tliese  facts  iinj)ress  oim  so  stroiifi'ly  tliat  it  is  safe 
to  say  that  no  on*'  can  pass  thnmyli  this  ('X|KM'i('ii('u  without  having-  the 
conviction  foircil  upon  liiin  that  thciv  is  a  great  structural  break  between 
the  quartz-shites  and  the  coniplex  series  to  the  soutli.  Such  a  general  rela- 
tion, it  seems  to  me,  is  more  significant  than  the  direct  abutment  of  the 
schistose  structure  of  one  rock  against  another;  more  signiticant  than 
actual  structural  breaks  seen  by  the  eye  in  any  single  exposure;  more  sig'- 
nihcant  than  an\'  basal  conglomerate;  yet,  as  will  be  seen,  all  of  these 
subordinate  proofs  of  unconformity  have  been  found  at  various  places  in 
this  district. 

The  above  paragraph  contrasting  the  lithological  charactei's  and  strati- 
graphical  relations  of  the  Southern  Complex  and  the  Quartz-slate  is  applica- 
ble with  almost  equal  force  to  the  relations  of  the  Southern  Com])lex  and 
Cherty  limestone.  The  only  modification  that  need  be  made  is  that  the 
Cherty  limestone  is  not  so  continuous  as  the  Quartz-slate,  so  that  one  is  not 
so  strongly  impressed  with  the  absolute  necessity  for  a  discordance  between 
the  two  in  order  to  adequately  explain  their  field  relations. 

The  relations  of  the  eruptive  massive  syenites  and  granites  to  the 
adjacent  schists  and  to  the  Cherty  limestone  and  Quartz-slate  are  such  as  to 
show  that  the  schists  and  massive  rocks  are  vastly  older.  That  the  granites 
should  intricately  intersect  the  schists  and  yet  for  many  miles  be  immedi- 
ately adjacent  to  the  Cherty  limestoue  and  Quartz-slate,  yet  never  cut 
them,  is  inconceivable  on  any  other  hypothesis  than  that  they  were  in  this 
position  before  the  deposition  of  that  belt  of  rocks.  It  is,  further,  all  but 
universally  believed  that  the  coarse  grained  granites  and  syenites  are  a 
class  of  rocks  which  have  crystallized  under  great  pressure  and  therefore 
at  depth.  If  this  be  true  they  must  have  been  subjected  to  great  erosion  in 
order  that  the  Cherty  limestone  and  Quartz-slate  could  have  been  deposited 
directly  upon  them,  and  they  are  therefore  immensely  older  than  the  latter 
rocks.  The  independent  relations,  also,  of  the  green  schists  to  the  quartz- 
slates  are  such  as  to  show  that  between  them  is  a  great  interval  of  time.  It 
has  been  said  that  the  fibers  of  the  schist  run  in  every  possible  direction  and 
that  in  certain  cases  they  abut  dii-ectly  against  the  slates.  These  schists 
may  be  eruptive  or  fragmeutal,  but  in  either  case  their  schistose  structure 


446  THE  PENOKEE  lEON-BEARING  SERIES. 

was  induced  before  the  deposition  of  the  slates  upon  them,  for  it  is  impossible 
that  rocks  could  be  so  profoundly  altered  and  yet  the  slates  of  much  the 
same  mineral  composition  in  contact  with  them  be  unaffected.  If  they  are 
eruptives  the  time  required  for  their  change  from  fresh  rocks  to  extremely 
foliated  altered  ones  must  have  been  very  great.  If  they  are  fragmental 
their  metamorphism  to  completely  crystalline  rocks  must  have  taken  as 
great  a  time.  So  that  on  either  hypothesis  they  are  immensely  older  than 
the  Quartz-slate  which  overlies  them,  for  the  rocks  of  this  formation  have 
neither  a  schistose  structure  nor  a  crystalline  character. 

West  of  Potato  river,  T.  45  N.,  R.  1  E.,  Wisconsin,  no  actual  contacts 
between  the  Southern -Complex  and  the  Penokee  rocks  are  known.  At 
Penokee  gap  the  green  schists  are  found  very  close  to  the  overlying 
Cherty  limestone.  The  latter,  as  usual,  dips  to  the  north  and  strikes 
approximately  east  and  west.  The  green  schists  to  the  south  also  have  a 
nearly  east  and  west  strike,  but  their  dip  is  southward.  This  being  the 
case,  the  fibers  of  the  gneisses,  unless  there  is  here  an  exceedingly  sharp 
fold  of  which  there  is  no  evidence,  abut  sharply  against  the  overlying  beds, 
and  thus  there  is  here  strong,  although  not  conclusive  evidence,  of  xmcon- 
formity. 

At  Potato  river,  fortunately,  the  underlying  schists  and  the  overlying 
Quartz-slate  are  found  in  direct  contact  with  each  other.  Here  the  Quartz- 
slate,  rising  in  low  cliffs,  is  exjjosed  throughout  its  entire  thickness  upon 
the  east  side  of  the  river  (Fig.  5).  For  some  distance  the  green  schists 
to  the  south  are  also  exposed,  and  the  actual  contact  between  the  two 
is  seen  on  the  bank  at  intervals  for  a  vei'tical  distance  of  75  feet,  while 
the  contact  is  continuous  for  the  lower  25  or  30  feet.  The  slates  are  all  of 
the  normal  character,  dipping  to  the  northward  at  an  angle  of  about  70°. 
The  green  schist  has  no  proper  dip  and  strike,  and  yet  it  has  a  strongly 
schistos.e  character.  Its  fibers  abut  almost  perpendicularly  against  the 
layers  of  the  slate,  as  shown  by  Fig.  6,  reproduced  from  a  carefully  made 
drawing  on  the  ground.  So  sharp  is  the  junction  line  between  the  two 
rocks  that  it  can  usually  be  located  to  the  fraction  of  an  inch,  and  hand 
specimens  were  obtained  in  part  from  both  formations.  The  surface  of  the 
green  schist  at  the  beginning  of  the  formation  of  the  conglomerate  is  seen 


GENERAL  CEOLOGY  OF  THE  inSTTJKT.  447 

to  have  been  somewhat  irreg-ular  (Fig.  7),  and  bcttwucii  ii  ;iiiil  tlie  ordinary 
slates  is  a  layer  of  basal  coug'lomerate  varying-  from  m  few  iiiclies  to  several 
feet  in  thickness.  It,  however,  quickly  grades  into  tlie  ordinary  slate  of  the 
district,  as  though  this  place  were  at  the  bottom  of  a  comparatwely  level 
shallow  sea,  rather  than  adjacent  to  a  shore.  '^Diis  conglomerate  contains  a 
few  white  quartz  and  jasper  pebbles,  the  former  sometimes  being  ten  inches 
in  diameter ;  but  the  great  mass  of  the  fragments,  and  especially  all  of  the 
large  bowlders  (which  reach  occasionally  five  feet  in  greatest  length),  are 
from  the  underlying  green  schist.  Moreover,  the  conglomerate  proves  that 
the  schistose  character  of  the  underlying  rock  had  been  fully  attained 
before  the  deposition  of  the  slates ;  for  the  schistose  structure  of  the  frag- 
ments is  as  well  developed  as  in  the  ledge  below.  A  fragment  broken 
from  such  a  schist  is  naturally  longer  in  the  direction  of  its  fibers  than 
transverse  to  them.  That  the  material  does  break  in  this  manner  was 
proved  in  obtaining  specimens.  The  green  schist  fragments  of  the  con- 
glomerate ai-e  generall}'  longest  in  the  direction  of  schistosity.  Now,  frag- 
ments of  this  sort,  when  broken  from  the  basement  rock  and  laid  down  as 
a  part  of  the  conglomerate,  would  naturally  lie  with  their  longer  direction 
parallel  to  the  horizon;  and  this  has  been  the  case,  for,  as  shown  in  Fig  7, 
nearly  all  of  the  larger  fragments  lie  with  their  greatest  length  parallel  to 
the  then  sea  floor,  that  is,  the  schistose  structure  of  the  fragments  is  at  right 
angles  to  that  of  the  ledge  from  which  they  were  derived.  Now,  if  this  schistose 
structure  had  been  developed  subsequently  to  the  formation  of  the  conglom- 
erate, it  would  have  been  parallel  in  both  the  fragments  and  basement 
rocks.  So  far  as  a  basal  conglomerate  and  unconformity  in  structure  at  a 
single  place  can  indicate  a  time  gap  between  two  formations,  this  locality 
indicates  it.  If  the  underlying  rock  was  a  sedimentary  one  it  must  have 
been  folded  and  extensively  eroded  to  change  it  to  a  crystalline  schist  and 
bring  its  foliation  in  a  vertical  position  at  surface.  If  it  was  an  eruptive 
rock  it  has  been  most  profoundly  altered,  and  its  alteration,  as  just  shown, 
must  have  taken  place  before  the  formation  of  the  conglomerate.  Such  an 
alteration  would  hardly  take  less  time  than  its  transformation  from  a  clastic. 
So  in  whatever  .light  it  is  regarded  this  basement  rock  is  vastly  older  than 
the  conglometate  and  slates  which  rest  upon  it, 


448  THE  TENOKEE  IRON-BEAEING  SERIES. 

At  the  west  branch  of  the  Moiiti'eal,  T.  46  N.,  R.  2  E.,  Wisconsin, 
green  schists  are  again  found  in  contact  with  the  rocks  of  the  quartz-slates. 
The  contact^  hei-e  exposed  is  of  much  smaller  size  than  at  Potato  river  and 
to  that  extent  is  less  satisfactory,  but  even  here  all  the  essential  facts  are 
the  same.  A  basal  conglomerate  is  found,  the  fragments  of  which  are  very 
largely  from  the  green  schist ;  this  conglomerate  quickly  varies  into  the 
normal  slate  of  the  regior ,  as  at  Potato  river ;  the  fibers  of  the  green  schist 
abut  against  the  conglomeratic  slates. 

Just  south  of  the  Aurora  mine  the  Quartz-slate  is  exposed  in  direct  con- 
tact with  the  granite.  The  basal  horizon  is  here  a  cherty  slate  which  does 
not  take  on  a  conglomeratic  aspect.  The  granite  surface  is  plainly  one 
of  erosion  and  into  its  pre-Penokee  joints  and  cracks  the  detritus  of  the 
Quartz-slate  has  sifted. 

A  short  distance  north  and  west  of  the  east  quarter  post  of  Sec.  15,  T. 
47  N.,  R.  46  W.,  Michigan,  on  the  railroad  spur  of  the  Palms  mine  is  another 
contact  between  the  Southern  Complex  and  the  Penokee  rocks.  Here  the 
junction  is  between  granite  and  a  conglomerate  belonging  to  the  Quartz- 
slate  member.  A  ledge  of  massive  granite  is  faced  by  a  conglomerate, 
which  penetrates  the  clefts  and  hollows  of  the  granite.  In  the  granite 
fresh  microcline  and  chlorite  are  abundantly  contained.  The  fragments 
of  the  conglomerate  are  mainly  granite,  but  chert,  jasper  and  quartz 
are  also  found.  These  fragments  are  in  part  well  rounded,  but  mostly 
are  angular,  as  is  common  when  near  their  source.  Much  of  the  feldspar 
of  the  fragments  is  beautiful,  fresh  microcline.  Green  chlorite  in  well 
defined  masses  is  also  plentiful.  It  is  certain  that  this  debris  was  derived 
from  the  granite.  The  coarse  grained  granite  at  this  place  plainly  consti- 
tuted tlie  surface  rock  when  the  conglomerate  was  formed  and  contributed 
material  to  it.  North  and  east  of  the  conglomerate,  after  a  short  covered 
space,  are  found  the  ordinary  phases  of  slates  of  the  Quartz-slate  member, 
dipping  north  at  their  normal  angle.  The  time  gap  here  between  the 
conglomerate  and  granite  must  be  taken  to  be  great  if  it  is  conceded  that 
the  presence  of  normal  granite  at  the  surface  is  proof  that  it  is  an  old  rock. 

About  700  steps  south  and  75  steps  east  of  the  northwest  corner  of 

iTWs  contact  -was  first  described  by  Dr.  T,  C.  Chamberlin.     See  Literature,  pp.  43,  44. 


GENi:i{AI-  (iKOLOCY  OF  THF,   DISTKHT.  449 

Sec.  13,  T.  47  X.,  U.  4(1  W.,  Mic^liigau — i.  e.,  jilxnit  '1  miles  cast  of  tlie  cdii- 
g'lotiieratc  just  <K'S(iil)cil — is  a  somewhat  similar  occurrence.  Here  a  sedi- 
mentary rock  in  a  layer  from  two  to  three  feet  in  thickness  is  found  upon 
the  northern  face  of  a  cliflP  of  granite.  The  specimens  were  unfortunately 
destroyed  by  tire,  so  that  thin  sections  have  not  been  studied,  but  the  field 
relations  are  the  same  as  near  the  Palms  mine. 

Passing-  eastward,  the  next  contact  between  the  Southern  Complex  and 
the  Penokee  series  is  east  of  Sunday  lake.  In  a  test  trench,  about  200 
steps  east  and  a  short  distance  south  of  the  north  quarter  post  of  Sec.  15,  T. 
47  N.,  R.  45  W.,  Michigan,  is  exposed  the  contact  between  the  lower  green 
schist  and  a  quartzite^  the  lowest  horizon  of  the  quartz-slate  at  this  point. 
The  schists  are  finely  foliated  and  contain  white  quartz  veins.  The  strike 
of  the  fibers  of  the  schist  does  not  correspond  to  that  of  the  overlying  suc- 
cession, but  abut  against  the  quartzite  at  a  rather  sharp  angle.  Here  the 
exposure  is  small  and  artificial,  and  the  conditions  do  not  appear  to  have 
been  favorable  to  the  formation  of  a  basal  conglomerate. 

In  the  NE.  \  of  Sec.  23,  T.  47  N.,  R  43  W.,  Michigan,  the  rocks  of  the 
Southern  Complex  approach  very  close  to  those  of  the  Penokee  series,  and 
a  beautiful  basal  conglomerate  is  exposed  in  the  latter.  The  underlying 
rock  is  here  a  coarse  gneissoid  granite.  It  is  separated  only  a  few  paces  from 
large  exposures  of  a  recomposed  granitic  rock.  This  recomposed  phase  of 
basal  conglomerate,  in  the  field  and  hand  specimen,  so  closely  resembles 
the  crystalline  rock  of  which  it  is  the  cemented  debris  that  the  two  might 
readily  be  confused;  but  Avhen  examined  in  thin  section  they  are  readily 
distinguished,  the  fragmental  character  of  the  one  and  the  thoroughly  crys- 
talline character  of  the  other  being  apparent.  This  recomposed  rock  varies 
into  a  conglomerate  containing,  besides  the  numerous  granite  fragments, 
pebbles  of  white  quartz  and  a  green  schist,  and  then  upward  into  the  oixli- 
nary  slate  and  quartzite  of  the  district.  That  the  material  of  this  frag- 
mental rock  is  here  derived  from  the  underlying  gneiss  and  gneissoid 
granite  there  can  be  absolutely  no  doubt,  the  major  portion  of  the  fragments 
being  precisely  like  these  rocks. 

A  short  distance  west  of  this  point  a  thin  layer  of  ferruginous  lime- 
stone is  found  but  five  steps  north  of  a  coarse  gneiss.     The  fact  that  the 

MON  XIX 29 


450 


THE  PENOKEB  lEON-BEARING  SERIES. 


^raruUe. 


conglomerate  adjacent  bears  fragments  of  both  the  Basement  Complex  and 
also  chert  fragments,  indicates  the  probability  that  this  limestone  belongs  to 
the  Cherty  limestone  and  that  it,  in  common  with  the  Basement  Complex, 
has  yielded  fragments  to  the  Quartz-slate. 

Passing  eastward,  in  Sec.  24,  T.  47  N.,  R.  43  W.,  Michigan,  test-pitting 
has  exposed  a  rock  which  is  again  a  recomposed  granite,  and  the  coarse 
granitoid  gneiss  itself  is  found  but  a  short  distance  to  the  southward.  The 
relations  here  are  plainly  the  same  as  those  in  the  northeast  part  of  Sec.  23. 
Near  the  center  of  Sec.  28,  T.  47  N.,  R.  42  W.,  Michigan,  the  eastern- 
most known  exposures  of  the  Penokee  succession  occur,  and  here  is  a 
beautiful  instance  of  a  basal  conglomerate  in  direct  contact  with  the  under- 
lying granite.  The  profile  of  thfe  rock  expo- 
sures here  is  shown  by  Fig.  12.  The  granite  and 
gneissoid  granite  have  evidently  formed  a  cliff 
against  which  the  basal  conglomerate  has  been 
deposited.  The  contact  between  the  granite  and 
conglomerate  strikes  north  30°  west,  and  the 
granite  dips  back  into  the  hill  at  an  angle  of 
75°,  so  that  the  contact  between  the  two  gives 
the  granite  an  appearance  of  ovei'hanging  the 
conglomerate.  If  the  strata  were  turned  back 
to  their  original  condition  this  would  not  be  the 

Fig.  12 Basal  coiiffloraerate  in  contact  with  i  i  i   i  i  ,  ,  ■ 

granite  in  s=c.  28,  T.  47  N.,ii.  42  w.,  Mich- case;  wc  slioiild  liavc  a  Conglomerate  restmg 
'e"""-  upon  an  ordinary  slope.     The  hill  faces  north 

of  east.  The  conglomerate  upon  tlie  face  of  the  cliff  at  its  upper  part  is 
about  6  feet  in  thickness  and  its  face  is  nearly  vertical.  The  granite- 
schist  ledge  continues  for  some  distance  beyond  the  point  at  which  the  con- 
glomerate is  found  in  a  direction  south  30°  east.  At  intervals  along  this 
face  conglomerate  is  found.  At  times  it  is  so  fine  grained  as  to  become 
an  ttrdinary  quartzite.  The  underlying  rocks  in  this  vicinity  are  of  both 
the  granitic  and  schistose  classes,  the  former,  however,  being  predominant; 
which  explains  the  fact  that  the  conglomerate  is  mostly  composed  of  granite 
debris,  but  also  contains  fragments  of  green  schist.  It  might  be  concluded 
from  this  exposure  that  the  conglomerate  was  underneath  the  granite  if  the 


Canff'omeroute. 


GENERAL  GEOLOGY  OF  THE  DISTKKT.  451 

nature  t)t'  rh(j  tV;i;^-iueiits  of  tho  coiif^loiuenitu  and  tliu  suhsequeut  tiltiny  ui' 
the  rocks  were  disregaa'ded.  We  have  here,  however,  nothing  Ijut  what  is 
known  to  occiu'  at  any  sea  clifF  in  whicli  its  detritus  becomes  a  part  of  an 
unconformable  newer  series  of  rocks.  In  the  immediate  vicinity  of  this 
chtf,  a  short  distance  to  the  northwest,  is  a  ravine  in  which  the  contact 
between  the  crystalhne  and  fragmental  series  is  again  found.  The  frag- 
mental  rock  is  at  tliis  phxce  a  recomposed  granite  Avhich  so  closely  resem- 
bles the  rock  from  which  it  is  derived  that  it  is  impossible  in  the  field  to 
locate  the  exact  junction  line.  In  thin  section  the  fundamental  difference 
between  .the  two  rocks  is  apparent  at  a  glance.  Here  practically  all  the 
material  of  the  fragmental  rock  was  derived  from  the  granite  immediately 
underlying  it. 

A  little  farther  to  the  northeast  (near  the  center  of  the  north  half  of  the 
NW.  4>  Sec.  28)  a  chert-conglomerate  is  found  in  several  places  but  a  few 
paces  from  the  granite.  These  chert-conglomerates  are  in  all  essential 
respects  like  that  found  near  the  Palms  mine,  already  described.  While  it 
includes  a  large  quantity  of  granite  debris,  including  quartz,  feldspar, 
biotite,  and  complex  areas  composed  of  these  minerals,  it  has  a  cherty 
background  which  contains  abundant  angular  chert  fragments.  It  appears 
that  here,  as  near  the  Palms,  the  Quartz-slate  member  has  derived  material 
from  both  the  Southern  Complex  and  the  Cherty  limestone. 

From  the  foregoing  it  appears  that  there  are  definite  proofs  of  a  strati- 
graphic  break  between  the  Southern  Complex  and  the  Penokee  series  at 
ten  places.  Two  of  the  localities  (see  PI.  ii),  both  being  actual  contacts 
accompanied  by  basal  conglomerates,  are  above  the  Western  schist,  the 
lowest  layer  of  the  upper  series  here  being  the  Quartz-slate.  Two  of  the 
localities,  both  again  being  contacts,  one  accompanied  by  a  basal  conglom- 
erate,  are  above  the  Central  granite,  the  lower  rocks  of  the  upper  series 
here  being  again  the  Quartz-slate.  One  of  the  places  is  above  the  Eastern 
schist,  the  contact  being  found,  but  no  basal  conglomerate  appearing.  The 
lowest  layer  of  the  upper  series  is  here  the  quartzite,  whicli  occasionally 
appears  at  the  base  of  the  Quartz-slate.  Finally,  five  of  the  localities  are 
above  the  Eastern  granite,  basal  conglomerates  or  recomposed  granites 
occurring  at  each  locality,  while  at  two  of  the  2)laces  are  actual  contacts. 


452  THE  PENOKEE  IKON-BEARING  SERIES. 

At  these  contacts  the  lowest  member  of  the  upper  series  is  the  Quartz-slate 
in  different  phases.  When  it  is  remembered  that  it  is  exceedingly  difficult 
and  unusual  to  find  actual  contacts  between  unconformable  series,  even 
when  there  is  the  same  certain  general  evidence  of  the  unconformity  that  is 
found  in  this  district,  the  above  number  must  be  considered  extraordinary. 
It  is  also  noticeable  that  the  contacts  are  widely  distributed,  and  in  about 
an  equal  number  of  cases  the  schists  and  the  granites  are  the  underlying 
formations. 

No  actual  contact  between  the  Southern  Complex  and  the  Cherty  lime- 
stone has  been  found.  However,  at  Penokee  gap  and  in  Sec.  23,  T.  47  N., 
R.  45  W.,  Michigan,  the  two  are  exposed  close  together.  At  the  first  place 
the  rock  of  the  Southern  Complex  is  a  schist;  at  the  second  it  is  a  granite 
gneiss.  At  the  former  the  schist  and  the  limestone  dip  in  opposite  direc- 
tions, the-  schist  inclining  30°  to  the  south  and  the  limestone  65°  to  the 
north. 

But  there  is  yet  further  evidence  as  to  the  magnitude  of  this  uncon^ 
formity.  It  is  certain  that  the  complex  basement  upon  which  the  newer 
series  was  laid  down  was  nearly  horizontal.  When  it  is  remembered  that 
this  Soutliern  Complex  is  here  resistant  massive  granite  and  there  soft 
foliated  schistose  rocks,  it  is  plain  that  the  forces  of  erosion  had  nearly 
exhausted  themselves,  i.  e.,  that  a  '"base  level"  of  erosion  had  been  nearly 
attained,  before  the  newer  series  began  to  form.  When  we  consider  what  is 
involved  in  this,  it  strongly  reenforces  what  has  gone  before  as  to  the  great 
time  gap  which  must  have  intervened  between  tliese  older  formations  and 
the  lower  fragmeiatal  rocks.  The  proof  of  the  existence  of  this  nearly 
level  plain  lies  in  the  small  variation  in  thickness  of  the  Quartz-slate 
and  the  Cherty  limestone.  The  difference  of  elevation  in  the  basement 
throughout  its  whole  extent  east  and  west  probably  was  not  more  than  a 
few  hundred  feet  at  the  time  the  Cherty  limestone  began  to  form.  This 
formation  is  found  here  and  there  in  detached  expostires  for  a  distance  of 
many  miles.  Its  maximum  thickness  is  now  300  feet.  In  the  sea  floor 
there  was  probably  at  the  time  when  tjie  limestone  was  deposited  no 
greater  variation  in  elevation  than  the  thickness  of  this  formation  as  it  then 
existed. 


(JElStKKAL  (JEOLOCJV  OK  TIIK   DISTRKX  453 

At  the  beginning-  of  tlic  (lc[)(isiti()n  of  tlie  Quartz-slntu  iiioinhor  of  the 
Peuokee  series  proper  tlie  ciis*?  \v;is  even  more  strikiiij^-.  'I'liis  layer,  avei'- 
ag'iiig  300  or  400  feet  in  tliickiu'ss,  Wiii  in  one  pl;u;e  reaching"  twice  this 
amount,  at  no  [lo'mt  is  known  to  l)e  much  U-ss  than  300  feet  in  thickness. 
It  forms  a  continuous  belt  throughout  most  of  the  extent  of  the  series,  and 
where  missing  has  been  swept  away  by  erosion,  as  have  the  higher  mem- 
bers at  such  places.  It  is  certain,  then,  that  the  variation  in  elevation  of 
this  ancient  land  surface  at  this  time  was  little  or  no  more  than  300  feet, 
except,  perhaps,  in  tlie  eastern  part  of  the  Eastern  area,  where  it  is  not  clear 
that  the  slates  there  occurring  as  the  lower  layers  of  the  Penokee  series 
are  the  equivalents  of  the  Quartz-slate  to  the  westward. 

A  rock  basin  composed  of  diversified  crystalline  schists  and  massive 
rocks  90  miles  in  diameter  and  having  no  elevations  greatly  exceeding  300 
feet  is  rare,  and  it  is  universally  believed  that  the  complex  areas  in  which 
this  is  the  case  have  been  subjected  to  enormous  denudation.  The  amount 
of  erosion  in  such  regions  is  often  compared  to  that  necessary  to  reduce 
lofty  mountains  to  mere  stumps.  Before  the  beginning  of  the  deposition  of 
the  Penokee  series  proper  it  is  clear  that  the  Southern  Complex  was  thus 
brought  nearly  to  a  plane. 

To  summarize,  the  proofs  of  unconformity  above  the  Southern  Com- 
plex are  as  follows:  First,  belts  of  sedimentary  rocks  strike  across  the 
coimtry,  being  now  in  contact  with  one  variety  of  underlying  rock,  now  in 
contact  with  another,  always  keeping  their  course,  never  being  penetrated 
or  interfered  with  by  any  of  the  southward  lying  rocks  (excluding,  of 
course,  the  later  basic  dikes),  whether  schistose  or  griinitic  intrusions  in  the 
schists.  Second,  these  underlying  rocks  are  either  massive  ones  which 
are  presumably  igneous  or  are  schists  in  which  the  extreme  of  foliation  and 
crystalline  character  is  found,  while  the  overlying  Penokee  rocks  are 
plainly  water  deposited  sediments.  Third,  in  ten  places  above  the  Base- 
ment Complex  are  basal  conglomerates  or  recomposed  rocks.  Seven  of 
these  places  show  the  actual  unconformable  contacts.  The  detritus  is 
mainly  identical  in  character  in  each  case  Avith  the  material  of  the  rock 
upon  which  it  rests,  showing, that  the  basement  rocks  must  have  reached 
.  their   present    condition    before    the    formation    of   the   lowest    overlying 


454  THE  PENOKEE  IROH-BEAKING  SERIES. 

member.  When  the  basement  rocks  are  green  schists  their  fohatioii  had 
been  developed  and  they  had  been  truncated ;  when  they  are  granite,  if 
formed  at  depth,  it  had  reached  the  surface  by  ei'osion.  Fourth,  the  fact 
that  the  horizon  at  which  the  underlying  complex  is  in  contact  with  the 
Penokee  series  proper  does  not  vary  more  than  300  or  400  feet  at  the  out- 
side is  the  clearest  sort  of  evidence  that  the  underlying  rocks  had  nearly 
reached  a  "base  level"  before  the  beginning  of  the  deposition  of  the  Peno- 
kee series.' 

The  unconformity  between  the  Cherty  limestone  and  the  Penokee  series 
proper. — In  the  description  of  the  Quartz-slate  (Chapter  iv)  it  was  seen  that 
at  a  number  of  places  this  member  contains  in  its  basal  j)ortions  abundant 
debris  derived  from  the  chert  of  the  Cherty  limestone,  becoming  in  several 
places  a  genuine  chert-conglomerate.  •  This  derivation  is  indicated  by  the 
lithological  likeness  of  the  pebbles  of  the  conglomerate  and  the  chert  of 
the  limestone,  as  well  as  by  the  fact  that  these  conglomerates  are  usually 
found  near  some  of  the  exposures  of  the  limestone.  At  Penokee  gap  the 
Quartz-slate  rests  directly  upon  the  Cherty  limestone,  being  here  a  recom- 
posed  quartz-rock  nearly  all  of  the  debris  of  which  comes  from  the  imme- 
diately subjacent  member.^ 

Beginning  at  the  west  and  passing  east  these  recomposed  rocks  or 
conglomerates  have  been  found  at  Penokee  gap,  mount  Whittlesey,  Potato 
river,  near  the  Palms  mine,  and  east  of  Sunday  lake  at  several  points  in 
Sees.  10,  14,  and  15,  T.  47  N.,  R  45  W.,  Michigan.  In  all  these  cases  the 
chert  pebbles  derived  from  the  Cherty  limestone  are  well  rounded  and  in 
exactly  the  same  condition  as  the  chert  now  is  in  the  limestone.  These 
pebbles  appear  to  be  clear  evidence  that  the  limestone  had  become  cherty 
before  the  deposition  of  the  lowest  member  of  the  Penokee  series  proper. 


'  The  time  gap  implied  l)y  such  an  unconformity  as  the  above  is  fully  discussed  by  Prof.  Irving, 
U.  S.  Geol.  Survey,  7th  Annual  Eeport,  pp.  390-443.  Suffice  it  here  to  say  that  it  is  clearly  shown 
that  an  nnconformity  of  this  sort  indicates  "a  lapse  of  time  long  enough  to  cover  (1)  the  folding  of 
the  lower  series,  (2)  its  elevation  into  a  land  surface,  (3)  a  long  continued  denudation,"  .and  (4)  its 
depression  under  the  sea.  "In  other  words,  it  indicates  an  interval  of  more  or  less  extended 
orogr.iphic  movement,  with  its  accompanying  »  *  «  denudation."  In  this  case  it  is  plain  that 
the  orographic  movement  is  of  a  most  extended  character. 

2  Geology  of  the  Eastern  Lake  Superior  District,  R.  D.  Irving.  Tlie  recomposed  rocli  is  Irving's 
IIB,  Geol.  Wis.,  vol.  iii,  pp.  109,  110. 


GENERAL  tlEOUKiV  OF  THE  DISTKHT.  455 


'^m 


he  parallrl  (listrilmtiipii  of  llic  (k^tiiclicd  t>ntcr()|)s  of  the  CliL-rty  lime- 
stone and  the  coiitiniious  belt  ot"Quartz-slat<'  is  evidence  tliat  tlie  oroyrapliic 
muveinent  between  the  two  in  the  Penokee  district  was  not  of  a  compli- 
cated character.  Also  tending-  in  the  same  direction  is  the  similarity  of 
the  dip  of  the  limestone  and  the  Peiiokee  series  proper.  While  in  some 
places  the  limestone  appears  to  have  a  flatter  (hp  than  the  Penokee  series, 
this  is  not  so  marked  as  tct  sug-g-est  an  unconformity  between  the  two  were 
it  not  that  other  data  point  in  this  direction.  Taking-  all  the  facts  into 
account,  it  is  concluded  that  there  was  a  considerable  time  interval  between 
the  Cherty  limestone  and  the  Penokee  series  proper.  In  this  inter\al 
the  limestone  was  consolidated,  and  if  the  chert  is  a  segregation  formed 
from  scattered  (irganic  remains,  or  was  introduced,  from  outside,  this  also 
occurred.  The  Cherty  limestone  and  Basement  Complex  were  raised  above 
the  sea  and  ei'osion  began.  The  amount  of  material  removed  it  is  impossi- 
ble to  estimate.  It  is  only  known  that  the  Cherty  limestone  in  some  places 
is  300  feet  thick,  in  others  is  absent.  The  presence  of  occasional  pebbles 
of  jasper  in  the  conglomerates  of  the  Quaitz-slate  suggests  that  upon  the 
limestone  was  once  a  higher  formation  of  a  different  character  which  was 
subsequently  wholly  carried  away.  After  this  time  of  erosion  the  land 
was  again  depressed  below  the  sea  and  the  lower  part  of  the  Penokee 
series  proper  began  to  be  deposited.  Tlie  lack  of  marked  discordance  in 
the  bedding  of  the  Cherty  limestone  and  the  Quartz-slate  is  no  evidence 
that  the  time  gap  between  the  two  was  not  long  enough  to  have  produced 
a  most  prononnced  discordance,  for  this  Penokee  area  may  have  been  a 
part  of  a  plain  removed  from  zones  of  important  folding  and  thrusting  which 
may  have  occurred  simultaneously  in  other  districts. 

The  Iron-hearing  and  Upper  slate  members. — Thus  far  the  relations  of  the 
Southern  Complex  to  the  two  lower  members  of  the  Penokee  series  and  the 
relations  of  the  latter  to  each  other  have  been  spoken  of  Above  the 
quartz-slate  follows  m  perfect  conformity  the  Iron-bearing  formation.  This 
member,  hke  the  major  part  of  the  Cherty  limestone,  has  been  shown  to  be 
of  a  nonfragmental  character.  The  change  from  the  fragraental  Quartz- 
slate  to  the  Iron-bearing  member  is  always  abrupt.  The  uppermost  mem- 
ber of  the  former  has  been  said  to  be  a  coarsely  crystalline  quartzite.  It  is 


456  THE  PENOKEE  IRON-BEAEmG  SERIES. 

plain  that  at  the  final  stage  of  the  formation  of  this  member  there  was  a 
clear  sea,  only  well  rolled  quartz  grains  being  deposited.  This  implies 
sorting  of  the  material  and  therefore  less  rapid  accumulation.  It  is  prob- 
able that  this  change  of  condition  was  accompanied  by  a' gradual  sinking  of 
the  sea  bed.  Naturally  following  the  clearing  up  of  the  water  have  come 
the  nonfragmental  sediments  of  the  iron  belt.  As  has  been  shown  (pp.  247- 
248)  tliey  are  analogous  to  limestone  formations  in  many  respects.  The 
uniformity  in  width  of  this  belt  is  noticeable  throughout  its  western  three- 
fourths.  In  its  eastern  part,  where  there  are  considerable  variations  in 
thickness,  the  irregularities  have  been  explained  to  be  due  to  contempora- 
neous volcanic  activity. 

Above  the  Iron-bearing  member  next  succeeded  the  great  thickness  of 
fragmental  Upper  slate.  At  Tylers  fork  these  slates  show  their  present 
maximuni  thickness,  nearly  13,000  feet.  East  and  west  of  this  point  the 
belt  gradually  narrows  until  it  is  cut  off,  at  the  Avest  near  Numakagon  lake 
and  at  the  east  near  Sunday  lake,  by  the  overlying  Keweenaw  rocks.  The 
relations  between  the  Iron-bearing  formation  and  the  upper  slates  are  the 
same  as  those  between  the  latter  and  the  underlying  quartz-slates;  that  is, 
they  are  two  formations  which  are  in  perfect  comformity,  as  is  shown  by 
the  strike  and  dip  of  many  exposures  in  both  belts.  The  change  from  this 
iron  formation  to  the  mechanical  sediments  of  the  Upper  slate  was  rather 
the  abrupt,  although  not  so  sharp  as  the  change  from  the  Quartz-slate  to 
the  Iron-bearing  member.  The  three  members  mentioned  with  contempora- 
neous eruptives  constitute  the  Penokee  series.  By  an  examination  of  PI.  ii, 
it  is  seen  that  they  are  not  all  continuous  throughout  the  district  traversed 
by  them;  bui  wherever  the  series  is  present  one  formation  follows  another, 
except  in  the  Eastern  area  in  the  order  mentioned  in  conformable  succes- 
sion, so  that  they  are  properly  placed  together  as  a  group  of  formations. 

The  unconformity  at  the  base  of  the  Keweenaw  series. — North  of  this 
Penokee  series  are  found  the  eruptive  and  fragmental  rocks  of  the  Kewee- 
naw series.  The  rocks  of  the  latter  immediately  north  of  the  former  are 
usuall}^  bedded  surface  eruptives ;  but  from  Black  river,  in  the  east  part  of 
T.  47  N.,  R.  46  W.,  Michigan,  to  near  the  Montreal  river,  in  the  basement 
layers  of  the  Keweenaw  series,  has  been  seen  at  various  points  a  red  sand- 


(IKNEKAL  (lEOLOCiY  Ol"  TIIK   DISTKICT.  457 

stone  <»r  (juiirtzitc.  Tliat  tluTc  is  a  (jTont  tiiiic  'j;i\\)  hctwccii  tlic  S(iiitli(irii 
Complex  and  the  Peuokee  scries  lias  just  been  slmwii.  That  tlicrc  is  also 
a  time  interval,  altlioug-h  not  so  vast  a  one,  hctwccii  the  intii-l)earing'  and 
the  overlying  Ke\>'eena\v  series,  is  equally  clear.  In  anv  single  section  this 
discordance  is  not  plain,  for  the  Keweenaw  rocks  nortli  of  the  Penokee 
series,  like  the  latter,  have  as  a  whole  a  northern  dip  at  a.  high  ang-le.^  The 
eruptive  character  of  the  basement  member  of  the  Keweenaw  series  for 
most  of  the  distance,  and  its  northern  inclination,  make  it  more  difficult  to 
prove  an  unconformity  between  the  tMo  than  l)etween  the  Penokee  series 
and  the  Southern  Complex.  The  p>oof  here  rests  entirely  upon  broad  field 
relations ;  iinless  the  cong-lomerate,  placed  provisionall}'  at  the  top  of  the 
Upper  slate  Qjp.  305,  326), -belongs  witli  the  Keweenawan.  This  conglom- 
erate lies  but  200  steps  south  of  the  Keweenawan  greenstone.  It  is  sep- 
arated from  the  typical  biotitic  slate  (p.  326)  of  the  Upper  slate  member  by 
an  unexposed  interval  of  280  steps.  The  pebbles,  ranging  up  to  eig'ht  or 
ten  inches  in  diameter,  are  mainly  of  white  quartz,  Hut  flint  and  black 
homstone  are  also  abundant.  The  most  probable  source  of  the  last  two 
are  the  Cherty  Limestone  and  Iron-bearing-  members  of  the  Penokee 
series.  While  it  can  not  be  asserted  whether  this  conglomerate  belongs  to 
the  Upper  slate,  or  at  the  base  of  the  Keweenawan,'  the  latter  alternative  is 
the  more  probable,  and  if  this  be  true,  the  rock  may  be  considered  a  basal 
conglomerate. 

Beginning  at  the  west  end  of  the  Penokee  succession,  and  following 
the  contact  of  the  two  series  of  rocks  to  the  eastward,  it  becomes  evident 
that  there  is  a  considerable  break  between  them.  Near  the  northeast  corner 
of  Sec.  20,  T.  43  N.,  R.  7  W.,  Wisconsin,  characteristic  eruptives  of  the 
Keweenaw  series,  as  found  by  Mr.  Charles  E.  Wright,^  are  upon  the  north 
side  of  Numakagon  river,  and  a  gneissoid  granite  of  the  underlying  complex 
upon  its  south  bank.  It  is  not  certain  that  rocks  belonging  to  the  Peuokee 
series  are  not  between,  but  there  is  no  proof  of  this  or  of  their  existence  to 
the  westward  for  a  long  way ;  so  this  point  may  be  taken  as  the  western- 


'  Coppei'-beariug  Rocks  of  Lake  Superior,  pp.  225-234,  R.  D.  Iiviug,  IT.  S.  Geol.  Survey,  Mono- 
graph, vol.  V. 

2  Geol.  of  Wis.,  vol.  Ill,  p.  300. 


458  THE  PENOKEB  IROiN-BEAEING  SERIES. 

most  one  at,  which  it  is  jjrobable  that  rocks  of  this  series  will  be  found.  As 
a  matter  of  fact,  the  westernmost  known  exposure  is  in  the  SE.  J  of  Sec. 
24,  T.  44  N.,  R.  6  W.,  Wisconsin,  about  11  miles  north  of  east  of  this 
point.  In  Sec.  16,  T.  44  N.,  R.  5  W.,  Wisconsin,  the  eruptive  rocks  of 
the  Keweenaw  series  at  on*  point  are  immediately  north  of  the  rocks 
of  the  Iron -bearing  member,  Avhile  between  them  there  is  little  space 
in  this  township  for  the  rocks  of  the  Upper  slate  member.  Proceeding  east- 
ward the  exposures  found  in  the  NE.  |-  of  Sec.  23,  T.  44  N.,  R.  5.  W.,  Wis- 
consin, are  the  last  known  in  the  Penokee  series  for  fullj-  6  miles.  For 
this  distance  the  countrj^  is  low  and  swampy.  The  next  exposures  to  the 
eastward  are  found  in  Sec.  24,  T.  44  N.,  R.  4  W.,  Wisconsin.  No  attempt 
is  made  to  map  the  Penokee  series  in  this  interval,  as  the  only  indications 
of  its  existence  are  somewhat  feeble  magnetic  attractions,^  which  may  be 
due  to  the  magnetite  of  a  basic  eruptive.  It  is  not  even  certain  but 
that  the  entire  succession  is  here  cut  oif  by  the  overlying  series.  There 
are  numerous  exposures  of  coarse  gabbro  in  the  southern  part  of  Sec.  24, 
T.  44  N.,  R.  5  W.,  Wisconsin  (see  PI.  v),  a  short  distance  east  of  the 
granites  in  the  south  part  of  Sec.  23.  In  Sees.  22  and  23,  T.  44  N.,  R. 
4  W.,  are  again  exposures  of  gabbros  a  short  distance  from  the  granite 
to  the  south  and  east.  We  have  no  evidence  which  will  enable  us 
to  locate  these  greenstones  as  a  part  of  the  Keweenaw  series  or  as  intrusive 
rocks  in  the  Penokee  or  underlying  series.'  If  they  are  regarded  as 
Keweenaw  rocks,  that  series  must  here  be  conceived  not  only  to  take  the 
place  of  the  Penokee  succession,  but  to  occupy  at  least  half  a  mile  of  the 
area  where  one  would  expect  to  find  the  underlying  complex.  This  struc- 
ture would  imply  an  erosion  not  only  sufficient  to  remove  all  of  the  rocks 
■*  of  the  Penokee  series,  but  to  have  cut  a  valley  of  very  considerable  depth 
into  the  Southern  Complex  in  the  interval  between  Penokee  and  Keweenaw 
time.  This  unusual  amount  of  erosion  is  not  incredible,  but  the  relative 
locations  of  the  ledges  in  the  south  part  of  Sec.  24  and  in  the  northeast  part 
of  Sec.  23  are  such  as  to  give  an  almost  incredible  steepness  to  this  hypo- 
thetical gorge.  Another  possible  explanation  of  the  relations  is,  that  the 
Penokee  rocks  are  absent  by  erosion  and  that  there  has  been  here  a  double 

iGeol.  of  Wis.,  vol.  Ill,  pp.  278-281. 


GENEHAL  CiKOL()(;V  OF  TUK  DISTKIOT.  •  459 

ff 

fault  witli  a  soiitliwiu-d  tlimw,  l)\  incniis  dt' wliicli  ;i  scctiim  <il'tlii'  Soutlieru 
C<mi|)lc\  lias  ])c'('n  set  off"  from  tlu?  corresponding  rocks  cast  and  west.  But 
this  iissinn[)tion  of  l)()tli  unusual  erosion  and  faulting  lins  no  j)ositive  facts  in 
its  fa\(>r,  Mud  it  seems,  in  the  lack  of  deKnite  evidence,  tli;it  it  is  probable  that 
the  Penokee  series  is  continuous  and  that  the  greenstones  adjacent  to  the 
granite  in  this  area  are  intrusive  within  them,  rather  than  a  part  of  the 
Keweenawan  basement  flow. 

,  About  half  a  mile  west  of  the  east  range  line  of  R.  4  W.,  AVisconsin, 
numerous  exposures  of  tlie  iron  formation  are  again  found.  The  known 
thickness  of  the  Penokee  series  is  here  about  half  a  mile,  while  it  ma}'  be 
considerably  thicker.  At  the  center  of  Sec.  18,  T.  44  N.,  R.  3  W.,  Wiscon- 
sin, the  series  is  at  least  three-fourths  of  a  mile  wide.  Just  east  of  English 
lake  this  width  has  become  more  than  a  mile.  This  widening  of  the  series 
goes  on  continuously  until  Tylers  fork  is  reached,  T.  45  N.,  R.  1  W.,  Wis- 
consin, where  its  maximum  thickness  is  found.  In  passing  eastward  from 
this  stream  tlie  series  gradually  narrows  with  great  uniformit}'  until  a  short 
distance  west  of  Sunday  lake,  in  the  west  part  of  T.  47  N.,  R.  45  W.,  Mich- 
igan, where  the  Upper  slates  have  disappeared.  From  this  point  to  the 
middle  of  T.  44  W.,  Michigan,  the  overlying  Keweenaw  rocks  are  in  direct 
contact  with  the  Iron-bearing  formation.  At  one  place  just  east  of  Sunday 
lake  this  formation  can  not  be  exjDosed  for  more  than  a  third  of  its  width. 
Through  townships  44  W.,  43  W.,  and  42  W.,  Michigan,  the  greenstone 
ridg'e  constituting  the  basement  of  the  Keweenaw  series  passes  nearly  in 
an  east  and  west  direction,  with  the  exception  of  1 J  miles  in  the  east  part 
of  T.  47  N.,  R.  44  W.,  Michigan,  and  here  the  discrepancy  is  explained  (pp. 
424,  425)  as  being  probably  due  to  a  fault.  In  these  townships  the  immedi- 
ately underlying  rock  is  a  mingled  fragmental  and  nonfragmental  one 
which,  as  explained  (p.  431),  is  probably  the  equivalent  of  the  upper  part 
of  the  Iron-bearing  member  to  the  west. 

That  the  Penokee  series  could  have  been  deposited  to  a  thickness  of 
about  14,000  feet  at  Tylers  fork,  and  at  other  points  be  only  1,000  feet 
thick,  or  entirely  absent,  is  not  at  all  probable.  This  is  especially  true,  as 
it  is  the  lower  1,000  or  1,500  feet  of  the  series  which  is  found  whenever 
any  part  is  present.     As  already  explained,  the  persistence  of  the  Quartz- 


460  '  THE  PENOKEE  lEON-BEAEING  SERIES. 

slate  and  Iron-bearing-  members  in  nearly  uniform  thicknesses  shows  that 
at  the  time  of  their  deposition  there  was  for  this  distance  an  approximately 
level  basin.  If  no  part  of  the  upper  slates  and  iron  formation  have  been 
swept  away  by  erosion,  we  must  believe  that  there  was  inaugurated  before 
the  end  of  the  latter  period  an  orographic  movement  which  formed  a  syn- 
clinal trough,  the  center  of  which  was  Tylers  fork,  and  thus  a  great  thick- 
ness of  upper  slates  -there  accumulated,  while  to  the  east  and 'west  the 
thickness  became  less  and  less  until  it  finally  disappeared,  as  well  as  a  part 
or  the  whole  of  the  lower  members  of  the  series.  That  a  great  earth  move- 
ment of  this  sort  could  occur  without  disturbing  the  perfect  conformity  of 
the  formation  of  the  series  is  improbable.  The  discordance  in  the  strikes  of 
the  layers  would  indicate  the  change,  but  as  all  the  members  are  in  appar- 
ent perfect  accordance,  one  layer  has  followed  upon  another  without  dis- 
turbance. This  being  the  case,  the  Penokee  series  was  originally  probably 
of  rather  unifomi  thickness,  and  doubtless  this  thickness  continued  both 
east  and  west  of  the  points  at  which  the  series  can  now  be  traced;  for  there 
are  many  reasons  for  believing  that  this  district  was  a  part  of  a  great  basin 
which  extended  to  the  Marquette  district  on  the  east  and  to  the  quartzites  of 
the  Chippewa  valley  on  the  west. 

The  relations  just  sketched  can,  then,  have  but  one  meaning — that  a 
great  unconformity  separates  the  Penokee  and  Keweenaw  series.  This 
is  the  only  possible  explanation  of  the  facts  that  the  Keweenaw  series  is 
above  an  Upper  slate  member  13,000  feet  thick  at  Tylers  fork,  and  east  and 
west  not  a  great  distance  is  above  a  belt  much  thinner,  while  a  little  far- 
ther east  and  west  this  slate  disappears  altogether,  and  yet  a  little  farther 
to  the  west  the  lower  members  are  hidden,  and  west  of  Numakagon  lake 
the  Keweenaw  series  apparently  rests  directly  upon  the  Southern.Complex. 
During  this  intervening  period  the  rocks  of  the  Penokee  series  were  raised 
above  the  sea,  and  then  suffered  long  continued  denudation  "until  in  places 
the  entire  succession  was  carried  away,  exposing  the  underlying  rocks. 
How  thick  the  series  as  a  whole  was  at  the  beginning  of  this  erosion  there 
is  no  available  evidence,  but  the  amount  of  material  swept  away  at  Numa- 
kagon lake  was  14,000  feet  more  than  at  Tylers  fork,  while  the  difference 
in  amount  of  erosion  between  the  last  place  and  Sunday  lake  is  scarcely 


GENERAL  CiKOLOdY  OF  TUE  DISTRICT.  461 

1,000  feet  less.  Probabl)  tliu  series  must  have  been  everywhere  thicker 
than  at  the  present  time  at  any  place.  Tlie  amount  of"  erosion  during  this 
period  was,  then,  in  some  places  measured  perhaps  b\'  twice  10,000  feet. 
During-  the  progress  of  this  erosion  it  is  possible  that  the  series  was  gently 
bent  into  a  synclinal  trough,  the  center  of  which  was  in  the  ^^cinity  of 
Tylers  fork  and  the  eastern  end  near  Sunday  lake.  This  suggestion  is 
made  as  an  explanation  of  the  diiference  in  the  amount  of  erosion, 
those  parts  naturally  being  eroded  most  which  were  at  highest  elevations. 

The  time  gap,  then,  between  the  Penokee  apd  the  Keweenaw  series 
must  have  been  sufficient  for  a  widespread  orographic  movement  and  deep 
denudation — a  vast  lapse  of  time ;  so  that  while  the  unconformity  between 
these  two  series  is  not  nearly  so  great  as  between  the  Penokee  series  and 
the  Southern  Complex,  yet  it  stands  as  one  of  the  greater  time  gaps  in 
geological  history. 

The  Eastern  sandstone  and  the  unconformity  at  its  base. — The  only  remain- 
ing teiTane  in  which  we  are  concerned  in  this  memoir  is  the  Eastern  sand- 
stone,  which  (as  shown  by  PI.  i),  extending  from  Keweenaw  bay  to  a  long 
distance  west  of  Gogebic  lake,  conceals  a  broad  strip  of  the  older  forma- 
tions. The  low  ground  north  of  the  trap  range,  in  the  north  part  of  T.  47 
N.,  R.  44  W.  and  43  W.,  Michigan,  is  the  southern  boundary  of  this  sand- 
stone. As  T.  47  N.,  R.  42  W.,  Michigan,  is  reached,  near  Gogebic  lake, 
this  low  ground  swings  rapidly  to  the  southward,  and  it  was  early  in  this 
study  suspected  that  here  the  Eastern  sandstone  is  the  surface  formation, 
but  no  natural  exposure  was  found  which  would  either  prove  or  disprove 
this  suspicion.  However,  test  pitting  in  the  NW.  -\  of  Sec.  28,  T.  47  N.,  R. 
42  W.,  Michigan,  has  shown  the  sandstone  to  there  exist.  The  basal  con- 
gloifterates  there  found  at  the  junction  of  the  Penokee  series  with  the 
underlying  granites  have  already  been  described,  pp.  394-395,  409.  In 
this  interesting  locality  is  still  another  basal  conglomerate  which  belongs  to 
the  Elastern  sandstone.  In  the  test  pits  close  to  the  outcrops  of  the  Peno- 
kee series  and  the  Southern  Complex  the  conglomerate  is  coarse,  and  its 
matrix  is  somewhat  indurated,  it  being  necessary  to  resort  to  blasting  in 
sinking.  Upon  exposure  to  the  air  the  matrix  of  the  conglomerate  dis- 
integrators, as  a  result  of  which  a  heap  of  sand,  pebbles,  and  bowlders  of 


462  THE  PENOKEE  lEON-BEAEHSTG  SERIES. 

various  sizes  is  found  about  each  test  pit,  although  parts  of  the  rock  hold 
together  with  sufficient  firmness  to  yield  large  specimens  of  the  conglomer- 
ate. The  pebbles  and  bowlders  are  well  rounded.  The  character  of  the 
pebbles  indicates  that  they  have  been  derived  from  the  Southern  Com- 
plex, the  Penokee  and  the  Keweenaw  series.  From  the  last  are  coarse  red 
indurated  sandstones,  amygdaloids,  coarse  grained  basic  eruptives,  quartz- 
porphyries,  and  other  varieties  of  rock.  From  the  Penokee  series  the  pebbles 
are  more  numerous  than  from  either  of  the  others,  as  would  be  naturally  the 
case  since  the  exposed  conglomerate  is  in  contact  with  these  rocks.  The 
pebbles  belonging  here  are  lean  ore,  banded  jasper,  magnetitic  schist,  chert, 
chert-breccia,  recomposed  granite,  and  quartzite,  the  induration  of  which  is 
due  to  the  enlargement  of  quartz-grains.  From  the  Basement  Complex 
pebbles  and  bowlders  are  also  very  numerous,  comprising  white  vein  quartz, 
gneiss,  granite,  and  many  vai'ieties  of  crystalline  schist.  The  pebbles  from 
all  of  these  som-ces  are  characteristic  of  the  series  from  which  they  are 
derived.  To  describe  them  in  detail  here  would  be  but  fo  repeat  the  litho- 
logical  descriptions  given  of  them  in  the  treatment  of  the  formations  of  the 
district.  It  is,  however,  to  be  remembered  that  in  this  area  no  crystalline 
schist,  gneiss,  or  granite  has  been  found  anywhere  except  in  the  Southern 
Complex;  no  jasper,  magnetitic  schist,  chert-breccia,  lean  m-e,  or  recom- 
posed granite  has  been  found  in  other  than  the  Penokee  series,  while  quartz- 
porphyry  and  certain  phases  of  the  basic  eruptives  have  been  found  nowhere 
but  in  the  Keweenaw  series;  so  the  evidence  is  as  clear  as  any  lithological 
evidence  can  be  that  the  various  pebbles  have  been  derived  from  the  sources 
assigned  them.  The  test  pits  show,  so  far  as  tlie  rock  has  any  stratifica- 
tion, that  it  lies  in  a  horizontal  position,  and  the  character  of  the  basal  con- 
glomerate alone  would  be  sufficient  to  prove  that  we  have  to  deal  with  the 
Eastern  sandstone.  Further,  the  relation  of  the  underlying  rock  to  the 
sandstone  is  shown  in  one  test  pit.  After  passing  through  a  thickness  of  24 
feet  of  the  conglomerate  just  described,  the  shaft  penetrated  the  chert-brec- 
cia of  the  Penokee  series  (p.  406)  clearly  proving  that  this  rock  is  earlier 
than  the  conglomerate.  Again,  a  short  distance  northeast  of  these  test  pits 
are  others  in  sandstone  which  in  every  respect  is  like  the  ordinary  red 
Eastern  sandstone,  and  which  show,  so  far  as  it  is  possible  in  test  pits,  the 


GEN?]RAL  (JEOLOGY  OF  THE  DISTRICT.  463 

rock  to  ho  in  a  horizontal  position.  The  Penokee  series  is  not  absolutely 
cut  oil'  b)'  the  sandstone,  but  its  breadth  at  one  point  in  the  northwest  part 
of  Sec.  28,  can  not  be  more  than  200  or  300  feet,  and  it  is  probable  that 
farther  south  the  entire  succession  is  overlain  by  it. 

It  is  clear,  then,  in  the  upward  passage  to  this  Eastern  sandstone  that 
another  great  unconformity  has  been  passed.  First,  the  sandstone  is  in  a 
horizontal  position,  lying  directly  iipon  the  upturned  edges  of  the  Penokee 
series  and  presumably  having  the  same  relations  to  the  Keweenaw  series. 
Second,  this  sandstone  contains  abundant  fragments  derived  from  the 
Southern  Complex,  Penokee-Gogebic,  and  Keweenaw  series.  From  what 
has  gone  before  it  is  manifest  that  in  order  that  this  should  be  the  case  the 
whole  Keweenaw  and  Penokee  series  must  have  been  tilted  and  have  suffered 
vast  erosion  in  order  that  the  base  of  the  latter  and  the  iinderlying  series — 
once  probabh'  buried  under  many  thousands  of  feet  of  sediments — could 
reach  the  surface  and  furnish  these  fragments.  Since  the  sandstone  is  hori- 
zontal it  is  plain  that  the  two  upper  series  were  tilted  to  their  present 
inclination  and  eroded  before. the  deposition  of  this  sandstone.  The  final 
part  of  the  erosion  furnished  the  material  of  which  the  sandstone  is  made. 
This  unconformity  is  one,  then,  which  in  magnitude  is  probably  equal  to 
if  not  greater  than  the  geological  break  between  the  Penokee  series  and 
the  Southern  Complex.-' 

Resume  of  geological  Mstory. — We  are  now  prepared  to  give  a  r^sumd 
of  the  geological  history  of  the  district  of  which  the  Penokee  series  is 
a  part.  The  oldest  rocks  are  the  great  Southern  Complex.  Of  the  origin 
of  a  part  of  them  little  is  known,  but,  whatever  their  genesis,  before 
the  beginning  of  the  deposition  of  the  Penokee  series  there  was  a  long 
period  during  which  earth  movements  and  erosion  acted  upon  them.     This 

'  The  discordance  between  the  eastern  sandstone  and  the  Keweenaw  series  lias  heen  long  main- 
tained by  eminent  geologists,  though  denied  by  others.  This  position  was  iirst  taken  l)y  Brooks  and 
Pumpplly  in  1872  (On  the  Age  of  the  Copper-Bearing  Rocks  of  Lake  Superior.  Am.  Jour.  Sci.,  3d 
series,  aoI.  in,  pp.  428-432).  The  last  exhaustive  and  convincing  treatment  of  this  question  is  by- 
Profs.  R.  D.  Irving  and  T.  C.  Chaniberliu  (Bull.  U.  S.  Geol.  Survey  No.  23).  Upon  Keweenaw  point  it 
has  been  maintained  that  the  pebbles  in  the  sandstone  do  not  imply  a  great  break  because  of  eruptive 
origin.  At  Gogebic  lake  are  nearly  all  the  phases  of  pebble.s  of  the  three  great  series  of  rock  of  the 
region.  That  they  should  be  abnudnnlly  coutaiiicd  in  the  saudstinic  in  t^ic  condition  which  they  are 
now  found  i«si(«  can  not  be  explained  upon  any  other  hypothesis  thau  a  great  unconformity, 


464  THE  PENOKEE  IROE^-BEAEING  SERIES. 

erosion  coptinued  until  they  were  reduced  nearly  to  a  plain  throughout 
the  distance  from  Numakagon  lake  to  Itike  Gogebic.  The  district  was 
then  submerged.  The  conditions  prevailing  were  quiet  ones,  for  almost 
immediately  there  began  forming  at  the  bottom  of  the  sea  the  nonfrag- 
mental  rocks  of  the  Cherty  limestone  membei'. 

Between  the  Cherty  limestone  and  Quartz-slate  there  was  an  ei'osion 
interval  which  ma}^  mark  a  time  break  of  considerable  magnitude.  It  is 
even  probable  that  above  the  Cherty  limestone  member  were  deposited 
other  formations  which  have  been  entirely  removed.  While  in  the  lower 
part  of  the  Quartz-slate  member  a  considerable  amount  of  material— even 
sufficient  to  form  basal  conglomerates — ^lias  been  derived  fromthe  Cherty 
limestone,  the  great  mass  of  the  materif^l  came  from  the  gneiss-granite 
Basement  Complex.  Where  the  Cherty  limestone  is  now  absent  the  basal 
conglomerates  found  contain  fragments  which  are  almost  wholly  from  the 
granite  and  gneiss.  These  conglomerates  have  been  discovered  at  a  num- 
ber of  localities  and  may  be  nearly  continuous.  The  character  of  the 
sediments  was  very  uniform  for  some  time,  although  there  were  to  a  certain 
extent  variations  in  conditions,  thin  beds  of  feldspathic  sandstone  being 
interlaminated  with  beds  of  shale. 

After  a  time  there  was  a  clearing  up  of  the  waters  and  an  assorting  of 
materials,  as  a  result  of  which  well  rounded  quartz  grains  only  were 
deposited,  and  this  layer  now  constitutes  the  pure  vitreons  quartzite  com- 
posing the  upper  50  feet  of  the  member. 

After  the  deposition  of  this  thin  layer  of  sandstone  there  was  again  a 
change  of  conditions  by  which  fragmental  sedimentation  ceased  and  the 
nonfragmental  chemical  or  organic  sediments  of  the  iron  belt  began  to 
.  form.  During  this  period  the  conditions  were  again  uniform,  the  material 
everywhere,  except  in  the  Eastern  area,  at  all  horizons  appearing  to  be  a 
cherty  ferriferous  carbonate.  During  the  time  of  the  accumulation  of  the 
800  feet  of  the  Iron-bearing  member  it  is  probable  that  the  bed  of  the 
ocean  continued  steadily  to  subside. 

Again  a  change  of  conditions  came  about,  resulting  in  the  deposition 
of  fragmental  layers.  The  passage  from  the  nonclastic  iron  formation  to 
the  clastic  slates  was  not  so  abrupt  as  the  change  from  the  quartz-slates  to 


GENKRAL  (;EOL()(iY  OF  Till';  DISTRICT.  465 

the  iron  formation,  but  in  most  localities  it  took  place  within  a  compara- 
tively short  distance.  From  this  time  onward  the  accumulatintj-  beds 
of  the  Penokee  series  were  very  uniform  in  character.  The  material  depos- 
ited constitutes  the  Ujjper  slate  member.  This  member,  in  places  more  than 
10,000  feet  in  thickness  and  making  up  at  least  six-sevenths  of  the  whole 
series  when  at  its  maximum  thickness,  is  in  turn  graywacke  and  graywacke- 
slate,  mica-schist  and  mica-slate,  with  less  frequent  clay-slates.  The 
original  material  for  all  of  these  phases  of  rocks  was  very  largely  quartz 
and  feldspar,  the  only  differences  being  the  relative  proportions  of  the  two 
minerals  and  their  fineness  of  comminution.  Thick  beds  of  fine  grained 
black  clay-slates  and  gray  wacke-slates  show  tlmt  through  a  large  proportion 
of  the  area  the  material  was  very  finely  pulverized.  From  Penokee  gap 
and  westward  the  original  rock  has  been  metamorphosed  into  a  mica-slate 
or  mica-schist.  Here  the  material  furnished  by  the  underlying  gneiss  and 
granite  was  almost  wholly  feldspar.  This  peculiarity  is  explained  by  the 
fact  that  the  area  of  granite  and  granitoid  gneiss  stretching  to  the  south 
and  west  is  veiy  strongly  feldspathic.  After  the  deposition  of  at  least 
12,000  feet  of  these  materials,  and  perhaps  other  kinds  of  sediments  which 
have  been  subsequently  swept  away,  came  the  end  of  Penokee  time. 

The  district  was  then  elevated  above  the  sea,  gently  folded,  and  suf- 
fered a  long  period  of  atmospheric  denvidation.  The  erosion  was  suffi- 
cient at  the  west  and  east  ends  of  the  district  to  entirely  remove  the  series. 
What  fraction  of  it  has  been  removed  at  its  place  of  present  maxinuim  thick- 
ness is  unknown.  Following  the  Penokee  series  came  the  great  succession 
of  eruptive  and  fragmental  layers  which  built  up  the  Keweenaw  series.  This 
series  has  been  already  treated  in  a  monograph  of  the  Geological  Survey,^ 
and  nothing  will  be  here  said  as  to  the  conditions  which  prevailed  or  as  to 
the  succession.  But  the  probable  connection  between  the  eruptives  of  this 
series  and  the  dikes  of  the  Penokee  series  is  to  be  noted.  The  little  altered 
diabases  of  the  Southern  Complex  and  those  of  the  Penokee  series  have 
been  described  in  detail  and  their  relations  to  the  containing  formation 
given  (chapter  vii).  It  has  been  seen  that  these  same  dikes  have  been  pro- 
foundly altered  in  the  iron  formation,  where  they  have  been  subjected  to 

'Copper-Bearing  Rooks  of  Lake  Superior;  by  R.  D,  Irving,  u.  S,  Geol,  purvey,  Monograph  v, 
MON  XIX 30 


466  THE  PENOKEE  IRON  BEAEING  SERIES. 

long  continued  leaching.  In  that  part  of  the  Penokee  series  which  has 
been  the  seat  of  mining  operations,  the  large  number  of  these  dikes  and 
the  fact  that  they  cut  the  containing  formations  perpendicularly  are 
shown  by  the  descriptions,  pp.  271-275  and  Pis.  xxx  and  xxxi.  That 
these  diabases  are  the  pipes  through  which  has  passed,  from  deep  within 
the  earth,  the  vast  amount  of  material  which  formed  the  basic  volcanic  flows 
of  the  Keweenaw  series  can  hardly  be  doubted.  The  trap  range  north  of 
the  Penokee  series  is  a  set  of  rocks  varying,  it  is  true,  greatly  in  structural 
character,  being  here  amygdaloids  and  there  diabases  and  gabbros,  but  m 
chemical  composion  these  rocks  are  practically  the  same  as  those  that  are 
found  in  the  form  of  dikes  within  the  Penokee  succession. 

■  After  or  during  Keweenaw  time  began  the  orographic  movement, 
accompanied  and  followed  by  erosion,  which  made  the  synclinal  trough  of 
lake  Superior,  and  which  upturned  and  truncated  the  whole  great  tlnckness 
of  formations  constituting  the  Keweenaw  and  Penokee  series.  We  have  no 
measure  by  which  to  estimate  the  time  required  for  this  work,  but  it  was 
sufficient  to  bring  to  the  surface  a  continuous  succession  of  beds  more  than . 
50.000  feet  thick. 

Prior  to  this- time,  during  it,  and  subsequent  to  it  went  on  the  alter- 
ations which  changed  the  rocks  of  the  Penokee  series  from  their  original 
condition  to  their  present  somewhat  metamorphosed  one.  There  is  no  means 
of  placing  the  time  at  which  the  change  from  the  feldspathic  fragmental  rocks 
to  the  mica-schists  occurred.  There  seems  to  be  tolerably  clear  evidence 
■that  the  transformation  of  tlie  cherty  iron  carbonates  to  the  many  phases  of 
rock  now  found  in  the  formation  took  place  during  the  uplifting  and  erosion 
or  subsequent  to  it. 

After  the  Penokee  and  Keweenaw  series  had  assumed  their  present 
inclined  position  and  had  suffered  vast  erosion,  there  was  de230sited  upon 
their  upturned  edges  the  Eastern  sandstone,  which  now  cuts  them  both  off 
just  west  of  Gogebic  lake. 

Why  the  district  is  given  a  separate  memoir. — The  reason  is  now  more 
apparent  than  at  any  time  before  for  giving  this  Penokee  district  a  separate 
memoir.  It  stands  out  in  the  lake  Superior  country  unique  in  its  simplicity 
and  isolation.     It  is  a  great  series  of  water  deposited  sediments,  the  origin 


GENEKAL  CiEOLOGY  OV  TIIK  DISTRICT.  467 

of  which  has  been  for  the  most  part  deterininud.  TIk;  locks  liave  simply 
been  tilted  to  the  northward  at  lui  angle  most  convenient  to  determine  the 
succession  of  l)i'hs.  It  is  without  any  subordinate  fold  whatever,  so  that 
traversed  anywhere,  excluding-  the  Eastern  area  and  Cherty  limestone,  if 
sufficient  exposures  are  found,  the  succession  of  its  belts  can  be  made  out, 
one  following-  the  other  in  conformity.  Tliis  series  is  terminated  on  the 
east  by  the  uuconformably  overlying  horizontal  Easteni  sandstone;  it  is 
terminated  on  the  west  by  its  being  entirely  swept  away  by  erosion,  the 
Keweenaw  series  directly  underlying  the  Southern  Complex.  It  is  marked 
off  from  the  underlying  granitic  and  gneissic  rocks  by  one  of  the  greatest 
unconformities  of  geology.  The  proof  of  this  unconformity,  as  evidenced 
by  broad  relations  and  by  numerous  localities  in  which  actual  contacts  are 
found,  is  of  the  clearest  possible  sort.  To  the  north  of  the  Penokee  rocks 
is  a  third  independent  set  of  formations,  the  Keweenaw  series,  which  is 
separated  from  the  former  by  a  time  gap  only  inferior  to  that  just  men- 
tioned. 

Depth  and  metamorphism. — The  Penokee  series  furnishes  an  instructive 
lesson  as  to  the  depth  to  which  rocks  can  be  buried  and  still  be  slightly 
affected  by  metamorphosing  processes.  The  series  itself  is  14,000  feet  thick. 
It  was  covered  before  being  upturned  by  a  great  thickness  of  Kewee- 
naw rocks.  This  series  at  the  Montreal  river  is  estimated^  to  be  50,000 
feet  thick.  Adding  this  to  the  known  thickness  of  the  Penokee  series 
we  have  a  thickness  of  64,000  feet,  or  more  than  12  miles.  The  Peno- 
kee rocks  were,  then,  buried  to  a  great  depth,  the  exact  amount  depending 
upon  their  horizon  and  upon  the  stage  in  Keweenaw  time  when  the  tilting 
and  erosion  which  brought  them  to  the  surface  was  inaugurated.  That  the 
synclinal  trough  of  lake  Superior  began  to  form  before  the  end  of  the 
Keweenaw  period,  and  consequently  that  the  Penokee  rocks  were  not 
buried  under  the  full  succession,  is  more  than  probable.  However,  they 
must  have  been  buried  to  a  very  great  depth — at  least  several  miles— and 
thus  subjected  to  high  pressure  and  temperature,  notwithstanding  which 
they  are  comparatively  unaltered.     In  the  quartz-slates  near  the  bottom 

1  Copper- Bearing  Kooks  of  Lake  Superior;  by  E.  D.  Irving,  p.  230,  U.  S.  Geol.  Survey,  Mon- 
ograph V. 


468  THE  PENOKEE  IRON-BEAEING  SEEIBS. 

of  this  pile  the  feldspars  have,  it  is  true,  in  large  measure  decomposed  to 
chlorite,  mica,  aud  quartz.  Also  the  quartzite  of  the  upper  part  of  this  for- 
mation has  been  indurated  by  the  enlargement  of  the  quartz-grains.  The 
alteration  in  parts  of  the  Iron-bearing  and  Upper  slate  formations  has  locally 
been  more  extensive.  But  the  clastic  character  of  the  fragmental  belts  is 
usually  seen  at  a  glance  under  the  microscope.  The  pressure  to  which  these 
rocks  have  been  subject,  the  amount  of  which  one  dares  hardly  to  estimate 
in  figures,  has  not  been  sufficient  to  distort  the  quartz-grains  in  any  degree ; 
nor  has  it  been  sufficient  to  give  a  schistose  structure  to  the  quartz-slates. 
It  is  a  probable  deduction  from  these  facts  that  the  weight  alone  of  any  ordi- 
nary amount  of  superincumbent  rock  is  not  sufficient  to  develop  schistose 
structure.  The  schistosity  found  in  the  various  fragmentals  of  the  Mar- 
quette, Menominee,  and  Vermilion  lake  districts,  and  in  some  places  in  the 
Eastern  area  of  the  Penokee  series,  must  have  been  developed  in  connection 
with  the  dynamic  action  of  folding  to  which  they  have  been  subjected. 

SECTION  III.— CORRELATION. 

Throughout  this  memoir  the  terms  Archean,  Laurentian,  Keewatin, 
Huronian,  etc.,  have  been  avoided.  Certain  of  these  words  have  been  so 
differently  used  by  different  writers  that  it  was  thought  best  to  free  the  dis- 
cussions involved  in  this  book  from  misapprehensions  which  would  result 
from  their  use.  The  structural  relations  of  the  series  considered  are  so  per- 
fectly distinct  that  no  evidence  or  use  of  names  from  other  localities  is 
necessary  in  order  to  make  out  the  succession  of  belts  within  the  Penokee 
district  itself,  or  the  relations  of  the  series  to  one  another.  Before  closing, 
however,  it  is  necessary  that  some  reference  be  made  to  other  series,  which 
have  been  designated  by  these  and  other  terms,  and  an  indication  be  given 
of  our  judgment  as  to  their  proper  use,  and  where  the  series  considered 
would  fall  under  that  usage. 

Equivalency  of  Penokee  series  proper  with  Animikie  series. — As  the  open- 
ing step  a  comparison  will  be  made  with  the  Animikie  series  (see  PI. 
xxxvii),  the  equivalency  of  which  with  the  Penokee  series  proper  is  as 
plain  as  the  equivalency  of  any  two  areas  of  detached  rocks  in  a  single 
geological  basin  can  possibly  be  in  which  clear  paleontological  evidence 
is  lacking.  * 


GENERAL  GEOLOdV  OK  THE   DISTUIOT.  469 

It  luis  been  set'u  tluit  jibttve  tliL'  Cliorty  liinostoiio  of"  the  Peiiokee  dis- 
trict is  an  erosion  interval  and  perhaps  a  considerable  structural  break.  In 
the  Aniniikie  district  we  know  of  no  equivalent  to  this  member,  and  in 
what  follows  it  is  excluded  ft-om  the  discussion.  The  Penokee  and  the 
Animikie  rocks  have  a  parallelism  in  lithological  characters  which  is 
remarkable.  This  parallelism  has  already  been  discussed,  but  the  main 
facts  are  here  repeated.  Not  only  is  there  a  general  likeness  between  the 
specimens  from  the  two  districts,  but  almost  every  phase  of  rock  from  the 
Animikie  series  can  be  matched  by  specimens  from  the  Penokee  series. 
In  the  Animikie  district  the  formations  underlying  the  iron-bearing  belt  are 
not  extensively  exposed,  and  consequently  little  is  known  of  the  Animikie 
equivalent  of  the  Quartz-slate  of  the  Penokee  series.  But  along  the  lower 
Current  river,  near  Port  Arthur,  Ontario,  occur  quartz-slates  underlying  the 
Iron-bearing  member  which  resemble  certain  phases  of  the  Penokee  quartz- 
slate.  Beginning  with  the  iron  formations,  the  parallelism  between  the  two 
series  is  almost  exact.  The  irony  beds  upon  Gunflint  lake,  whei'e  are  found 
the  best  known  exposures  of  the'  formation,  are  in  their  lower  parts  jasper, 
magnetite-actinolite-schist,  and  cherty  ferruginous  rocks  containing  more  or 
less  iron  carbonate.  Higher  up  are  thick  deposits  of  thinly  bedded  cherty 
iron  carbonate.  All  these  varieties  of  rock  are  found  in  the  iron  formation 
of  the  Penokee  series,  and  at  many  places  the  order  of  succession  is  the 
same.  Above  the  iron-bearing- belt  in  both  districts  is  a  great  thickness  of 
fragmental  clay-slates  and  graywacke-slates  which  are  again  practically 
identical  in  character.  It  is  true  that  in  the  western  part  of  the  Penokee 
district  mica-schists  have  developed  from  these  slates,  but  the  original  con- 
dition of  these  rocks  was  essentially  like  that  of  the  unaltered  phases. 

Underlying  both  the  Animikie  and  the  Penokee  series  is  a  complex  of 
granites  and  schists,  the  unconformity  between  which  and  these  series  is  of 
the  most  pronounced  character.  That  the  Animikie  series  is  thus  separated 
from  the  underlying  rocks  has  been  seen  by  all  who  have  studied  it,  and, 
considering  this  general  agreement,  the  proof  of  this  unconformity  will  not 
be  here  repeated.  Above  both  series  follow  the  Keweenaw  rocks.  In 
both  districts,  in  passing  at  any  place  from  the  underlying  rocks  to  the 
Keweenaw  series  in  section,  the  two  are  in  apparent  conformity;  but  when 


470  THE  PENOKEE  lEON-BEAEING  SEEIES. 

the  lines  of  contacts  between  the  iron-bearing  and  the  Keweenaw  series 
are  followed  for  some  distajice,  both  with  the  Animikie  and  Penokee  series, 
this  apparent  conformity  is  found  to  be  illusory  ;  that  is,  the  Keweenaw 
series  is  now  in  contact  with  one  member  of  the  iinderlying  series 
and  now  with  another,  until  in  both  districts  at  one  or  more  places  the 
entire  iron-bearing  series  is  entirely  cut  oflP,  the  Keweenaw  rocks  com- 
ing directly  in  contact  with  the  Basement  Complex.-'  This  means  that 
between  the  deposition  of  the  Penokee  and  Animikie  series  and  the  outflows 
of  Keweenaw  time  there  intervened  a  period  of  erosion  which  was  sufficient 
in  places  to  entirely  remove  the  two  former  and  to  cut  in  some  places  deeply 
into  the  older  rocks  themselves.  There  is,  then,  an  immense  time  gap 
between  these  series  and  the  overlying  Keweenaw  rocks,  although  this 
unconformity  does  not  approach  in  the  length  of  time  involved  to  that 
separating  them  from  the  underlying  schists  and  granites. 

The  Animikie  series,  in  its  most  typical  development,  extends  from 
Gunflint  lake,  along  the  national  lioundary  between  Minnesota  and  Ontario, 
to  Thuilder  bay,  lake  Superior.  The  Penokee  series  lies  upon  the  opposite 
side  of  lake  Superior.  -  The  latter  is  a  simple  imfolded  succession,  dipping 
to  the  northward  under  the  lake.  The  Animikie  is  another  such  siiccession, 
dipping  to  the  southward  under  the  same  body  of  water.  There  is,  then, 
little  doubt,  considering  all  the  facts,  that  the  two  series  represent  a  single 
period  in  the  history  of  the  synclinal  trough  which  forms  the  basin  of  lake 
Superior.^  The  relations  and  likeness  of  the  Penokee  and  the  Animikie 
series  have  been  dwelt  upon  at  length  as  showing  the  breadth  of  the 
geological  basin  in  which  the  deposition  of  like  rocks  Avas  taking  place 
simultaneously.  The  equivalency  here  shown  is  a  long  step  in  under- 
standing the  equivalency  of  other  rocks  in  the  lake  Superior  basin. 

Equivalency  of  Penokee  and  Marquette  series. — A  comparison  of  the 

'  For  full  discussion  of  the  proof  of  the  uncpnformity  hetween  th'e  Animikie  antl  Keweenaw 
Beries  see  E.  D.  Irving :  On  the  Classification  of  the  Eai'ly  Cambrian  and  pre-Cambrian  Formations, 
7th  Auunal  Report  U.  S.  Geo].  Survey,  pp.  417-423.  If  the  Keweenaw  series  simply  were  in  contact 
with  the  gneisses  and  granites  it  might  be  hehl  that  we  have  only  to  do  with  an  overlap,  but  its  con- 
tact— now  with  one  horizon  of  the  Animikie,  now  with  another — can  only  be  explained  by  an  uncon- 
formity. 

^E.  D.  Irving:  Copper-Bearing  Rocks  of  Lake  Superior,  U.  S.  Geol.  Survey,  Monograph  v,  1883, 
pp.  410-418. 


GENERAL  (ilCOl.OCiV   <)K  TIIK   DISTKICT.  471 

Penokoe  aiul-  .M,ii-([iu'tte  .successions  shows  that   hctwccu  the   iwo  tlierc  is  u 
very  close  correspoudeiice. 

Uiicouforuiably  below  the  elastics  of  tliosc  districts  is  a  crystalline 
Basement  (!()ni[)lex  composed  of  schists,  g-neisses  and  granites.  Witliin  the 
pre-Keweeuavvan  cla.stics  in  (ia-cli  district  is  a,  second  physical  bn^ak.  In 
the  Peuokee  district  the  series  below  the  break  is  known  to  be  represented 
only  by  a  single  formation,  the  Clierty  limestone.  That  other  higher 
formations  once  here  existed  is  indicated  by  the  presence  of  fragments  of 
jasper  and  quartzite  in  the  lowest  horizon  of  the  Quartz-slate.  These  jasper 
frag'ments  occur  in  the  basal  conglomerate  of  the  Quartz-slate  at  Potato 
river,  at  two  localities  near  the  Palms  mine,  and  at  one  place  in  the  Eastern 
area.  Usually  the  pebbles  are  not  of  large  size,  but  occasionally  they  are 
several  inches  in  diameter.  Quartzite  pebbles  are  even  more  abundant 
than  those  of  jasper.  It  is,  therefore,  probable  that  the  three  sedimentary 
formations  of  the  Lower  Marquette  series  were  once  represented  by  equiva- 
lent members  in  the  Penokee  district. 

The  correspondence  of  the  members  of  the  Penokee  series  j)roper 
with  those  of  the  Upper  Marquette  is  complete.  The  Upper  Marqnette  and 
Penokee  series,  looked  at  broadly,  are  great  slate  formations,  both  of  which 
contain,  near  the  base,  an  iron-bearing  horizon.  In  the  Penokee  series 
that  portion  of  the  slate  overlying  the  ore  formation  has  been  called  the 
Upper  slate  member,  and  that  below  it  the  Quartz-slate  member.  The 
lower  part  of  the  Quartz-slate  is  a  quartzite  and  conglomerate  which  cor- 
responds to  the  quartzite  and  conglomerate  forming  the  base  of  the  upper 
Marquette  series.  The  upper  most  horizon  of  the  Penokee  Quartz-slate  is  a 
narrow  layer  of  persistent  quartzite  which  does  not  appear  to  be  repre- 
sented in  the  Marquette  district.  The  character  of  the  ore-bearing  mem- 
ber is  identical  in  both  districts,  being  unquestionably  derived  from  a  lean 
cherty  carbonate  of  iron.  The  characteristic  rocks  of  both  are  now  the 
iron  carbonates,  cherts  containing  bands  and  shots  of  ore,  and  the  iron  ores. 
The  chief  differences  between  the  two  are  that  in  the  Penokee  district  the 
actinolite-magnetite-schists  are  more  prevalent,  that  the  iron-bearing  for- 
mation is  more  persistent,  and  that  its  ore  bodies  are  more  abundant.  Con- 
nected with  these  facts  is  perhaps  the  presence  of  the  upper  horizon  of 


472  THE  PENOKEE  lEON-BEAEING  SEEIES. 

quartzite  in  the  Quartz-slate,  which  shows  that  a  clearing  up  of  the  waters 
occurred  before  the  beginning  of  deposition  of  the  iron-bearing  sediments. 
A  still  further  analogy  between  the  Penokee  and  Upper  Marquette  series 
is  the  presence  in  both  of  abundant  surface  volcanics.  We  have,  then,  in 
the  two  districts  the  following  parallel  descending  pre-Keweenawan  suc- 
cession: 


Fenohee. 

Upper  slate,  locally  mica-scMst. 

Iron-bearing  formation. 

Quartz-slate;  upper  liorizou  persistent 
quartzite;  central  mass  a  slate;  lower 
part  often  conglomeratic,  bearing  frag- 
ments of  lower  series,  either  Cherty 
limestone  or  Basement  Complex,  and 
locally  a  quartzite. 

Unconformity. 

Eroded  away. 

Limestone. 

Unconformity. 

Basement  complex. 


Marqiiette. 

Upper  slate,  rather  extensively  mica-schist. 

Iron-bearing  formation. 

Lower  slate;  lower  part  quartzite  or 
quartzite  conglomerate,  bearing  frag- 
ments of  lower  series,  either  lower  Mar- 
quette or  Basement  Complex. 


Unconformity. 
Iron-bearing  formation. 
Limestone  and  lower  quartzite. 
Unconformity. 
Basement  comiilex. 


Comparison  with  other  series. — As  is  well  known,  in  the  region  about 
lake  Superior  are  other  areas  containing  limestone,  quartzites,  graywacke, 
gray  wacke-slates,  mica-slates,  mica-schists,  volcanic  elastics,  and  the  peculiar 
phases  of  rock  of  the  iron-bearing  formations.  The  positions  of  these  various 
areas  as  shown  upon  PL  i,  are  designated,  respectively,  by  H,  H2,  H3,  H^, 
etc.  These  have  been  known  in  the  past  as  the  original  Huronian  district, 
the  Marquette,  Felch  Mountain,  Menominee  iron-bearing  districts,  the  St. 
Louis  slates,  the  Chippewa  valley  quartzites,  the  Black  river  Iron-bearing 
schists,  the  Baraboo  quartzites,  the  Sioux  quartzites,  and  the  folded  schists 
of  Canada,  including  the  Vermili  on  series. 

The  relations  of  these  series  to  one  another  and  to  the  Penokee  series, 
less  closely  related  with  the  Penokee  district  than  are  the  Animikie  and  the 
Marquette,  are  fully  considered  in  a  bulletin  of  the  United  States  Geologi- 
cal Survey  on  the  Algonkian  and  Archean.^  As  a  result  of  the  discussion 
there  sfiven,  the  relations  of  the  series  of  these  difFerent  districts  are  tabu- 
lated  as  follows : 


iC.  E.  Van  Hise:  Correlation  papers,  Algonkian  and  Archean.     Bull.  86,  U.  S.  Geol.  Survey. 


GENEUAL  GEOLOGY  OF  THE  DISTRICT. 


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474  THE  PENOKEE  lEON-BBAEING  SERIES. 

To  give  in  full  the  reasons  for  the  positions  assigned  to  these  various, 
series  and  the  underlying  and  overlying  rocks  would  be  but  to  repeat  the 
discussion  before  referred  to.  In  the  above  table  the  Penokee  series  proper, 
including  the  Quartz-slate,  Iron-bearing,  and  Upper  slate  members,  is  placed 
as  the  equivalent  of  the  Upper  Huronian,  Upper  Marquette,  Upper  Ver- 
milion, and  the  quartzites  of  Wisconsin,  Minnesota,  and  Dakota,  primarilj^ 
on  structural  and  secondarily  on  lithological  grounds,  while  the  Cherty 
limestone  at  the  base  of  the  series,  being  separated  by  a  considerable  erosion 
interval  from  the  Penokee  series  proper,  is  regarded  on  grounds  of  a 
similar  character  as  equivalent  to  some  part — probably  a  lower  part — of 
the  Lower  Huronian,  Lower  Kaministiquia,  Lower  Vermihon,  Lower  Mar- 
quette, and  equivalent  series.  The  position  assigned  to  this  formation  is 
warranted  because  of  the  very  considerable  time  break  between  the  Cherty 
limestone  and  the  Penokee  series  proper. 

The  Penokee  series  and  those  series  equivalent  to  it  constitute  a  part 
of  the  great  Algonkian  system,  and  the  Southern  Complex  is  Archean. 


PLATE    XIV. 


475 


Plate  XIV. — From  the  Southern  Complex. 

Fig.  1.  Biotite-granite.  Specimen  9639,  slide  4228.  From  the  NE.  i  of  the  NE.  i  Sec.  20,  T.  44  N.,  E. 
3  W.,  Wiscousin.  In  polarized  light,  x  25.  The  section  shows  the  strongly  feldspathic 
character  of  the  rock  and  the  abundance  of  nonstriated  feldspar,  although  striated  feldspar, 
both  microcline  and  plagioclase,  are  seen.     The  dark  areas  are  largely  biotite. 

Fig.  2.  Biotitic  granitoid  gneiss.  Specimen  9674,  slide  3394,  0  steps  N.,  160  steps  W.,  of  the  south- 
east corner  of  Sec.  23,  T.  44  N.,  E.  5  W.,  Wisconsin.  Upon  the  Marengo  river.  In  polarized 
light,  X  25.  The  section  shows  the  alteration  of  feldspar  into  biotite  and  quartz.  Large, 
irregular,  much  altered  areas  of  orthoclase  and  plagioclase  are  contained  in  a  fine  ground- 
mass  composed  of  quartz  and  biotite.  The  aggregates  of  these  minerals,  besides  filling  the 
interspaces,  cut  into  the  larger  feldspar  areas,  so  that  there  is  a  gradation  from  the  fine 
grained  background  into  the  feldspar.  In  certain  cases  this  alteration  has  extended  quite 
to  the  centers  of  the  feldspar  individuals.  This  figure  suggests  that  the  rock  was  once  very 
much  more  strongly  feldspathic  than  at  present — perhaps  as  strongly  feldspathic  as  the  rock 
in  the  previous  figure — and  that  the  alterations  have  changed  it  into  a  strongly  biotitic  and 
quartzose  rock.  The  foliation  which  is  now  apparent  in  the  rock  may  also  be  one  of  the 
results  of  this  alteration. 

Fig.  3.  Hornblende-schist.  Specimen  9050,  slide  2776.  From  near  the  northwest  corner  of  Sec.  35,  T. 
46  N.,  R.  2  E.,  Wisconsin.  In  polarized  light,  X  25.  The  groundmass  of  the  section  is  com- 
posed of  finely  crystalline,  closely  interlocking  quartz  and  hornblende.  Within  this  ground- 
mass  are  nimierous  large  roundish  areas  of  feldspar,  which  are  now  on  their  outer  parts 
apparently  altering  into  quartz  and  hornblende.  The  appearance  of  the  feldspar  suggests 
a  fragmental  character,  but  however  closely  examined,  nothing  else  in  the  section  gives 
additional  light  upon  this  question.  If  any  of  the  hornblende  schists  of  the  Southern  Com- 
plex are  clastic  (but  this  is  doubtful),  this  is  probably  one  of  them. 

Fig.  4.  Hornblende-gneiss.  Specimen  9060,  slide  2921.  From  the  south  part  of  the  SW.  i  Sec.  33,  T. 
.  46  N.,  E.  2  E.,  Wisconsin.  In  ordinary  light,  X  25.  The  light  background  is  composed  of 
finely  crystalline  quartz  mingled  with  a  good  deal  of  feldspar,  the  latter  comu  only  being  in 
larger  areas  than  the  quartz  and  often  having  roundish  outlines.  These  facts  are  not 
apparent  in  the  figure.  The  slide  is  reproduced  to  show  the  peculiar  character  of  the  horn- 
blende individuals.  They  run  from  minute  fibers  up  to  tolerably  large  blades  which  have 
extremely  ragged  outlines.  They  cut  the  quartz  and  feldspar  through  and  through.  Their 
appearance  is  such  as  to  suggest  that  they  are  now  in  the  process  of  grovrth,  and  in  thin 
section  the  relations  of  the  hornblende  and  .quartz  to  the  feldspar  farther  suggest  that  from 
the  feldspar  these  minerals  have  developed.  If  all  of  the  minerals  now  present  are  original, 
the  hornblende  must  certainly  have  been  the  first  to  crystallize,  so  thoroughly  does  it  pene- 
trate the  others ;  but  this  is  very  much  less  probable  than  that  it  has  developed  as  tl  e  last 
mineral  of  the  rock  and  subsequent  to  its  consolidation. 

476 


U.   S.   CeOLOOICAt.  fiunvEY 


MONOORAPH  XIX      PLATE  XlV 


Biotite-granite, 


Fig.  2.— Biotitic  granitoid  gneiss. 


Fig.  3. — Hornblende-schist. 


Hornblende-schist. 


THIN   SECTIONS   FROM   THE  SOUTHERN    COMPLEX. 


PLATE   XV. 


477 


Plate  XV. — Feom  the  Southern  Complex. 

Fig.  1.  Hornblende-granite.  Specimen  12873,  slide  5504.  From  the  NE.  i  of  the  SW.  i  Sec.  23,  T.  47 
N.,  R.  47  W.,  Michigan.  South  of  Aurora  mine.  In  polarized  light,  X  25.  The  strongly 
feldspathic  character  of  the  granite  of  the  Southern  Complex,  shown  by  PI.  xiv,  Fig.  1,  is 
again  brought  out,  although  more  quartz  is  present  than  in  the  previous  figure.  A  larger 
proportion  of  the  feldspar  is  microcline.  The  pegmatitic  structure  of  quartz  and  feldspar  is 
nicely  shown  iu  one  place.  The  dark  areas  are  mostly  hornblende.  In  one  individual  twin- 
ning is  seen. 

Fig.  2.  Horublende-biotite-syenite.  Specimen  7615,  slide  2070.  From  near  the  northeast  corner  of 
Sec.  27,  T.  47  N.,  R.  47  W.,  Michigan.  In  ordinary  light,  X  25.  A  coarsely  crystalline 
feldspathic  l)ackground  is  apparent.  The  individuals  of  feldspar  are  large  and  fit  in  much 
the  same  manner  as  does  the  feldspar  in  the  massive  syenites  and  granites  from  this  vicinity ; 
but  in  this  rock  it  is  cut  through  and  through  with  hornblende  and  biotite,  which  are 
arranged  approximately  with  their  longer  axes  in  a  common  direction  and  thus  give  the 
rock  its  foliation.  The  relations  suggest  the  secondary  development  of  the  hornblende  and 
biotite  within  the  feldspar  by  dynamic  metamorphism. 

Fig.  3.  Biotite-gneiss.  Specimen  7529,  slide  1963.  From  a  short  distance  south  of  the  north  quar- 
ter post  of  Sec.  18,T.  47  N.,  R.  45  W.,  Michigan.  In  polarized  light,  X  25.  The  section  is 
composed  of  small  particles  of  nearly  uniform  size,  consisting  of  quartz  and  feldspar  min- 
gled with  biotite.  The  roundish  and  yet  closely  fitting  character  of  the  quartz  and  felds- 
par is  nicely  shown.  There  are  also  seen  a  few  larger  roundish  grains  of  quartz.  This  is 
one  of  the  rocks  of  the  Southern  Complex  which  has  a  strong  fragmental  appearance,  and 
yet  there  is  no  certain  evidence  that  it  is  clastic.  The  particles  now  perfectly  fit  one  another, 
therefore  they  could  not  thus  have  been  mechanically  deposited.  None  of  them  show  evi- 
dence of  enlargement,  so  the  crystalline  appearance  can  not  be  accounted  for  iu  this  way. 

Fig.  4.  Hornblende-gneiss.  Specimen  9458,  slide  3077.  From  near  the  south  quarter  post  of  Sec. 
16,  T.  47  N.,  R.  45  W.,  Michigan.  In  ordinary  light,  X  25.  The  background  of  the  section 
is  composed  of  small,  perfectly  fitting,  roundish  granules  of  quartz  and  feldspar  in  nearly 
equal  quantity.  Contained  within  this  groundmass  are  numerous  large  crystals  of  horn- 
blende, which  in  transverse  sections  often  have  well  developed  crystal  outlines,  the  forms 
being  usually  the  pinacoid  and  unit  prism.  Each  individual  of  hornblende  includes  many 
grains  of  quartz  and  feldspar.  The  hornblende  must  have  been  here  the  last  mineral  to 
develop,  since  it  includes  so  large  a  proportion  of  the  other  minerals.  Its  relations  to  the 
other  minerals  taken  in  connection  with  its  crystal  forms  make  its  occurrence  analogous 
to  such  metamorphic  minerals  as  garnet  and  stauiolite. 
478 


U.   8.   OEOLOOICAL  SURVEY 


MONOGRAPH  XIX      PLATE  XV 


Fig.  1. — Hornblende-granite. 


Fig.  2.— Hornblende  biotite-syenite. 


Fig.  3. — Biotite-gneiss.  Fig.  4.~Hornblende-gneiss. 

THIN  SECTIONS  FROM  THE  SOUTHERN   COMPLEX. 


PLATE   XVI. 


479 


Plate  XVI. — From  the  Cherty  Limestone  Member. 

Fig.  1.  Tremolitic  dolomite.  Specimen  9678,  slide  3165.  From  the  NW.  i  of  Sec.  22,  T.  44  N.,  E.  5  W., 
Wisconsin.  lu  ordinary  light,  x  60.  The  finer  grained  part  of  the  section  shows  the  evenly 
granular  appearance  characteristic  of  the  massive  limestones  and  dolomites.  In  one  part 
of  the  figure  is  coarsely  crystalline  carbonate  and  a  broad  blade  of  tremolite.  The  sec- 
tion chances  to  be  so  cut  that  this  mineral  appears  in  part  as  a  mere  film,  below  which  the 
vague  outlines  of  the  particles  of  dolomite  are  seen.     (See  p.  135.) 

Fig.  2.  Cherty  limestone.  Specimen  7485  A,  slide  1934.  From  the  SE.  J  of  Sec.  18,  T.  47  N.,  R.  44  W. , 
Michigan.  In  polarized  light,  X  60.  The  central  band  of  the  figure  is  largely  granular 
dolomite,  in  which  is,  however,  a  considerable  quantity  of  chert.  This  dolomite  is  inter- 
laminated  with  two  layers  of  nearly  pure  chert,  one  of  which  is  much  more  finely  crys- 
talline than  the  other.      (See  p.  187.) 

Fig.  3.  Concretionary  chert.  Specimen  9434,  sUde  3131.  From  the  NW.  i  of  Sec.  41,  T.  47  N.,  E. 
45  W.,  Michigan.  In  polarized  light,  X  60.  A  fine  grained  chert.  The  minute  mosaic  char- 
acteristic of  chert  when  viewed  in  polarized  light  is  nicely  shown.  The  material  is 
arranged  in  a  vague  concretionary  fashion,  areas  composed  of  very  finely  crystalline  silica 
being  surrounded  by  borders  of  more  coarsely  crystalline  quartz.  Although  imperfectly 
shown  in  the  figure,  the  latter  has  to  some  extent  the  radial  fibrous  arrangement  of  chal- 
cedony.    (See  p.  137.) 

Fig.  4.  Chert.  Specimen  9424,  slide  3064.  From  the  NW.  i  of  Sec.  14,  T.  47  N.,  E.  45  W.,  Mich- 
igan. In  jjolarized  light,  X  60.  The  section  shows  very  well  the  variations  in  fieneness  of 
grain  of  the  more  coarsely  crystalline  varieties  of  chert.  It  is  wholly  composed  of  com- 
pletely individualized  quartz,  the  intricate  interlocking  of  which  is  well  shown  and  which 
is  in  strong  contrast  to  the  appearance  presented  by  quartzites  in  which  the  interspaces 
have  been  filled  by  the  enlargements  of  fragmental  quartz.  (See  p.  137.) 
480 


U.  6.  QEOLOQICAU  SURVEY 


MONOGHAPH  XIX      PLATE  XVI 


Fig.  1. — Tremolitic  dolomite. 


Fig.  2. — Cherty  limestone. 


Fig.  3. — Concretionary  chert,  pjg.  4 Chert. 

THIN    SECTIONS    FROM   THE   CHERTY   LIMESTONE   MEMBER. 


PLATE  XVII. 


4gi 

MON   XIX 31 


Plate  XVII. — From  the  Base  of  the  Qtjartz-Slatb  Member. 

Fig.  1.  Chert  containiug  fragmeutal  quartz.  Specimeri  9534,  slide  3140.  From  the  NW.  i  of  See. 
14,  T.  44  N.,  R.  3  W.,  Wisconsin.  In  polarized  light,  x  30.  The  background  of  the  section 
is  composed  of  finely  crystalline  quartz.  In  it  are  contained  large  well  rounded  grains  of 
quartz,  oup  of  which  has  plainly  received  a  second  growth.  In  the  lower  part  of  the  figure 
is  a  complex  rounded  area  of  chert  not  greatly  different  from  the  matrix  in  which  it  is  set. 
(See  pp.  157-15&.) 

Fig.  2.  Quartzose  chert.  Specimen  953.5,  slide  3141.  From  the  same  place  as  Fig.  1.  In  polarized 
light,  X  60.  The  center  of  the  figure  shows  in  the  cherty  background  a  large  quantity 
of  magnetite.  In  the  exteriors  of  the  large  fragmeutal  grains  of  quartz^re  very  numerous 
minute  needles  which  are  taken  to  be  actinolite.  In  certain  cases  these  actinolite  needles 
are  included  only  in  the  enlargements  of  the  quartz  grains,  but  in  others  appear  to  penetrate 
rather  deeply  into  the  cores.  This  occurrence  strongly  suggests  that  actinolite  may  develop 
in  such  a  manner  as  to  penetrate  quartz  the  latter,  of  course,  being  simultaneously  removed. 
(See  pp.  157-158.) 

Fig.  3.  Chert-conglomerate.  Specimen  9419,  slide  3127.  From  the  SW.  i  of  Sec.  14,  T.  47  N.,  E.  45 
W.,  Michigan.  In  i)olarized  light,  X  60.  The  background  of  the  conglomerate  is  a  chloritic 
quartz-slate,  the  (luartz-grains  of  which  at  many  places  distinctly  show  the  enlargements. 
The  large  roundish  area  with  the  spotty  appearance  is  a  part  of  one  of  the  smaller  of  the 
chert  fragments  which,  iu  the  form  of  pebbles  and  bowlders,  are  very  abundantly  contained 
in  the  rock.     (Seep.  169.) 

Fig.  4.  C4reeu-schist  conglomerate.  Specimen  0175,  slide  2994.  From  the  SE.  ^  of  Sec.  19,  T.  45  N., 
R.  1  E.,  Wisconsin.  In  polarized  light,  X  60.  The  figure  is  from  the  Potato  river  basal 
conglomerate.  For  the  most  part  it  consists  of  three  large  fragments  of  green  schist,  which 
in  (me  case  very  distinctly  shows  the  scJiistose  structure.  These  fragments  are  set  in  a 
u»atrix  consisting  very  largely  of  small  roundish  quartz-grains,  mingled  with  which  are 
quite  numerous  well  defined  crystals  of  magnetite. 
(See  p.  159.) 
482 


U.   S.   OEOLOGtCAL  SURVEY 


MONOGRAPH  XIX      PLATE  XVII 


Fig.  1. — Chert  containing  fragmental  quartz. 


Fig.  2.^Quartzose  chert. 


Fig.  3.— Chert-conglomerate.  Fig.  4.— Green  schist  and  conglomerate. 

THIN  SECTIONS  FROM   THE  BASE  OF  THE  QUARTZ-SLATE  MEMBER. 


PLATE    XVIII. 


i83 


Plate  XVIII.— From  the  Qtjautz-Slate  Member. 

Fig.  1.  Graywaoke-slate.  Specimen  9442,  slide  3071.  From  the  SW.  J  of  Sec.  10,  T  47  N.,  E.  45 
W.,  Michigan.  In  ordinary  light,  x  60.  The  figure  represents  a  typical  average  grained 
gr.iywaoke-slate.  The  rounded  fragments  are  in  about  equal  quantity  quartz  and  feldspar. 
Chlorite  is  the  chief  insterstltial  mineral.     (See  pp.  165-166.) 

Fig.  2.  The  same  in  polarized  light.  The  orthoclase  is  in  most  cases  separable  from  the  quartz, 
in  that  it  lacks  the  perfect  clearness  and  uniformity  of  color  which  each  grain  of  that 
mineral  shows.     The  striated  feldspars  are  nicely  shown.     (See  pp.  165-166.) 

Fig.  3.  Cherty  slate.  Specimen  9641,  slide  3310.  From  the  NW.  i  of  Sec.  13,  T.  47  N. ,  K.  46  W., 
Michigan.  In  polarized  light,  X  60.  The  background  of  the  section  is  an  ordinary  chlo- 
ritic  slate.  It  contains,  however,  in  certain  layers  numerous  large  well  rounded  frag- 
ments of  (luartz  and  chert,  both  of  which  are  well  shown  in  the  figure.     (See  pp.  164-165.) 

Fig.  4.  Sericitic  and  chloritic  slate.  Specimen  9523,  slide  3091.  From  the  SE.  i  of  Sec.  16,  T.  47  N., 
R.  46  W.,  Michigan.  In  polarized  light,  X  60.  The  .section  illustrates  one  of  the  rather  uni- 
form fine  grained  fcldspathic  quartz-slates.  The  small  roundish  areas  are  in  part  quartz 
and  iu  part  feldspar.  The  dark  material  is  mostly  chlorite  and  grains  of  quartz  and  feld- 
spar, which  chance  to  be  near  the  point  of  extinguishment.     (See  p.  164.) 

484 


U.  8.  QEOLOQtCAL  SURVEY 


MONOGRAPH  XIX      PLATE  XVIII 


Fig.  1.— Grayv»acke-slate. 


Fig.  2.— Graywaclie-slate  in  polarized  light. 


Fig.  3.— Cherty  slate, 


Fig.  4.— Sericitic  and  chloritic  slate. 
THIN   SECTIONS   FROM   THE   QUARTZ-SLATE   MEMBER. 


PLATE   XIX. 


485 


Plate  XIX.— From  the  Quaktz-Slate  Member. 

Fig.  1.  Biotitic  chlorite-slate.  Specimen  9565,  slide  3098.  From  tlie  NW.  J  of  Sec.  14,  T.  44  N.,  E.  3 
W.;  Wisconsin.  In  polarized  light,  X  60.  Many  small  oval  and  angular  particles  of  quartz 
are  contained  in  a  matrix  consisting  very  largely  of  chlorite,  with  which  is,  however,  a 
small  amount  of  biotite.  The  angularity  of  the  quartz-grains  is  A'ery  noticeable.  This  is 
in  some  cases  due  to  the  enlargement  of  the  quartz-grains,  but  often  they  were  thus  angu- 
lar when  deposited.  The  figure  has  a  more  crystalline  appearance  than  the  section  from 
which  it  was  taken,  from  the  fact  that  the  cores  of  the  quartzes  which  have  undergone  a 
second  growth  are  not  strongly  marked  off  from  the  enlargements,  although  in  thin  section 
they  are  easily  distinguished.     (Seep.  158.) 

Fig.  2.  Biotite-slate.  Specimen  9644,  slide  3154.  From  the  NE.  i  of  Sec.  17,  T.  44  N.,  R.  3  W.,  Wis- 
consin. In  ordinary  light,  x  60.  A  few  grains  of  quartz  with  general  roundish  forms, 
although  now  minutely  angular  by  enlargement,  are  contained  in  a  background  which 
consists  almost  wholly  of  biotite  and  quartz.  In  the  light  irregular  areas  quartz  is  pre- 
dominant, and  elsewhere  biotite.  While  the  fragmental  character  of  this  rock  is  not 
evident  in  the  figure,  it  is  plain  in  thin  section.  The  rock,  however,  is  one  of  the  most 
crystalline  of  the  biotite-slates  which  are  found  in  the  Quartz-slate  member.  It  is  wholly 
possible  that  the  irregular  whitish,  roundish  areas  now  composed  of  quartz  and  mica  rep- 
resent original  fragmental  grains  of  feldspar.     (See  p.  156.) 

Fig.  3.  Sandstone.  Specimen  9004,  slide  288a  From  the  NW.  i  of  Sec.  27,  T.  47  N.,  R.  47  W.,  Mich- 
igan. In  ordinary  light,  x  60.  Quartz  in  roundish  and  irregular  fragmental  grains,  is 
the  predominating  constituent,  although  feldspar  is  important.  The  abundant  interstitial 
material  is  so  heavily  stained  with  oxide  of  iron  that  it  is  difficult  to  determine  what  other 
luineruls  ;ire  present.     (Seep.  163.) 

Fig.  4.  Argillaceous  shale.  Specimen  7504,  slide  1946.  From  the  NE.  i  of  Sec.  15,  T.  47  N.,  R.  45  W., 
Michi'4:m.  In  ordinary  light,  x  60.  The  section  shows  one  of  the  finer  grained  and  more 
clayey  phases  of  the  Quartz-slate  member.  On  one  side  of  the  figure  recognizable  frag- 
ments of  quartz  and  feldspar  are  the  chief  constituents.  Upon  the  other  side  extremely 
finely  divided  clayey  minerals  are  preponderant.     (Seep.  167.) 

486 


U.  S.  OEOUOQICAL  SURVfY 


MONOCBAPH  XIX      PLATE  XIX 


Fig,  1. — Biotitic  chlorite-iilate. 


Fig.  2.— Biotite-slate. 


Fig.  3,— Sandstone.  pig,  4,_Argillaceous  slate. 

THIN   SECTIONS   FROM   THE   QUARTZ-SLATE   MEMBER, 


PLATE    XX. 


487 


Plate  XX. — Pbom  the  Upper  Horizon  of  the  Quartz-Slate  Member. 

Fig.  1.  Quartzite.  Specimen  9082,  slide  2780.  From  tlie  east  side  of  Sec.  19,  T.  45  N.,  R.  1  E., 
Wisconsiu.  lu  ordinary  light,  X  25.  The  rock  is  a  vitreous  quartzite,  and  jet  the  figure 
shows  with  perfect  distinctness  the  outlines  of  each  of  the  rounded  grains  of  sand  just 
as  they  were  origiually  deposited.     (See  p.  160.) 

Fig.  2.  The  same,  in  polarized  light.  The  cause  of  the  present  strong  and  vitreous  character  of  the 
quartzite  is  clearly  shown  by  this  figure.  Each  of  the  grains  of  quartz  has  added  to  itself 
other  quartz  until  the  grains  have  met  and  interlocked.  This  is  a  fine  instance  of  the 
induration  of  a  sandstone  liy  simple  enlargement  of  the  original  grains.     (See  p.  160.) 

Fig.  3.  Ferruginous  quartzite.  Specimen  9154,  slide  2804.  From  the  SW.  i  of  Sec.  27,  T.  46  N.,  R." 
2  E.,  Wisconsin.  In  polarized  light,  X  25.  The  rock  is  again  a  vitreous  quartzite,  which 
has,  however,  a  Lrown  color.  The  outlines  of  the  original  grains»are  distinctly  seen  as 
in  Fig.  1.  The  cohu'  of  the  rook  is  due  to  the  oxide  of  iron  located  in  the  interstices.  (See 
p.  162.) 

Fig.  4.  The  same,  in  polarized  light.  As  in  Fig.  2,  each  of  the  rounded  grains  of  quartz  has  been 
enlarged.  The  interlocking  of  these  enlargements  has  been  interfered  with  by  the  included 
iron  oxide,  so  that  the  Vock  is  not  so  vitreous  and  strong  as  that  from  which  Figs.  1  and  2 
are  taken.     (See  p.  162.) 


U.   S.   QEOLOOICAL  SURVEY 


MONOOHAPH  XIX       PLATE  XX 


Fig.  l.-Quartz(te 


Fig.  2  — Quditzite,  in  polarized  light. 


Fig.  3 — Ferruginous  quartzite.  Fig.  4.— Ferruginous  quartzite,  in  polarized  light. 

THIN  SECTIONS  FROM  THE  UPPER  HORIZON  OF  THE  QUARTZ-SLATE  MEMBER. 


PLATE    XXI. 


489 


Plate  XXI. — Sideritic  Eocks,  prom  the  Iron-Bearing  Member  and  prom 

Lawrence  County,  Ohio. 

Fig.  1.  Sideritic  chert.  Specimen  9814,  slide  3880.  "Limestone  ore."  Lawrence  county,  Ohio. 
In  ordinary  light,  X  25.  The  section  has  a  cherty  background,  which  contains  large  oval 
and  roughly  rhombohedral  areas  of  iron  carbonate.  It  is  introduced,  here  in  order  to  com- 
pare this  carbonate  with  those  of  the-Penokee  and  Animikie  districts. 

Fig.  2.  The  same,  in  polarized  .liglat.  The  fine  mosaic  character  of  the  cherty  background  is  here 
exhibited.  The  outlines  of  the  carbonate  areas  become  less  distinct  than  in  the  previous 
figure. 

Fig.  3.  Sideritic  slate.  Specimen  9191,  slide  29S6.  From  the  NE.  i  Sec.  6,  T.  45  N.,  R.  2  E.,  Wis- 
consin. In  ordinary  light,  x  60.  The  section  is  cut  transversely  to  the  lamination  of 
the  rock.  The  hand  specimen  is  very  thinly  and  regularly  laminated.  The  laminae  are 
seen  to  be  somewhat  irregular.  The  obscure  dark  and  light  portions  are  both  iron  car- 
■*  bonate,  which  differ  chiefly  in  the  inclusions  which  they  contain.  The  minute  white 
particles  are  chert  and  amorphous  silica.     (Seep.  237.) 

Fig.  4.  Sideritic  and  ferruginous  chert.  Specimen  9474,  slide  3082.  From  Sec.  13,  T.  47  N.,  R.  46 
W.,  Michigan.  In  ordinary  light,  x  60.  The  section  shows  the  alteration  of  iron  carbon- 
ate into  iron  oxide.  The  background  is  chert.  Upon  one  side  of  the  figure  are  rhombo- 
hedra  of  little  altered  siderite.  Upon  the  other  are  black  pseudomorphous  areas  which 
are  composed  of  somewhat  hydrated  hematite.  Between  the  two  are  gradation  phases. 
(See  p.  237.) 
490 


U.  8.   OEOLOOICAL  SURVEY 


MONOGRAPH  XIX      PLATE  XXI 


Fig.  1.— Sideritic  chert. 


Fig.  2. — Sideritic  chert,  in  polarized  light. 


%;;;^^SS^^'^i»^''    *' 


.^-^■^;      '..r-^-y'^- 


^;:-'.. 


^^'mi--^^'f' 


.T^^f^a^:-- 


:;''%^ 


Fig.  3.— Sideritic  slate.  Fig-  4.— Sideritic  and  ferruginous  chert, 

THIN   SECTIONS   OF  SIDERITIC   ROCKS   FROM   THE   IRON-BEARING    MEMBER   AND    FROM    LAWRENCE  COUNTY,   OHIO. 


PLATE    XXII, 


491 


Plate  XXII.— Ferbu&inous  Cherts  prom  the  Iron-Bearing  Member. 

Fig.  1.  Concretionary  cliert.  Specimen  9048,  slide  2886,  From  the  SE.  i  Sec.  27,  T.  46  N.,  E.  2  E., 
Wisconsin.  In  ordinary  light,  X  25.  In  a  cberty  background  are  beautiful  concretions, 
wlilch  are  composed  of  concentric  rings  of  iron  oxide  and  chert.  One  concretion  particu- 
larly is  very  fine,  showing  many  closely  packed  concentric  rings.  Silica  is  seen  breaking 
across  these  rings  in  a  few  places.     (See  pp.  227-228. ) 

Fig.  2.  The  same,  in  jiolarized  light.  Here  the  cherty  backgrotind  appears  as  a  fine  mosaic.  The 
quartz  in  and  about  the  concretions  is  more  coarsely  crystalline  than  the  average  of  that  in 
the  matrix.  One  concretion  has  as  a  nucleus  comparatively  closely  crystalline  quartz.  This 
variation  in  the  character  of  the  silica  is  suggestive  that  the  flue  spotty  silica  is  perhaps 
original.  The  concretions,  as  shown  by  subsequent  plates,  have  been  produced  from  iron 
carbonate,  and  as  the  iron  oxide  formed  from  the  carbonate,  the  remaining  space  was  occu- 
pied by  silica,  which  crystallized  in  larger  particles  than  the  supposed  original  silica.  (See 
pp.  227-228.) 

Fig.  3.  Brecciated  chert.  Specimen  7622,  slide  2072.  From  the  Montreal  river,  between  Michigan 
and  Wisconsin.  In  polarized  light,  x  25.  The  background  is  again  cherty,  and  contains 
within  it  many  small,  rather  perfect  rhombohedra  of  siderite,  which  have  altered  to  a 
greater  or  less  degree  to  iron  oxide.  Contained  in  this  groundmass  are  irregular  areas 
which  do  not  have  a  concretionary  structure,  but  appear  to  be  true  fragments,  in  this 
matrix.  One  of  the  areas  is  severed  in  every  direction  liy  numerous  ramifying  veinlets  of 
silica.  The  same  thing  is  to  a  less  degree  noticeable  of  others  of  these  areas.  That  this 
cutting  silica  is  secondary  can  hardly  be  doubted.     (See  pp.  230-231.) 

Fig.  4.  Ferruginous  and  brecciated  chert.  From  the  same  locality  as  the  last.  In  polarized  light, 
X  25.  The  background  is  chert,  as  in  the  previous  figure.  In  this  background  are  found 
perfect  concretions  and  brecciated  areas.  One  of  the  latter  shows  plainly  its  fragniental 
character.  It  is  built  up  of  laminse  which  are  approximately  parallel  and  have  evidently 
been  broken  from  a  regularly  laminated  rook  and  here  deposited.  In  this  figure  and  the 
previous  one  we  have  clearly  a  mingling  of  fragmental  and  nonfragmental  material,  the 
fragmental  portion  of  which  is  possibly  derived  from  the  immediately  underlying  iron- 
bearing  beds.    (See  pp.  230-231.) 

492 


U.   S.   GEOLOOICAU  SURVfv 


MONOGRAPH  XIX      PLATE  XXII 


Fig.  1. — Concretionary  chert. 


Fig.  2. — Concretionary  chert,  in  polarized  light. 


Fig   3.— ceiaecd  trBchert.  Fig.  4. — Ferruginous  and  brecciated  chert. 

THIN  SECTIONS  OF  FERRUGINOUS  CHERTS  FROM  THE  IRON-BEARING   IVIEMBER, 


PLATE    XXIII. 


493 


Plate  XXIII.— Fekeuginotjs  Cherts  and  Actinolite  Slates  from  the  Iron- 
Bearing  Member. 

Fig.  1.  Ferruginous  chert.  Specimen  9081,  slide  4206.  From  the  SE.  i  of  Sec.  24,  T.  45  N.,  E.  1  W., 
Wisconsiu.  lu  ordinary  light,  X  25.  The  cherty  background  contains  .areas  which  are  in 
part  roughly  oval  or  roundish,  but  are  more  largely  exceedingly  irregular.  These  areas 
are  composed  of  quartz  .and  iron  oxide,  the  latter  being  mostly  hematite,  the  remainder 
magnetite.  The  regular  areas  suggest  concretions,  the  history  of  which  has  been  given. 
The  irreo-ular  areas  resemlile  fragments,  but  are  probably  of  chemical  and  dynamic  origin  and 
have  formed  within  the  rock  itself.  They  doubtless  represent  original  iron  carbonate  areas. 
This  carbonate  has  largely  changed  to  oxide,  but  has  also  to  some  extent  been  leached  out, 
thus  leaving  cavities.  After  or  before  the  comiiletion  of  this  process  silica  has  entered  and 
filled  the  cracks  and  cavities.  The  result  of  this  oxidation,  solution,  and  silicification,  com- 
bined with  movement,  has  been  to  put  in  the  place  of  well  defined  areas  of  iron  carbonate 
the  exceedingly  irregular  forms  presented  by  the  figure.     (See  p.  223). 

Fig.  2.  The  same,  in  polarized  light.  The  relations  just  mentioned  are  here  again  observed.  It  is 
further  seen  that  the  background,  instead  of  being  finely  spotty  and  perhaps  partly  amor- 
phous, as  in  the  previous  plate,  is  completely  cr^ystalline.  That  there  has  been  an  exten- 
sive rearrangement  and  entrance  of  silica,  is  shown,  even  more  plainly  than  in  the  previous 
£gure.     (See  p.  223.) 

Fig.  3.  Actinolitic  schist.  Specimen  9555,  slide  3190.  From  Peuokee  gap ;  NW.  i  of  Sec.  14,  T.  44  N., 
R.  3  W.,  Wisconsin.  In  ordinary  light,  X  25.  A  quartzose  background  contains  very  numer- 
ous minute  needles  of  actinolite  and  many  particles  of  magnetite,  the  latter  being  roughly 
concentrated  into  bands,  one  of  which  with  a  part  of  another  is  shown  in  the  figure.  The 
figure  represents  a  typical  rock  of  this  kind.     (See  pp.  218-219). 

Fig.  4.  The  same,  in  polarized  light.     The  completely  crystalline  character  of  the  quartzose  back- 
ground appears,  and  the  intricate  maimer  in  which  this  material  is  cut  by  needles  of  actino- 
lite is  perceived  by  comparing  this  figure  with  the  preceding.     (See  pp.  218-219). 
494 


U.   8.   GEOLOGICAL  8URVEV 


MONOGRAPH  XIX      PLATE  XXIII 


Fig.  1. — Ferruginous  chert- 


Fig.  2.— Ferruginous  chert,  in  polarized  light. 


Fig,  3— Actinolitic  schist.  Fig.  4.— Actinolltic  schist,  in  polarized  light 

THIN   SECTIONS   OF   FERRUGINOUS   CHERTS   AND   ACTINOLITIC  SLATES   FROM    THE    IRON-BEARING    MEMBER. 


PLATE   XXIV. 


495 


Plate  XXIV. — Actinolite  Slates  from  the  Iron-Beauing  Member  of  the 
Penokee  Series  and  Cherty  Iron  Carbonates  from  the  Vermilion 
Series. 

Fig.  1.  Actinolite-magnetite-schist.  Spocimeu  9558,  slide  3191.  From  Penokee  gap;  NW.  J  of  Sec. 
14,  T.  44  N.,  R.  .3  W.,  Wisconsin.  In  polarized  light,  X  25.  Magnetite  and  quartz  are  the 
most  abundant  constituents,  but  minute  needles  of  actinolite  are  seen  radiating  from  the 
magnetite  areas.  The  rock  is  a  regularly  banded  one,  and  while  this  is  not  strongly  marked 
in  the  thin  section  its  distinct  lamination  is  apparent.  The  rhombic  form  of  many  of  the 
magnetite  areas  is  seen.    Theseprobablyrepresent  sections  of  octahedra.     (Seep.  218.) 

Fig.  2.  Actinolitic  slate.  Specimen  9680,  slide  3167.  From  the  SE.  J  of  See.  20,  T.  44  N.,  R.  5  W., 
Wisconsin.  In  polarized  light,  X  25.  This  is  an  actinolitic  slate  in  -which  the  con- 
cretionary arrangement  of  the  various  constituents  is  shown.  The  close  association  of 
actinolite  and  magnetite  is  apparent,  the  minute  actinolite  needles  frequently  radiating 
from  the  magnetite  particles.  The  actinolite  is  manifestly  prior  to  the  quartz  in  crystal- 
lization, as  the  individuals  of  tlie  latter  are  penetrated  in  every  direction  by  the  actinolite 
needles.  This  figure  is  particularly  noticeable  on  account  of  the  very  coarsely  crystal- 
line ciiaracter  of  the  quartzose  background.  It  runs  between  the  concretions  iu  such  a 
manner  as  to  make  it  evident  that  it  must  have  crystallized  subsequent  to  their  formation. 
We  have  here,  then,  an  illustration  of  the  complete  rearrangement  of  the  silica  originally 
present  and  the  probable  introduction  of  a  good  deal  of  silica  from  an  extraneous  source. 
(See  p.  216.) 

Fig.  3.  Cherty  iron  carbonate.  Specimen  8726,  slide  3367.  From  Vermilion  lake,  Minnesota.  In 
ordinary  light,  X  25.  This  shows  very  finely  the  alternation  of  bands  of  chert  and  iron 
carbonate.  The  darker  belts  are  nearly  pure  siderite,  and  the  lighter  ones  almost  jiure 
chert.  The  cherty  belts  are  seen  to  contain  minute  rhombohedra  of  carbonate,  and  they 
cut  across  the  carbonate  belts  in  such  a  manner  as  to  imply  a  rearrangement  of  silica 
originally  present  and  the  introduction  of  additional  silica,  or  both.  The  figure  is  here 
introduced  in  order  that  the  cherty  carbonates  of  other  districts  may  be  compared  with 
those  from  the  Penokee  series.     (See  p.  260.) 

Fig.  4.  The  same,  in  polarized  light,  x  50.  It  is  seen  that  the  cherty  background  is  completely  crys- 
tallized; that  is,  contains  no  amorphous  material  and  is  made  ui>  wholly  of  quartz. 
The  intricate  manner  in  wliich  the  quartz  anastomoses,  cutting  the  iron  carbonate,  shows 
conclusively  that  to  some  extent  it  is  later  than  the  siderite,  while  it  has  probably  been 

extensively  rearranged.    (See  p.  260.) 

•» 

496 


U.   S.   GEOLOGICAL  SURVEY 


MONOGRAPH  XIX      PLATE  XXIV 


Fig.  1. — Actinolite-magnetite  schist. 


Fig.  2, — Actinolitic  slate. 


Fig.  3. — Cherty  iron  carbonate. 


Fig.  4. — Cherty  iron  carbonate,  in  polarized  light. 


THIN   SECTIONS  OF  THE  ACTINOLITIC  SLATES   FROM    THE    IRON-BEARING    MEMBER   OF  THE   PENOKEE  SERIES,   AND    CHERTY    IRON   CAR- 
BONATES   FROM   THE  VERMILLION   SERIES. 


PLATE   XXV. 


MON   XIX 32  497 


Plate  XXV. — Sideritig  Slates  prom  the  Animikte  Series. 

Fig.  1.  Chert.y  iron  carbouiite.  Speciiiieu  10172,  slide  3422.  Froui  Diuvsoii'.s  road,  Port  Artlmr, 
Canada;  Auimikic.  series.  In  ordinary  liglit,  x  60.  The  figure  well  illustratis  tlie 
apjiearance  ol"  one  of  the  ricliur  Animikie  chcrty  carbonates.  Throughont  most  of  the  sec- 
tion iron  carbonate  is  the  cliief  constituent,  and  the  manner  in  whicli  the  individuals  are 
packed  together  is  well  shown  ;  wheu  surrounded  by  chert  they  show  perfect  rlunnliohedral 
forms.     Alter.ation  of  iron  carbonate  to  iron  oxide  has  begun.     (See  p.  264.) 

Fifi.  2.  Sideritie  chert.  Specimen  6138,  slide  1173.  From  north  shore  of  Gunflint  lake,  T.  65  N.,  R.  3 
W.,  Minnesota;  Aniraikh;  series.  lu  ordinary  light,  X  2.5.  The  iigure  illustrates  the  for- 
mation of  iron  oxides,  iisoudomorphous  after  siderite.  A  background  of  chert  contains 
numerous  small  roundish  ami  rhombohedral  areas  of  siderite  and  iron  oxide.  Between  tfe 
little  altered  and  wholly  altered  siderite  a  complete  gradation  is  seen.     (See  ji.  264.) 

Fig.  3.  Actinolite-siderite-slate.  Specimen  10579,  slide  5188.  From  the  east  side  of  north  arm  of 
fJuutlint  lake,  Minnesota;  Animikie  series.  In  ordinary  light,  X  25.  This  section  is 
from  one  of  the  typical  Animikie  slates.  The  thinly  laminated  character,  of  the  roik  is 
well  showu.  Its  background  consists  in  about  equal  jjroportions  of  actinolite  and  siderile, 
mingled  with  a  little  chert.  The  dark  colored  material  comprises  all  three  of  the  oxides  of 
iron.     (Sec  p.  263.) 

Fig.  4.  The  same,  in  polarized  light,  X  25.  The  figure  is  from  another  part  of  the  sectioi],  which 
shows  the  termination  of  a  thick  layer  of  nciirly  pure  chert.  Such  lozenge-shaped  chcrty 
areas  within  the  Animikie  sla.tes  are  very  common  and  are  frequeutly  of  large  size,  lu  the 
clu^rty  background  are  seen  rhombic  outlines  of  siderite.  The  light  colored  border  between 
the  slaty  lamina'  and  the  chert  is  mostly  actinolite.  This  rel.ation  is  suggestive  that  the 
actinolite  has  resulted  fVom  a  reaction  between  the  silica  and  siilerite.  The  lamina-  of  the 
slate  are  seen  to  curve  about  the  chert  area.  A  short  distance  from  this  nodule  the  lam- 
ina) are  parallel,  as  represented  in  the  previous  figure.  (See  pp.  263,  265.) 
498 


U.   8     QFOLOOICAL  SURVrv 


MONOGRAPH  XIX      PLATE  XXV 


\:mJ 


.  y 


Fig.  1, — Cherty  iron  carbonate. 


Fig.  2. — Slderitic  chert. 


Fig.  3.— Actinolite-siderite  slate.  Fig.  4.— Actinolite-siderite  slate,  in  polarized  light 

THIN   SECTIONS  OF  SIDERITIC   SLATES   FROM   THE  ANIMIKIE  SERIES. 


PLATE   XXVI. 


499 


Plate    XXVI. — Ferruginous    Cherts    and   Iron    Carbonates    from   the 

Animikie  Series. 

Fig.  1.  Concretionary  chert.  Specimen  10577,  slide  4997.  From  north  side  Gunflint  lake;  Animikie 
series.  In  ordinary  light,  x  25.  The  complex  concretionary  structure  so  characteristic 
of  the  ferrnginous  cherts  is  here  beautifully  shown.  The  concretions  are  closely  jiacked 
together,  the  amount  of  material  between  them  being  relatively  small.  This  interstitial 
substance  is  cliiefly  chert,  but  it  contains  numerous  rhombohedra  of  siderite.  The  oxides 
of  iron  are  mostly  limonite  and  hematite,  but  with  them  is  mingled  some  magnetite.  The 
arrangement  of  these  iron  oxides  with  reference  to  one  another  is  usually  somewhat  irregu- 
lar, but  occasionally  the  magnetite  and  hematite  are  in  alternate  layers.    (See  p.  264.) 

Fir.  2.  The  same,  in  polarized  light,  X  60.  Another  part  of  the  same  section  is  here  shown,  the 
enlargement  being  greater.  One  of  the  concretions  is  seen  to  be  compound ;  that  is,  the 
larger  belts  of  iron  oxides  inclose  two  smaller  concretions.  Within  the  concretions  the 
quartz,  like  the  iron  oxide,  is  seen  to  have  a  banded  arrangement.  The  background  is 
very  finely  crystalline,  but  the  silica  is  mostly  or  wholly  individualized.     (See. p.  264.) 

F'iG.  3.  Ferruginous  chert.  Specimen  10576,  slide  5186.  From  the  Gunflint  beds.  In  ordinary 
light,  X  25.  The  section  illustrates  the  sharp  alterations  which  sometimes  occur  between 
tine  grained  evenly  laminated  cherty  carbonate  and  ferruginous  chert,  with  a  well  devel- 
oped concretionary  structure.  The  fine  grained  part  is  composed  of  exceedingly  crystalline 
and  amorphous  silica,  and  of  siderite  in  minute  rhombohedra.  In  the  concretionary  part 
of  the  section  no  carbonate  remains.  Within  the  concretions  the  quartz  is  somewhat 
coarsely  crystallized.    (See  p.  264.) 

Fig.  4.  Siderltic  chert.  Specimen  6138,  slide  1173.  Also  from  the  Gunflint  beds.  In  polarized  light, 
X  25.  The  semiamorphous  and  chalcedouic  phases  of  silica  which  are  found  iu  the  fer- 
ruginous rocks  are  nicely  shown.  The.  darker  colored  part  of  the  tigure  consists  chiefly  of 
■siderite  and  iron  oxide,  pseudomorphous  after  it.     (See  p.  264.) 

500 


U.    S.   QEOtOOtCAL  SURVEY 


MONOGftAPH  XIX      PLATE  XXVI 


Fig.  1  .—Concretionary  chert. 


Fig.  2.— Concretionary  chert,  in  polarized  light. 


Fig.  3. — Ferruginous  chert.  Fig.  4. —  Sideritic  chert. 

THIN   SECTIONS"OF   FERRUGINOUS   CHERTS   AND    IRON    CARBONATES    FROM    THE  ANIMIKIE  SERIES. 


PLATE    XXVII. 


501 


Plate  XXVir. — Formation  op  Concretions  in  Iron- Bearing-  Member. 

Fi6.  1.  Sitleritic  ohe.rt.  Specirapu  1250S,  slide  5522.  From  Sec.  16,  T.  47  N.,  R.  46  W.,  MichisiUi,  In 
ordiuary  liglit,  X  25.  A  clie.rty  background  coutaiii.s  iniinerons  riniTidisb  and  rboralio- 
liedral  areas  oT  siderite.  This  siderite  lias  Iipgun  to  alter  to  opaipie  lilai'k  oxide  of  iron. 
In  pLaces  in  the  section  this  alteration  lias  gone  far,  and  ronndisli  forms  are  prodnced 
which  are  imitative  of  the  shape  taken  by  the  original  carlionate,  althongh  to  a  consideralile 
extent  this  carbonate  has  been  replaced  in  its  alteration  by  silica.      (See  p.  232.) 

Fig.  2.  Sideritic  clicrt.  Specimen  7622,  slide  2072.  Fi'om  tlie  Montreal  river,  between  Wisconsin 
and  Michigan.  In  ordinary  light,  X  25.  A  l)aclcgronnd  of  cliert  ciintains  nnmeroiis 
rhoinbohedi'a  of  siderite  of  greatly  varying  sizes.  The  siderite  ha.s  altered  to  n  consider- 
able extent  to  oxide  of  iron,  and  illustrates  the  formation  of  concretions  as  in  the  ]>revioiis 
tig'ire,  but  the  stage  of  growth  is  more  advanced.  In  oni'  case  a  nearly  solid  oval  area  of 
oxide  of  iron  has  beeji  produced  by  the  alter.ation  of  one  of  siderite.  In  several  other 
areas  the  oxide  of  iron  forms  a  ring  about  the  siderite  inclosed.  In  still  others  the  oxide 
of  iron,  while  it  is  scattered  somewhat  irregularly  througli  the  areas, 'as  a  whole  retains  the 
form  of  the  original  siderite  areas,  altliongh  thospacc  once  occupied  by  thi^  siderite  is  taken 
in  part  by  silica.  In  the  large  comidex  concretion  the  series  of  alteration  thus  illustrated 
has  been  com])leted.  It  contains  at  present  no  siderite,  liut  consists  of  a.  series  of  concen- 
tric rings  of  iron  oxide  which  have  a  silice(ms  background.  This  concretion  is  cut  by  a 
vein  of  silica,  as  are  also  three  other  areas  on  one  side  of  the  figure.  These  veinlets  have 
clearly  formed  after  the  development  of  the  concretionary  areas.      (See  p)).  :  30-231). 

Fig.  3.  Another  part  of  the  same  section,  x  25.  The  figure  again  illustrates  the  form:.ition  of  con- 
cretions of  iron  oxide  from  original  siderite  areas.  ■  This  concretionary  arrangement  of 
the  iron  oxide  is  shown  in  nearly  all  of  the  siderite  individuals,  but  is  shown  in  a  particu- 
larly fine  manner  by  the  Large  area  in  the  middle  of  the  figure.  Enongli  siderite  remains 
so  that  its  rhombohedral  cleavage  is  nicely  shown.  Upon  the  outer  part  of  the  area,  is  a 
tolerably  continuous  ring  of  bl.ack  oxide  of  iron,  beyond  which  areotlu'r  imperfect  larger 
rings  of  red  and  black  oxides  of  iron.  The  alteration  in  the  interior  of  the  a-rea  has  to 
some  extent  followed  the  cleavage  lines,  and  we  tlius  liave  a.n  (explanation  of  tlie  irregular 
forms  in  the  interiors  of  the  concretions  of  tlie  iirevious  figures.  It  is  to  be  furtlier  noted 
that  the  forms  of  tlie  rings  in  the  concretions  do  not  conform  to  those  of  tine  original  sider- 
ite area.,  but  form  regular  ovals.      (See  pp.  230-231.) 

Fig.  4.  Fernigiuons  chert.  Specimen  9009,  slide  2765.  From  the  same  exposure  as  Figs.  2  and  3.  In 
ordiuary  light,  X  25.  This  figure  illustrates  the  rcsulti  of  an  almost  complete  oxidation 
of  the  siderite  areas,  only  a  trace  of  that  mineral  remaining.  The  whole  space  that  i  t 
once  occiiijied  is  taken  by  reddish  brown  hematite  and  a  small  amount  of  silica.  The 
greenish  areas  are  probably  chlorite.    (See  pp.  228-230.) 

502 


U.  S.  GEOLOGICAL   SURVEY. 


MONOGRAPH    XIX.  PL. XXVII 


SscicetuWilhclmsLilhoCc  N  Y 


PLATE    XXVIII, 


503 


Plate    XXVIII. — Magnetitio    and    Actinolitic    Slates    from    the    Ikon- 

Beaeing  Membee. 

Fig.  1.  Actinolitic  slate.  Specimen  9620,  slide  3147.  From  Tylers  fork,  NE.  i  of  Sec.  33,  T.  45  N., 
R.  1  W.,  Wisconsin.  In  ordinary  light,  X  165.  The  figure  shows  the  rhombohedral  shape 
of  a  complex  .irea  of  actinolite  and  magnetite  and  the  close  association  of  these  minerals. 
The  exierior  of  the  area  in  the  center  of  the  figure  is  mostly  magnetite,  mingled,  however, 
with  some  .actinolite,  while  the  interior  is  pure  actinolite.  That  this  area  represents  an 
original  rhomliohedron  of  iron  carbonate  is  very  probable.     (See  pp.  220-221.) 

Fig.  2.  Magnetitic  concretionary  chert.  Specimen  9625,  slide  3150.  From  Tylers  fork,  NE.  i  of  Sec. 
33,  T.  45  N.,  R.  1  W.,  Wisconsin.  In  ordinary  light,  x  60.  A  cherty  background  contains 
roiindish,  oval,  and  roughly  rhombic  outlined  areas.  The  iron  oxide  in  these  areas  is  mostly 
V  magnetite,  in  the  form  of  crystals.     The  magnetite  is  often  concentrated  upon  the  exteriors 

of  the  areas,  projecting  somewhat  into  the  cherty  background.  The  forms  of  the  areas  at 
once  suggest  the  carbonate  areas  of  PI.  xxi,  Figs.  1  and  2,  and  PI.  xxvii,  Fig.  1,  and  each 
doubtless  now  occupies  the  place  once  taken  by  siderite.  In  the  alteration  to  magnetite  the 
individuals  grew  beyond  the  outer  borders  of  the  siderite  areas,  but  the  forms  as  a  whole 
were  maintained.  A  considerable  amount  of  silica  must'  have  entered,  as  the  magnetite 
occupies  but  a  small  part  of  the  space  once  taken  by  the  carbonate.    (See  pp.  222-223.) 

Fig.  3.  Banded  magnetitic  jasper.  Specimen  12791,  slide  .5477.  From  Sec.  11,  T.  47  N.,  R.  45  W., 
Michigan.  In  ordinary  light,  X  25.  The  white  background  is  a  completely  individualized 
but  finely  crystalline  chert.  The  part  containing  the  abundant  red  hematite  is  in  hand 
specimen  a  brilliant  red  jasper.  These  jaspery  bands  alternate  with  the  black  ones,  which, 
instead  of  hematite,  consist  of  magnetite,  mostly  in  well  defined  crystals.  The  change  from 
magnetite  to  hematite  is  gradual.     (See  pp  239-240.) 

Fig.  4.  Actinolitic  slate.  Specimen  9555,  slide  3190.  From  Penokee  gap,  NW.  J  of  Sec.  11,  T.  44  N., 
R.  3  W.,  Wisconsin.  In  polarized  light,  x  165.  The  section  is  a  typical  actinolitic  slate. 
The  quartz  is  completely  crystallized.  The  magnetite  has  mostly  ■well  defined  crystal  out- 
lines and  is  manifestly  the  first  mineral  to  crystallize,  being  scattered  uniformly  through 
the  section  without  any  regard  to  the  actinolite  and  quartz,  and  therefore  included  by  both 
of  them.  The  actinolite  is  in  its  characteristic  blades  and  sheaf-like  forms,  having  a  radial 
arrangement  of  its  fibers.  It  is  as  plainly  the  second  mineral  to  crystallize,  as  needles  of 
actinolite  everywhere  penetrate  the  quartz,  but  never  the  magnetite.  The  quartz  constit- 
utes a  background  for  the  magnetite  and  actinolite,  and  includes  them  in  such  a  manner 
as  to  make  the  conclusion  certain  that  it  must  in  the  main  have  crystallized  subsequently 
to  the  formation  of  the  magnetite  and  actinolite.     (See  pp.  218-219). 

504 


U.  S.  GEOLOGICAL    SURVEY. 


MONOGRAPH    XIX,  PL.  XXVI 1 1 


Fig. 


FiG.2. 


FiG.3.  FiG.4-. 

THIN  SECTIONS   OF  MA6NETIT1C  AND  ACTINOLITIC  SLATES   FROM   IRON-BEARING   MEMBER. 


Shc1c«ii  »  Wl(l«IlTlsLll^clCo  N  Y 


PLATE   XXIX. 


505 


Plate    XXIX. — Prom  the  Iuon-Fokmation  op  the  Animikib    Seeies    and 

FROM  Lawrence  County,  Ohio. 

Fig.  1.  Conoretionarj'  chert.  Specimen  10577,  slide  4997.  From  north  side  Gunflint  lake;  Auimikie 
series.  In  ordinary  light,  x  GO.  The  photographs.  Figs.  1  and  2,  PI.  xxvi,  are  repro- 
ductions from  the  same  section  as  this  figure.  It  is  drawn,  however,  to  show  more  exactly 
the  relations  of  the  various  oxides  of  iron  to  the  cherty  hack'ground.  In  the  first  place 
veins  of  chert  intricately  intersect  the  oxides  of  iron  in  such  a  manner  as  to  sliow  that  the 
silica  entered  after  the  iron  oxides  h.ad  formed.  The  magnetite  in  the  concretions  is  sonie- 
timos  in  nearly  continuous  hands,  butin  other  parts  is  mingled  intimately  with  the  liem- 
atite.  If  the  large  area  takes  the  place  of  one  of  iron  earhouate,  at  difi'erent  times  tlie 
circumstances  wer<'  favorable  for  the  production  of  each  of  the  iron  oxides  iu  prepohderat- 
iiig  ([uantity.     (.See  pp.  2i;r)-2GG. ) 

Fig.  2.  Aetiuolitic  slate.  ■■Specimen  7012,  slide  2081.  From  the  SW.  i  of  .Sec.  23,  T.  65  N.,  R.  4  W., 
Minnesota;  Aniiuikie  series.  In  ordinary  liglit,  X  25.  The  reliitiuns  ol'  the  magnetite, 
actinolite,  and  quartz  are  here  nicely  shown.  Tlie  magnetite  is  plainly  the  first  mineral  to 
crystallize,  fre(inently  liaving  crystal  outlines,  and  being  included  within  botli  the  other 
minerals.  Thl^  close  assoi'iation  of  a(^tiuolite  and  magnetite  is  again  illustrated,  the  two 
combiniMl  having  usually  roundish  or  oval  ibrms.  Tlie  silica  cuts  the  .section  in  such  a 
maimer  as  to  give  it  in  places  vein-like  forms,  which  must  have  developed  subsequently 
to  the  formation  of  the  magnetite  and  actinolite.     (See  p.  266.) 

Fig.  3.  Aetiuolitic  and  sideritic  slate.  Specimen  10.580,  slide  5189.  From  the  Gunllint  beds;,Animi- 
kie  series.'  In  ordinary  light,  x  60.  The  section  illustrates  a  phase  of  the  iron  formation 
in  which  all  of  the  minerals,  quartz,  siderite,  magetite,  -and  actinolite  are  together.  The 
colorless  part  is  the  cherty  background,  which  is  completely,  although  finely  crystallized. 
The  siderite  is  represented  by  the  vague  rhombohedral  areas,  which  inchule  numerous 
minute  particles  of  gray  and  black  material,  presumably  oxides  of  iron-.  This  mineral  is 
the  most  abundant  one  iu  the  section.  The  magnetite  occurs  as  usual  with  its  cliaracteiistic 
crystal  outlines,  and  in  places  is  included  in  such  a  maniu'r  in  the  siderite  as  to  sug- 
ge.st  its  fcn-mation  from  that  mineral.  The  actinolite  is  scattered  here  and  there  iu  miiinte 
blades  .and  neeiUes,  being  particularly  abundant  upon  one  side  of  the  figure.     (See  p.  265.) 

Fig.  4.  Cherty  iron  carbonate.  Specimen  9814,  .slide  3880.  From  Lawrence  county,  Ohio.  In  ordi- 
nary light,  x  25.  The  photographic  Figs.  1  and  2,  PI.  xxi,  are  from  this  section.  The 
beginning  of  the  alteration  of  siderite  to  limonite  and  hematite,  and  also  the  beginning  of 
the  formation  of  concretions,  are  here  better  seen.  The  background  is  fincdy  crystalline 
and  ainoii)hous  silica,  the  nature  of  which  is  better  shown  .by  the  figures  referred  to.  This 
cherty  iron  carbonate  from  Ohio  is  remarkahly  like  that  from  the  Pouokee  series  shown 
in  PI.  XXVII,  Figs.  1  and  3,  thc^  only  dirt'erence  being  that  the  alteration  of  the  siderite  is 
of  a  diflercnt  character,  in  one  case  black  oxide  of  iron  being  ])rO(lueod,  and  in  the  other 
red  and  brown  oxides.  (See  p.  247.) 
506 


U.  S. GEOLOGICAL   SURVEY. 


MONOGRAPH    XIX.  PL.  XXIX. 


Fig. 


FiG.2. 


'i^^.:^%^^lMffs^ 


FiG,3. 


FiG.4-. 


THIN   SECTIONS   FROM   THE    IRON  FORMATION   OF   THE    ANIMIKIE    SERIES,AND  FROM   LAWRENCE  COUNTY,  OHIO, 


SackeluWilhilmsLjL'ioCoN  ^ 


PLATE   XXX. 


507 


Plate  XXX.— Ore  Deposits. 

Fig.  1.  Longitudinal  section  of  soutli  deposit,  Montreal  mine.  Above  the  main  dike  rock  rich  ore 
is  found,  while  between  ih'in  and  some  smaller  dikes  is  mixed  ore. 

Fig.  2.  Longitudinal  section  of  nnrtli  deposit,  Montreal  mine.     Ore  is  seen  above  each  of  two  dikes 
at  different  depths.     The  fact  is  noted  that  ore  which  was  origin.illy  fonnd  at  the  surface 
.  and  mined  in  an  open  pit  has  in  both  cases  been  carried  under  the  rock  surface.     In  this 
case  two  parallel  dikes,  very  close  to  each  other,  have,  in  both  cases,  above  them  a  moder- 
ate sized  ore  deposit. 

Fig.  3.  Cross  section  of  south  deposit,  Montreal  mine,  at  No.  8.  shaft.     The  relations  of  the  foot  wall 
^      quartzite,  dike  rock,  ore  and  drift,  are  well  illustrated. 

Fig.  4.  Cross  section  of  south  and  north  deposits,  Montreal  mine,  on  line  of  No.  7  shaft  of  south 
deposit.  The  quartzite,  both  dike  rocks  of  the  south  <leposit,  and  the  dike  rock  of  the 
north  deposit  are  all  seen. 

Fig.  5.  Longitudinal  section  of  Pence  mine.  A  strong  dike  carries  above  it  a  heavy  deposit  of  ore, 
which  was  reached  by  two  shafts  immediately  upon  passing  through  the  drift.  Just  above 
this  same  dike,  a  little  farther  to  the  west,  no  ore  is  found. 

Figs.  6,  7,  and  8.  Cross  sections  of  Pence  mine,  at  Nos.  1,  2,  and  Father  Hennepin  shafts.     The  rela- 
tions of  the  dike  rock,  quartzite,  ore  deposits,  drift  material,  and  ferruginous  cherts  in  this 
mine  are  by  these  sections  clearly  shown. 
508 


U.  8.  QfOLUOICAL  auHVEY 


MONOGRAPH  XIX      PLATE  XXX 


■'/> 


fti> 


^liSj-  OloNo,4 


No3 


Open   pit 


_SuRFflcrMflTEmAL  REMOVED 


No2  N0.1 


No  I 


"". 

'^ 

%  ^ 

"'^^ 

r-  '# 

No  7 


Wo  3 


~i?^^s^g^i    r^'    ^ 


XjongituxicnaZ    Section^ 


FflTHE^HENNEPIN 
r-SHAFT 


S^ 


ORE   DEPOSITS. 


Fig.  1. — Longitudinal  section  of  south  deposit,  Montreal  mine. 
Fig.  2.— Longitudinal  section  of  north  deposit,  Montreal  mine, 
Fig.  3.— Cross  section  of  south  deposit,  Montreal  nnine. 


Fig.  4 — Cross  section  of  south  and  north  deposits,  Montreal  mine 
Fig.  5.— Longitudinal  section  of  Pence  mine. 
Figs.  6,  7,  8. — Cross  sections  of  Pence  nnine. 


PLAICE    XXXI 


509 


Plate  XXXI. — Ore  Deposits. 

Fig.  1.  LougitiUdinal  suctiou  nl'  south  deposit,  Colby  niiuo.     This  iigure  shows  a  very  large  deposit 

of  ore,  Nvhieii  was  found  at  tlic  siirlare.  but  whleli,  in  following  eastward,  was  found  to  pass 

■    under  the  ferruginous  chert.     So  far  as  tliis  figure  goes,  no  indication  of  a  dilce  is  seen,  but 

a  drill  liole  put  down  from.oue  of  its  deeper  levels  has  passed  through  a  thicli  dike  reel;. 

Fig.  2.  Loiigituilniiil  section  of  north  deposit,  Colljy  luiue.  This  deposit,  before  it  had  beea  devel- 
oped to  sucli  a.  deptli  a.s  tlie  south  deposit,  readied  a  dike,  and  a  little  fartlier  north  this 
same  dike  reaches  tlio  surface  of  the  ground. 

Fig.  3.  Cross  section  ol'  luuth  and  south  deposits,  Colby  mine.  The  perpendicular  line  running 
tlirough  tlie"ligurc  is  drawn  liecause  the  two  parts  of  the  section  are  not  exactly  upon  tlie 
same  plane.  Tluiy  dilier  Irom  this  so  slightly,  however,  that  the  true  relations  of  the  ore 
bodies  to  tlie  suriouniling  rocks  are  shown  by  the  combined  hgnrc. 

Fig.  4.  Longitudinal  .section  of  Trimble  miue.  This  is  a  case  in  which  the  main  deposit  of  ore  is 
above  three  parallel  <likes  very  close  to  one  another.  This  deposit,  like  many  others,  is  i'onncl 
at  the  surlace,  but  when  traced  eastward,  passes  below  the  ferruginous  chert.  Above  the 
lower  dike,  a  little  lartlier  west,  is  a  considerable  ore  deposit. 

Fig.  ,5.  Cross  section  of  Trimljle  mine,  showing  quartzites,  ferruginous  chert,  and  dike  rocks. 

Fig.  6.  Cross  section  of  Miunewawa  mine,  showing  the  same  relations  of  quartzite,  dike  rock,  and 
ore  bodies;  .and  also  a  faulting  of  the  foot  wall  quartzite  where  cut  by  the  dike  rock. 

Fig.  7.  Section  designed  to  show  variation  from  unaltered  carbonate  to  ferruginous  chert  and  ore 
bodies  in  passing  Irom  higher  to  lower  horizons,  and  to  illustrate  the  manner  of  ore  con- 
centration.    S.  Upper  slate;  Q.  Quartzite;  F.  Q.  Feldspathic  Quartzite. 
510 


U.  a.  ttEOLOOIOAL  suRvev 


MONOGRAPH  XIX      PLATE  XXXI 


No.3 


No2 


^NE  6'.cfy'f  vi 


0     □□    0  QC^ 


„t,^_,..j;o.«sxH  ^,___. -^.„..,_  _ ,.> 


ICZDEZD 


^SiHi 


FiG.3. 


FiG.6. 


fbrruffLnoutChcr^. 


Fig. 4. 


,/ 


FiG.5. 


ORE   DEPOSITS. 


Fig.  1. — Longitudinal  section  of  south  deposit,  Colby  mine. 
Fig.  2.— Longitudinal  section  of  north  deposit,  Colby  mine. 
Fig.  3. — Cross  section  of  north  and  south  deposits,  Colby  mine. 
Fig.  4. — Longitudinal  section  of  Trimble  mine. 


FlG.7. 


Fig.  5. — Cross  section  of  Trimble  mine. 

Fig.  6. — Cross  section  of  Minnewawa  mine. 

Fig.  7. — Theoretical  section  to  show  variation  from  unaltered  carbonate. 


PLATE    XXXII. 


511 


Plate  XXXII. — Graywackes  prom  the  Upper  Slate  Member. 

Fig.  1.  Micaceous  graywncke.  Specimen  9118,  slide  2906.  From  the  SE.  i  of  Sec.  11,  T.  45  N.,  R.  1 
W.,  Wisconsin.  In  polarized  light,  x  25.  The  section  shows  the  general  rounded  character 
of  the  quartz  grains,  some  of  which  are  now  minutely  angular  by  enlargement.  The  com- 
plex, minutely  crystalline  areas  represent  single  fragments  of  feldspar,  which  are  now 
largely  altered  to  mica  and  (juartz  and  thus  appear  more  like  fln^y  crystalline  interstitial 
material  than  simple  Iragmental  grains.      (See  p.  326.) 

Fig.  2.  Biotitic  and  muscovitic  graywackc.     Specimen  9544,  slide  3092.     From  NW.  i  of  Sec.  11,  T. 

44  N.,  R.  3  W.,  Wisconsin.  In  polarized  light,  X  60.  The  figure  illustrates  a  more 
quartzose  part  of  the  section.  It  shows  nicely  the  alteration  of  feldspar.  In  the  center 
of  the  figure  was  a  simple  fragment  of  feldspar,  which  has  largely  altered  into  mica  and 
quartz,  as  a  result  oC  which  the  rounded  area  is  now  a  completely  crystalline  mass  of  mica, 
quartz,  and  feldspar.  Its  trno  nature  is,  however,  shown  by  its  rounded  appearance  and 
by  the  fact  that  the  many  detached,  small,  irregular  areas  of  feldspar  extinguish  together. 
(See  p.  316.) 

Fig.  3.  Biotitic  graywacke.  Specimen  9598,  slide  3334.  From  SE.  i  of  Sec.  15,  T.  45  N.,  R.  1  W., 
Wisconsin.  In  polarized  light,  a  60.  The  figure  illustrates  the  same  thing  as  Fig.  2. 
The  large  roundish  area  in  the  center  of  the  figure  was  ouce  a  single  fragmeutal  feldspar, 
but  it  has  almost  wholly  decomposed,  and  thus  forms  an  interlocking  mass  of  mica  and 
quartz.  In  this  case  detached  areas  of  quartz  thus  formed  are  seen  to  be  a  unit  over  a  con- 
siderable area.  This  figure  and  the  previous  one  illustrate  the  crystalline  character  which 
can  result  from  the  alteration  of  fragmeutal  feldspar.  Many  individuals  of  mica  and  (juartz 
have  been  produced  from  a  single  feldspar,  and,  as  they  form  simultaneously  by  the  altera- 
tion, they  interlock  as  completely  as  if  they  were  original  crystallizations,  and  also  with  the 
residual  feldspar  in  case  there  is  any.     (See  pp.  324-325.) 

Fig.  4.  Biotitic  and  chloritic  graywacke.     Specimen  9109,  slide  4418.     From  the  NE.  i  of  Sec.l2,  T. 

45  N.,  R.  1  W.,  Wisconsin.  In  polarized  light,  X  25.  A  typical  fine  grained  biotitic  and 
chloritic  graywacke.  The  fragmeutal  character  of  the  quartz  is  plain,  although  many  of 
the  smaller  particles  are  quite  angular.  The  fragmeutal  feldspar  was  originally  as  abun- 
dant as  the  quartz,  but  the  figure  docs  not  well  show  tliis,  as  most  of  the  grains  of  this 
mineral  are  extensively  altered  to  biotite  and  chlorite.     (See  p.  325.) 

512 


U.  8.   OEOLOOICAL  SURVtY 


MONOGRAPH  XIX       PLATE  XXXIl 


Fig.  1.— Micaceous  graywacke. 


Fig,  2. — Biotitic  and  muscovltic  graywacke, 


F'g.  3.— Biotitic  graywacke. 


Fig.  4.— Biotitic  and  chloritic  graywacke. 


THIN    SECTIONS   OF  GRAYWACKES   FROM   THE   UPPER  SLATE    MEMBER. 


PLATE   XXXIII. 


HON  XIX 33  513 


Plate  XXXIII.— The  Development  of  Mica-Slates. 

Fig.  1.  Biotitic  and  muscovitic  graywacke.  Siiecimen  9544,  slide  3092.  From  the  NW.  i  of  Sec.  11,  T. 
44  Jf.,  R.  3  W.,  Wisconsin.  In  polarized  light,  X  60.  The  .section  is  one  of  the  least 
altered  feldspathic  fragineutal  rocks.  Large  rounded  areas  of  feldspar  make  njp  most 
of  the  section,  although  smaller  iiarticles  of  fragmeutal  feldspar  aiid  ([uartz  are  contained. 
The  incipient  alteration  of  the  feldspars  to  the  micas  is  ohserved,  and  the  smooth  out- 
lines of  the  fragmental  grains  have  already  been  lost  by  alteration.  This  is  the  original 
phase  of  rock,  the  alterations  of  which  carried  to  the  extreme  liave  produced  the  crystalline 
mica-schists.    (See  p.  316.) 

Fig.  2.  Biotite-slatc.  Sitecimen  126,  Wright.  From  the  NE.  i  of  Sec.  9,  T.  44  N.,  R.  3  W.,  Wisconsin. 
In  polarized  light,  X  25.  The  section  is  fr<nn  a  rock  which  was  originally  like  Fig.  1. 
The  alterations  of  the  feldspar  to  the  micas  and  quartz  have  been  carried  to  a  greater  degree. 
The  regular  outlines  of  tlie  original  areas  are  wholly  lost,  so  that  while  they  have  a  general 
roundish  or  oval  form  their  exteriors  arc  minutely  angular.  The  micas  are  much  more 
abundant  at  and  near  the  exteriors  of  the  grains  of  feldspars  than  in  the  interiors,  although 
the  altei-ations  have  extended  quite  to  the  centers.      (See  p.  310.) 

Fig.  3.  Biotite-slate.  Specimen  128,  Wright.  From  the  NW.  i  of  Sec.  9,  T.  44  N.,  R.  3  AV.,  Wis- 
cou.siu.  In  ]iolarized  light,  X  25.  The  original  condition  of  this  rock  was  precisely  like 
that  of  Figs.  1  and  2.  The  series  of  alterations  there  shown  have,  however,  gone  much 
farther.  The  Iragmeutal  character  of  the  ielclspars  is  quite  lost.  They  have  been  pene- 
trated by  secondary  muscovite  and  biotite  until  they  arc  most  irregular  in  form,  only 
a  single  grain  approximately  retaining  its  oval  outline.  As  a  further  result  of  this  process 
a  considerable  proportion  of  interstitial  crystalline  ([uartz  and  mica,  as  compared  with  the 
two  previous  ttgurcs,  is  found,  which  has  come  directly  from  the  decomposition  of  the 
feldspar.    (See  p.  310.) 

Fig.  4.  Biotite-slate.     In  polarized  light,  x  2.5.     This  iignre  is  taken  from  part  of  the  same  section 
as  the  last,  in  which  the  alterations  just  described  have  gone  still  farther.     The  cryst;J- 
line  character  of  the  developing  rock  is  better  shown.     There  rcmaJn,  however,  areas  of  . 
feldsjiar  which  act  as  a  unit,  although  they  are  cut  iu  every  direction  by  the  alteration 
products,  quartz  and  mica.     (See  p.  310) 

514 


U.  S.  GEOLOGICAL  SURVEy 


MONOOriAPH  XIX      PLATE  XXXIII 


Fig.  1. — Biotitic  and  muscovitic  graywacke 


F'g.  2.~Biotite  slate. 


Fig.  3.-Biotite  slate.  pig  4,_Biotite  slate. 

THIN   SECTIONS  SHOWING   DEVELOPMENT  OF   MICA-SLATES. 


PLATE    XXXIV. 


515 


Plate  XXXIV. — The  Development  of  Mica-Schists  and  Mica-Slates. 

Fig.  1.  Muscovitic  biotite-schist.  Specimen  2039,  Wisconsin.  From  the  NE.  i  of  Sec.  6,  T.  44  N.,  R. 
2  W.,  Wisconsin.  In  poliirized  light,  X  60.  The  processes  of  alteration  described  in  the 
previous  plate  has  here  gone  so  far  as  to  leave  but  little  trace  of  original  fragnien'tal 
.feldspar.  Areas  of  feldspar,  however,  remain  in  the  section,  which,  while  cut  by  quartz 
and  mica,  so  as  to  give  all  of  the  minerals  a  most  crystalline  material,  yet  polarize  as  a  unit. 
(Seepp.  320-32L) 

Fig.  2.  Biotite-schist.  Specimen  1.53,  Wright.  From  the  NE.  i  of  Sec.  4,  T.  44  N.,  R.  3  W.,  Wis- 
consin. In  polarized  light,  X  60.  The  section  is  a  typical  biotite-schist  from  the 
Upper  slate  member.  Tlie  examination  of  the  thin  section  shows  no  evidence  of  frag- 
mental  origin.  It  is  composed  almost  wholly  of  mica  and  quartz,  the  two  minerals  inter- 
locking in  the  most  intricate  fashion.  Taken  alone  it  could  not  be  shown  that  this  rook 
and  the  preceding  were  derived  from  fragmental  feldspathic  sediments,  but  taken 
in  connection  with  the  i)revious  figures  aud  the  formation  in  which  they  are  found,  it  is 
certain  that  they  have  been  produced  by  the  alteration  of  such  a  rock.  The  figures  of 
the  previous  plate  and  1  and  2  of  this  plate  constitute  a  graded,  series  which  illustrates 
well  tlie  series  of  alterations  by  which  there  has  been  produced  from  a  completely  frag- 
mental rock,  composed  chiefly  of  feldspar,  a  rock  which  is  a  typical  crystalline  schist.  (See 
p.  311.) 

Fig.  3.  Biotite-slate.  Specimen  130,  Wright.  From  SE.  i  of  Sec.  10,  T.  44  N.,  R.  3  W.,  Wisconsin. 
in  polarized  light,  X  60.  The  section  is  one  of  the  exceedingly  finely  crystalline  black 
biotite-slates.  That  it  has  benn  derived  from  a  fragmental  feldspatliic  rock  like  the  mica- 
schists  is  shown  by  the  vaguely  oiitlined  fragmental  feldspars,  the  alteration  of  which  here 
produces  the  finely  crystalline  biotite  and  quartz.     (See  p.  315.) 

Fig.  4.  Biotite-slate.  Specimen  9113,  slide  2903.  From  the  SE.  i  of  Sec.  12,  T.  45  N.,  R.  1  W.,  Wis- 
consin. In  polarized  light,  X  60.  The  section  is  one  of  the  typical  finely  crystalline 
biotite-slates.  It  was  doubtless  produced  from  a  fragmental  feldspar  rock,  as  in  the  previ- 
ous figure,  although  it  shows  no  renuiining  fragmental  feldspar.  However,  in  other  parts  of 
the  thin  section  and  in  the  hand  specimen  the  vague  outlines  of  rounded  feldspars  are  seen. 
(Seep.  326.) 
516 


0.  S.  QEOLOaiCAt.  sufivr> 


MONOGRAPH  XIX      PLATE  XXXlV 


Fig.  I.— Muscovitic  biotite  schist. 


Fig.  2,— Biotite  schist. 


Fig.  3.— Biotite  slate. 


Fig.  4.— Biotite  slate 


THIN   SECTIONS  SHOWING   DEVELOPMENT  OF  MICA-SCHISTS  AND   MICA-SLATES. 


PLATE    XXXV. 


517 


tLATE  XXXV. — Feom  the  Easteen  Area. 

Fig.  1.  Ferruginous  chert  aud  quartzite.  Specimen  12937,  slide  6547.  From  near  the  center  of 
Sec.  23,  T.  47  N.,  E.  44  W.,  Michigan,  In  pohirized  light,  x  60.  The  section  shows  the 
sharp  tran.sition  which  frequently  occurs  between  noufragmental  and  fragmental  sediments 
in  the  iron-bearing  belt  of  the  Eastern  area.  U})0u  one  side  of  the  figure  is  seen  a  small 
part  of  a  hanfl  of  pure  nonfragmental  chert.  This  abruptly  changes  into  fragmental  quartz 
set  in  a  matrix  consisting  of  nonlragmental  chert  and  iron  oxide.  A  portion  of  the  frag 
ments  of  quartz  are  simple  aud  two  grains  are  plainly  enlarged.  Other  grains  are  of  chert. 
In  the  change  from  nonfragmental  to  fragmental  sedimentation  the  underlying  nonfrag- 
mental material  has  been  broken  up  to  some  extent  and  has  yielded  fragments  which  are 
associated  with  the  sim])le  grains  of  quartz.     (See  p.  363.) 

Fig.  2.  Greeustoue-conglomerate.  Specimen  9313,  slide  4929.  From  the  SE.  i  of  Sec.  14,  T.  47  N.,  E. 
44  W.,  Michigan.  In  ordinary  light,  x  25.  The  figure  shows  the  extraordinary  struc- 
ture so  constant  in  tlie  greenstone-conglomerates.  It  is  from  one  of  the  phases  in  which 
chlorite  aud  quartz  are  the  i)reponderatiug  coustituents.  The  anastomosing  jiarts  which 
contain  the  irregular  areas  ant  largely  compt)Sed  of  (|uartz,  while  tlie  irregular  areas  are 
chiefly  made  U])  of  quartz,  chlorite,  and  epidote.  This  represents  a  pliase  of  rock  in  which 
the  extreme  of  alteration  has  taken  place.  The  products  are  minerals  which,  with  the 
exception  of  chlorite,  are  ultimates  in  the  series  of  transformations  of  I'ocks.  (See  pp.  384- 
385.) 

Fig.  3.  Greenstone-conglomerate.  Specimen  9369,  slide  3036.  From  the  SW.  i  of  Sec.  15,  T.  47  N.,  E. 
44  W.,  Michigan.  In  ordinary  light,  X  25.  The  figure  shows  another  phase  of  the 
remarkalile  structure  characteristic  of  these  rocks.  It  contains  apparent  fragments  which 
are  exceedingly  angular  and  which  are  set  in  an  anastomosing  matrix,  consisting  largely 
of  finely  ci-j'stallinc  quartz.  The  fragments  in  this  sectiou  are  gray  amorphous  material, 
in  which  are  numeroiis  minute  tabular  crystals  of  plagioclase.     (See  pp.  381-382.) 

Fig.  4.  The  same,  in  polarized  light,  x  25.     This  figure  shows  the  reticulating  quartzose  background. 
The  almost  completely  a.niori)hous  character  of  the    major  portion  of  the  fragments  is 
sharjily  brought  out.     "Within  them,  as  in  the  previous  figure,  the  tabular  crystals  of 
plagioclase  stand  out.    (See  pp.  381-382.) 
518 


U.  8.  CEOLOGICAl.  SURVEY 


MONOGRAPH  XIX      PLATE  XXXV 


Fig.  1.— Ferruginous  chert  and  quartzite. 


Fig.  2. — Greenstone  conglomerate. 


Fig.  3. — Greenstone  conglomerate. 


Fig.  4. — Greenstone  conglonnerate. 


THIN  SECTIONS  FROM  THE  EASTERN  AREA. 


PLATE   XXXVI. 


519 


U.  S.GEOLQGrCAL    SURVEY. 


400  feel  above  L  Superioi 


Qr.    L     Ha..    SI  '^Q-  MgSct.        GSchBISI. 


tt 


General    Se<nic 
Bl  SI    0    Bl    SI     Q.      QL 


■ 


t 


SOUTHFPn     i;. IMPLEX 


Cherty  Limeetone-  Member 


u  p         p         e 


Slat 


M  e         rn         b         e 


KEWEENAWAN 


PENOKEE    SERIES. 


SOUTHERN   COMPLEX. 


Upper  Slate  Member.         Iron-bearrng  Member  Quartz-Slate  Member         Cherty  Limestone  Member       Eruptives 


/ftXN   Ga.Gabbro      ^=  jmw  M  S  Mico-slote  ^S  G  Sch  Garnetiferous  actmolite  schist 

South  limit,          ^=     Bl  51  Black  slate  at  summrt  of  ihe  member 

=^     0        QuaHzite  ^^  Mg  Sch  Magnetic  and  actmoliticquartjrschis' 
South  limit                 Limitj  of  surface  distribution 


VO  Vitreous  quarlzito  at  summil  ^  WCh  White  chert  '.'M'.  D  Diabase  ^  Gn.  Schistose  chlontic  hornblende  gneiss 

of  the  member  ^  L    Limestone  iM4W  Gr  Granite 

SI    Mam  body  of  the  slate  South  limrt 

B  B  Basal  t>reccia 
South  limit 


Diiecttons  of  coiorecJ  hnes  show  stnke  of  layers 


Scale  of  Map  and  Sections  I  inch- 900  feet 


Contours    20  feel    vertical   distance 


MAP    SHOWING    THE    DISTRIBUTION    OF   EXPOSURES  AT   PENOKEE  GAP,  WISCONSIN. 


PLATE    XXXVII. 


521 


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INDEX, 


A. 

Actiuolite,  alteration  of,  210, 21S-'>14. 
from  feldspar.  305. 

of  actiuoliri^'  slate.  193,  198.  210-215,  257,  362,  364. 
of  chlorite  lui-k,  111. 
of  ferrngiiuMia  chert,  203,  26U. 
of  greenstom'-coiislomenite.  375,  376,377. 
of  irou-liejiring  meiiiher,  141, 2GS. 
of  iron  r-arltnriHte,  266. 
origin  of,  258,  2.')9.  26li,  267. 
jiseudoniorphs.  258. 
relations  tn  magnetite,  258. 
relation.s  to  sitleritc,  433. 
Actinolite-rock.  descrihed,  366. 
Aetinollte-sehist,  469, 471. 

dese.rib(Hl,  215, 21G,  217,  219.  220.  243,  366,  367,  382,  494, 4fl6. 
(See  Actinolitic  date.) 
Actinolitic  Mhite,  190, 195, 198. 199,  279,  297,  364,  365. 
analysis  (tf,  197. 

concretions  iu,  212.  • 

crystallization  of,  267-268. 

described,  194-198,  210-215, 266-267,  496,  498,  504,  506. 
grades  into  iron  carhonate,  246,257,258,260,  262.266,267. 

294.362. 
origin  of,  257-260. 
relation.s  to  eruptives,  259. 
relations  to  ferruginous  chert,  200,  202  203,212,213,246, 

257,  258.  259,  266. 
relations  to  limestones,  259,  267. . 
relation.s  to  npper-.slale.  214. 
(See  AotiitoUte-schist .) 
Agglomerate,  use  of  term,  374. 
Agnotozoic.  proposed  by  Irving,  87. 
Agogebic  lake.     (See  Gogebic  lake.) 
-Agogehic  district,  Wright  on,  83. 
Albite  of  quartz  slate,  150, 
Algonkian,  472,  473, 474. 
Alteration  of  actinolite,  210. 213, 214. 
of  amphibole,  354. 
ofaugite,S56,414,435. 
o'f  chlorite,  353. 

of  diabase,  348,  355,  356,  357,  358,  359. 
of  diallage,  115,353. 
ofdik:es,255,27],290. 
of  eruptives,  294. 

of  feldspar,  107,  108,  110,  118,  122, 125, 148, 150, 151, 155, 179 
180,  305,  306,  333   334,  335,  336,  337,  338,  339,  340,  342,  343, 
353,  356,  444. 
of  hornblende,  122. 
of  iron  carbonate,  199,  201,  204,  205,  254,  283,  284,  286-290,  291, 

292,294.295,393,434. 
of  magnetite,  350, 358. 


AltiTiiIitui  ut  iiieiiiiccanite,414. 

of  microcliiie.  335. 

of  oligoclase,  335, 

of  orfclioclase.  335, 338. 

of  plagioclaae,  115,  353, 413. 

of  pyroxene,  115,  353,  354. 

of  siderite,  201,  20^  253,  283,  362. 
Ampliiliole,  alteration  of,  354. 

from  angite.  356,  414. 

from  diallage,  353. 

from  ]iyroxcne,  354, 

included  in  feldspar. 412. 

of  actinolitic  slate,  197. 

of  diabase,  356. 

(See  llornbleiide  and  Sinaraijdite.) 
Amygdaloid,  410,434,462,460. 

described,  418. 

of  Keweenaw  an,  349. 

relations  to  gTeenstou«  eimgloraerate,  380.  418,  419. 
Analysi."^,  actinolitic  slate,  197. 

audesine,  352. 

anorthoclase,  352, 

biotite-schist,  336. 

clay-slate,  306. 

feldspar,  352. 

garnet,  214. 

Iron  carbonate,  192. 

iron  ore,  281. 

labradorite,  352. 

liinestones,  ISO-lSl. 
Andesino  of  diabase,  352, 357. 
Animikie  district,81. 192,  212,  248,  259.  260-267,  =^94,  433.  472,  473, 

498,  500.  506. 
Animikie  series,  relations  to  Keweenawan,  261,  469-470. 

relations  to  Huronian,  281. 

relations  to  Peuokeo  series,  66.  261,  262,  468-170. 

unconforniably  above  granite,  261 ,  262. 

uu conformably  above  schist,  261, 262. 
Anterite,  described,  367. 

of  actinolitic  slate,  364. 
Anorthoclase,  analysis  of,  352. 

of  diabase,  352. 
Anvil  mine,  179. 
Apatite  of  diabase,  350. 

of  gabbro,  115. 

of  syenite,  115. 
Apostle  islands,  188. 
Arcbean,  81-83,  87,  468,472,  473,  474. 
Ashbiud  mine,  83-84,  91, 154, 1G2, 1G3, 277, 281. 
Atkins  lake,  128.  216,  443. 
Atigite,  alteration  of,  356, 414, 415, 417, 435. 

enlargement  of,  86,  353, 411. 

of  au^ite-ijorpbyrite,  415, 417. 

523 


524 


INDEX. 


Augite  of  diabase,  350,  354,  856,  358,  410-411,  412, 435. 

of  gabbro,  115. 

of  greeiistDne-cougloTiiera,te,  375,  377. 

relations  to  siuaragdite,  413,  435. 
Augite-porphyri'te,  described,  414-419. 

of  Keweenawan,  349. 

relations  to  diabase,  415. 

relations  to  greenstone-conglomerate,  416. 
Aurora  mine,  94, 101, 102, 144, 146, 154, 163, 174,  281,  357, 448. 
Axels  island,  chert  of,  251,  252. 
Azoic  slates,  Whitney  on,  13. 
Azoic  system,  73-75. 


Bad  river,  36,  107, 108,  129,  138, 139,  176,  183,  185,  188,  301,  302, 

303,  304,  309,  343,  344,  438,  440. 
Bad  river  area,  7,  8,  9, 10, 17-19,  22,  28,  30,  33,  34-36,  37,  38,  39-40, 

42,  48,  67,  84, 104, 107-108, 144, 145, 146. 
Baraboo  district,  1,  473. 
Barnes  referred  to,  5,  6, 13, 14, 15. 
Basal  conglomerate.    (See  Conglomerv^te,  basal.) 
Base  level,  452, 454. 
Basement  complex,  428,  438, 455, 471, 472. 

relations  to  Hnronian,  81-82,  86. 

relations  to  Keweenawan,  470. 
(See  Southern  Complex  and  Laurentian.) 
Bayley  referred  to,  214. 
Beaver  bay,  81. 
Becke  referred  to,  86, 411, 412. 
Bedding,  26,  95,  29(i. 
Bessemer,  200. 
Biotite  from  actinolite,  210. 

from  amphibole,  354. 

from  aiigite,  414, 417. 

from  diallage,  115. 

from  feldspar,  118,  125,  180,  305,  333,  334,  335,  336,337,338, 
339,  342,  343,  353,  413, 414. 

from  pyroxene,  115. 

of  actinolitic  slate,  195, 210, 213. 

of  biotite-schist,  33(j,  339,  340,  341. 

of  biotite-slate,  338,  339. 

of  chert-conglomerate,  451. 

of  clay -.slate,  305. 

of  diabase,  351,  359. 

of  gneiss,  108, 116, 118, 119, 120. 

of  granite,  106, 107, 112, 113, 

of  gray  wacke,  304,  333,  334,  335,  336,  337. 

of  hornblende-gneiss,  110-111. 

of  mica-schist,  307,  308, 309. 

of  mica-slate,  307,  308,  309,  342,  343. 

of  Quartz-slate,  148, 152. 

of  syenite,  114, 115. 

of  syenite-schist,  125. 

of  Upper  slate,  303. 

relations  to  hornblende,  119. 

relations  to  magnetite,  354. 

relations  to  plagioclase,  354. 
Biotite-gneiss,  108, 116, 470,  478. 
Biotite-graywacke,  described,  323,  324^325. 
Biotite-schist,  309. 

analysis  of,  336. 

described,  310-311,  318,  330-321,  339-341, 516. 

origin  of,  339-341. 
(See  Mica-schist.) 
Biotite-slate,  described,  309,  310,  311,  312,  313,  315, 319-320,  321, 
322-324,  326,  338-339, 486,  514,  516. 


Biotite-slate,  origin  of,  338-339. 

(See  Mica-slate.)  ^ 

Birkinbine  referred  to,  87-92,  92-96,  280. 
Black  river,  81, 165, 179, 187,  233,  297,  298,  299,  304, 330,  456. 
Black  river  area,  37-38. 

east  branch,  145. 

■west  branch,  145, 189. 
Black  river  district,  Wisconsin,  1, 472, 473. 
Bladder  lake,  129. 
Blue  Jacket  mine,  164. 
Bonney  mine,  269. 
Bradley  referred  to,  41. 
Brainerd  referred  to,  247. 
Breccia.    (See  Chert-breccia.) 
Breeciation  of  actinolitic  slate,  266. 

of  ferruginous  chert,  194,  207,  209, 254,  264,  265. 

of  iron  carbonate,  263,  264. 

of  limestone,  207. 
Bronzite  of  diabase,  35-1. 
Brooks  referred  to,  7,  8,  15,  30-31,31-32,33,34-38,44,47,56,66, 

78,  79,  81,  463. 
Brotherton  mine,  269. 
Brotberton  referred  to,  9, 84. 
Brule  mountain,  81. 


C. 


Calcite  amygdules,  377,  418. 

of  chlorite  rock.  111. 

of  gneiss,  109, 118. 

of  gray  wacke,  307,  333,  336, 

of  limestone,  131. 

of  porphyrite,  418. 
Cambrian,  Irving  on,  86,  87. 
Canada,  2,  261,  472. 
Carbonaceous  material,-190, 193,  200,  203,  250,  307,  308,  333,  337, 

343. 
Carbonate.     (See  Iron  carbonate.) 
Carboniferous.  140,  247, 250,  252. 
Carries  creek,  176. 
Chalcedony  amygdules,  418. 
Chalcedony  of  actinolitic  slate,  211.' 

of  ferruginous  chei-t,  192, 194,  203. 

of  veins,  202. 

oi-gin  of.  252. 
Chamberlin  referred  to,  8,  9-10,  43-44,  59-66,  463. 
Chanuing,  indebtedness  to,  xiv. 

referred  to,  12,  270,  271,  279, 441. 
Chatard,  analysis  by,  351,  357. 

referred  to,  191, 192,  214. 
Chert,  32,  363,  379,  392,  423,  471. 

described,  132-134,  136-138,  225,  226,  227,  228,  230,  233,  239, 
240,  241,  243,  244, 367,  368,  399,  400,  403,  480,  482,  506. 

character  of,  132-134. 

of  limestone,  139-140,  205. 

of  actinolitic  slate,  266. 

of  clay-slate,  434. 

of  conglomerate,  174, 180,  448. 

of  ferruginous  chert,  205,  209. 

of  Iron  carbonate,  190,  200, 201, 254, 262, 263, 264. 

of  iron  ore.  283. 

of  quartzite,  395. 

of  veins,  202. 

origin  of,  140-141, 142,  248, 249,  250,  251,  252, 253. 

relations  to  limestone,  127-128, 132, 139, 141. 

relations  to  Quurtz-slate,  171, 172. 

replaced  by  iron  oxide,  283. 


INDKX. 


525 


Clint  bnoriii.  i:m.  1  III.  :i91.  :(9-t,  39o.  462. 

ili'sorilud.  ir>7.  Ifiit.  io;i. 40r>-407. 
(MuM't  coiicrot inns.  205. 
CluTt-con^'lnmrnitc,  I39,:iyi,-I54. 

aesrrilH'd,  UiT.  109.482. 
Clicrt-sclii.st,  22">. 

(lesiTibcd.  226,  24.5. 

(Si'o  ('hertij  limcstom\    Fi-miijuioiis  fhtrt,  Flint,  linna- 
titic  c/u-rt,  Jaspci\  Limrstoiif,  Silica,  Quartz.) 
Chorty  carbimalo.     (See  Imn  carbonate.) 
Cherfy  iron  rarbouate.     (Soo  Iron  carhouate.). 
Cborty  limrstono.  3, 137-142.  2IS.  420,  4:t:!,  4:15,  4HS,  443-444,  449, 
451.  452,  457.  4G4.  467,  469.  471,  472  -47;i,  474. 

fragments  of,  423. 

relatious  to  Easteru  area  rtx'.ks,  422, 42.'I. 

n-latioiis  to  gneiss,  446. 

relations  to  granite,  127-128,  445. 

relations  to  Quartz-slate,  130, 134. 139, 141. 142, 143. 144. 147, 
171,180,181,443,454,464. 

relations  to  schist,  127-128,  446. 

relations  to  slate,  128, 134, 139,  445. 

thicknessof,  130, 141. 

iiu conformably  below  Penokee  seri('s  proper,  454-455. 

uucontbrniabie  above  Sontberu  Complex,  444-454. 
(See  Chert,  Limestone.) 
Chippewa  valley  district,  1,  460,  472,  473. 
Chlorite,  alteration  of,  353. 

from  actiuolite,  210:213-214. 

from  augite,  356,414,  415,  417. 

from  feldspar,  118,  122,  151,   179,  180,  333-336,353,413,414, 
415,  41S. 

from  hornblende,  322. 

from  plagioclase,  115, 417. 

from  pyroxene,  115. 

of  actinoUtic  slate,  196, 210, 213,  364. 

of  aagiteporphyrite,  415. 

of  chert,  134. 

of  clay-slate,  305. 

of  coii^lomerate,  448. 

of  diabase,  356.  359. 

of  ferruginous  chert.  203. 

of  ferruginous  slate,  203. 

of  gneiss.  108-109,  111,  116,  US.  119. 120. 

of  grauite,  106, 107, 113, 123, 123. 

of  graywacke,  307,  333,  334,  335.  336. 

of  greenstone-conglomerate,  375,  376,  377. 

of  iron  carbonate,  190.  200. 

of  mica-schist,  307, 308,  309. 

of  mica-slate,  307,  308,  309. 

of  microgranite.  112. 

of  porpliyrite,  418. 

of  quartzite,  153-154. 

of  Quartz-alate,  148, 151, 152. 468. 

of  schist,  121. 

of  slate,  364,  365,  370. 

of  syenite,  122. 

of  Upper  slate,  303. 

of  veins,  202,  418. 

relations  to  hornblende,  119. 
Chlorite  amygdtiles,  418. 
Chlorite-gneiss.  108, 116. 
Chlorite-rock,113. 
Chlorite-schist,  308,  309. 
Chlorite-slate,  described,  382, 384, 391, 486. 

Easteru  area,  369,  391. 
Claiborne  iron  carbonate,  247. 
Clay-shale,  147, 149, 170. 


Clay-slate,  147. 302. 303,  3(I4. 344.  363, 365.  389,  391.  465.  469. 

analyHJH  of,  306. 

cleavage  of,  434. 

described,  305-306. 327-328, 330, 331 ,  372, 398, 399, 400. 

dip  of.  434. 

relatioiiM  to  gray  wackoa,  305. 333. 

relafions  to  grecn.Htonc-r(inglonn.'rate,  374,  375. 

I'olatlonsto  iron  on-.  285, 286. 
(Seo.S7ff/c,  Phyllitc.) 
(Meavage  of  actin()lili(!  slate,  J95. 
CU^avage  of  slate,  296.  390,  426,  427,  434 . 
C(dby  mine,  IGl,  232,  269.  270.  273,  281, 3.55,  .510. 
Concenlration  of  iron  oxide,  254. 
( 'oncentral  ion  of  silica,  255. 
Concretions,  205-209,  268. 

of  actinolitic  slate,  212, 258,  266.  267. 

of  ferruginous  chert,  212. 254,  255,  257,  265. 
(.'onglomerate,  147,  148,  302,  303,  304-305,  364, 393.  394,  395.  454. 
455. 

basal,  172,  173,  174,  180,  181,  388,  391,  443,  447-451.  4.57,  461- 
462,464,471,472. 

basal,  relations  to  ^athern  Complex,  422. 

described,  159, 161. 164-165.  326,  371, 400,  401.  409,  482. 
(Sec  J'asper-eoiiglomerato,  Vhcrt-congloinvrate.) 
Conglomerate- slate,  unconformably  above  schist,  147. 
Conover  referred  to,  9-10,  44. 
Copper-bearing  series,  47, 48,  66-67. 

relations  to  grauite,  104. 
(See  Keweenatvan.) 
Coutchiching,  473.' 
Credner  referred  to,  56. 
Cretaceous  cliert,  252. 
Crystalline  rocks  of  "Wisconsin,  41. 
Crystallization  of  grauile.  113. 

of  microgranite,  112-113. 

of  syenite,  114. 
Currant  river,  469. 

D. 

Dakota,  473,  474. 

Dawson's  road,  498. 

Decomposition  of  rocks,  effect  of  enTironment  on,  358, 359. 

Devonian,  86. 

Diabase,  103,  115,  122,  261,  268,  346, 347,  425,  431,  435.  465.  466. 

alteration  of,  348, 355, 356, 357,  358,  359. 

analysis  of,  357. 

description  of,  348,  354.  410-414. 

grades  into  gabbro,  348,  350,  3.58, 410. 

of  Kcweonawan.  52,  349,  377. 

of  Southern  Complex.  349,  358. 

ophitic  structure  of,  350, 351, 410. 

relations  to  augite-porphyrite,  415. 

relations  to  greenstone  conglomerate  area,  410. 
Diabase-por]3hyrite,  described.  416,417. 

of  Keweenawan,  377. 
Dial  compass,  "Wright  on,  52. 
Diallage,  alteration  of,  115, 353. 

of  g.abbro,  115,  350,  353. 
Dikes,  346,  347,  .3^8,  358,  359. 

alteration  of,  255,  271,  290. 

dip  of,  279. 

of  iron  formation,  35.5-358,359. 

of  Southern  Complex,  349. 

of  Upper  slate,  358. 

pitch  of.  278. 

relations  to  Iron-bearing  member,  12,  271-274,  465,  466. 

relations  to  Keweenawan,  465, 


526 


INDEa 


Dikes,  relations  to  quaitiite,  272,  374. 

relations  to  ahutt.  277,  278. 

thickness  of,  273. 
Dike-rock,  relation  to  iron  ores,  92,  274-275,  276,  277,  287,  289. 

290,  291,  292,  295,  3S,5. 
Diorite,  12,  52. 

I'rom  diabase,  355. 

Irom  Iron-bearing  member,  271. 

of  Keweenawan,  349. 
Diorite-porpliyrito,  370. 
Dioritic  rocks.  07,  09. 
Dip  of  clay -slate,  434. 

of  (likes,  272,  273, 276,  279. 

of  quarbzite.  270. 

of  slate,  427-428.  ♦ 

Dipping  needle,  52. 
Dolomite,  described,  13.0-137, 480. 

of  chert,  133. 

of  gray  wacke,  333,  33G. 

of  limcstolae,  130, 131,132. 
Drifts  in  iron  ore,,  278. 
Dulnth  gabbro,  37,  81.  v 

Dnmkowski,  von,  referred  to,  140,  251. 


Eakins,  anal.ysis  by,  306. 
Eastern  area,  300-436, 450, 467,  518. 

iron  ore  of,  305,  366. 
Eastern  area  rocks,  relations  to  cherty  limestone,  422, 423. 

relations  to  eastern  sandstone,  431. 

relations  t(t  greenstone  range,  422. 
Eastern  sandstone,  434,  430,  407. 

relations  to  eastern  area  rocks,  431. 

rolati(nis  to  gneiss  and  granite,  389. 

relatiorrs  to  Keweenawan,  47, 461-463,  460. 

relations  to  Penokeb  series,  12,  301. 401-403, 466. 

relations  to  Sonthern  Complex,  461-403. 
(See  Lake  f^uperior  sandstone.) 

unconformity  below,  388,  461-463. 
Emmons  (E.)  referred  to,  79. 
Emmons,  S.  E.,  on  lead  ores,  293. 
England,  chert  of,  231, 252. 
English  lake,  7,  300,  301,  303,  309,  459. 
English  lake  area,  8, 17-19. 
Enlargement  of  angite,  80,  353,  411. 
Enlargement  of  feldspar,  180, 395, 409, 
Enlargement  of  hornblende,  80,  353,  410,  411. 
Enlargement  of  quartz,  112, 132,150,152,  153,  180,  185,  209,  5 

288,  305, 33 1,  335,  339.  343,  345,  377,  395,  396,  444.  468. 
Enrironment,  effect  of,  on  decomposition  of  rock.  358,  359. 
Epidosito,  119. 
Epidote  aniygdnles,  377. 
Epidote,  from  cidorite,  353,  414,  415. 

from  feldspar,  118,  413, 414,  417,  418. 

of  augite-porphyrite,  415. 

of  gneiss,  108, 109,  111,  1 16, 118, 119. 

of  greenstone-conglomerate,  375,  376,  3T7. 

of  porphyrite,  417. 
Epidote-gneiss,  116. 
Epidote  veins.  418. 
Eruptives,  340.  359,  304,  305,  374-387, 410-419. 

of  Iron  bearing  member,  271-274. 

relations  to  actinolitie-slate,  259. 

relations  to  greenstone  conglomerate,  374. 

relations  to  Iron-bearing  memlier,  346,  361. 

relations  to  Upper  slate,  347,  348,  358. 


Y. 


Eault  at  Bad  river,  26,  77, 188, 438-440. 

at  Potato  river,  440^141. 

in  Eastern  area,  424-425,  441. 
Eaultiug  along  dikes,  347. 
Federal  mine,  273. 

Felch  mountain  district,  300,  472,  473. 

Feldspar,  alteration  of,  107,  108,  110,  118,  133,  125, 148, 150, 151, 
155,  179,  180,  305,  306,  333,  334,  335,  336,  337,  338,  339,  340, 
342,  343,  353,  356, 415.  418,  444. 

analysis  of,  352. 

enlargement,  180,  395,  4U9. 

growth  of,  110. 

included  in  hornblende,  119, 413.  ., 

of  augite  iiorphyrite,  415. 

of  biotite-slate,  338,  339. 

of  biotite-schist,  339,  340,  341. 

of  chert-cimglomerate,  451. 

of  clay-slate,  305. 

of  conglomer.ate,  305, 305, 448. 

of  diabase,  350,  351,  353,  354,  356,  3.V7,  359. 

of  gabbro,  115. 

of  gneiss,  108, 109-110,  111,  110, 119, 120, 121, 123, 125. 

of  granite,  100, 107,  111,  112, 122,  123. 

of  gray  wacke,  300,  333,  333,  331, 336, 337. 

of  greenstone-conglomerate,  379. 

of  mica-schist,  307,  308,  345. 

of  mica-slate,  307,  308,  343,  343. 

of  microgranite,  112, 113. 

of  novaculito,  147. 

of  porphyrite,  417. 

of  (luartzite,  153,  305,  369,  391. 

of  Qnartz-slate,  134, 143, 147,  148, 149, 150, 151,  1.12,  443. 

of  schist,  117, 118, 121,  122,  125. 

of  slate,  380. 
•      of  syenite,  11 4, 115, 133, 125. 

of  ijpper  slate,  303,  343,  344,  345,  465. 
porph.vritic,  418. 

(See    Plaf/loclatie,    Ortkoclase,    MicroeUn<^,    Labyadorite, 
Atioythlte,  Oligoclase,  Ajldesine,  Anorthoclase.) 
Ferrite  of  biotite-slate,  338. 

of  gray  wacke,  335. 

(See  Iron  oxide.) 
Ferro-doloraite,  300, 393. 

described,  399,  400. 
Perrnginons  chert,  190,  192,  198,  379,280,294,362,364. 

breociation  of,  207,  209. 

concretions  in,  205-209,  212. 

crystallization  of  207-268. 

described.  194,   202-209,  264-205,  490,  492,  494,  500,  502,  504, 
518. 

grades  into  ferruginous  slate,  194,  205. 

grades  into  fragmentalrock.  209. 

grades  into  iron  carbonate,  199,  203.  246,  256,  259.  260,  294. 

rel.ations  to  actiuolitic  slate,  200, 203.  2:i  i,  2 12-213, 246,  257, 
258,  259,  266. 

relations  to  iron  ore,  303,  203,  275,  270,  283,  287, 295. 

origin  of,  249.  254-257. 
Ferruginous  schist,  origin  oi",  85-80. 
Ferruginous  slate,  190, 198,  203,  279. 

described.  192-194,  202-205,  264. 

grades  into  ferruginous  cliert,  194,  305. 

grades  into  iron  carbonate,  193,  201-202,  205,  253,  294, 

origin  of,  253. 
First  National  mine,  164, 
Flint,  256, 


INDEX. 


527 


Flint,  tloHcrihed.  228-230, 231. 233. 

(Set-  Clifit.) 
Foliation  oCschisI.  in:;.  III.  117. 125, 30K.  442. 
I'tMH  -«  nil  of  iron  ore.  'J70-'J71.  274, 276,  277. 
Fo«tor  rrtiM-ml  toA^.  lH.  14.  15-17. '.H,  71. 
Fni;;mcntal  ami  uonrraf.!;inont!U  rocks,  t^radationa  lictwiu'ii, 

209.  24(5. 
Fra;;uiuntal  rofU.  grceuatoiio- i-onj^loiiieratf  ;;iaih;^  iiit.o.;i7!)- 

rolutioDS  to  j^rimitu,  ll'J."). 


Cf. 


Gabbro,  67,  105,  111,113,  116-^116,  261,262.208,348,351,358,410, 
425,  435,  458,  466. 

grades  into  diabase,  348.  350, 358. 410. 

of  Kewtt'Uawan,  3t",  37.  38.  HO,  129.  349. 

relations  to  Iruai-beariug  member,  185. 

relations  to  slates,  36. 
Garnet,  analysis  of,  214. 

of  actinolitic  slate,  195, 213, 214. 

ofniica-sebist,  119. 

of  Upper  slate,  214. 
Geiliie.  definition  of  graywacke,  306. 
Geodes  in  ferrnginous  ebert,  205. 
Germania  mine,  162, 228. 
Geyserite,  origin  of.  251. 

Gneiss,  2.  6.  20,  36,  37,  47,  5'J.  80,  81. 103. 107-111. 116-122, 123, 139, 
388,  428.  434, 435.  442.  443,  449, 452,  462,  465, 467, 471,  473. 

de.scribed,  107-111, 116-122. 

foliation  of,  117. 

grades  into  granite,  107, 123-124, 125. 

origin  of.  122.  125-126. 

relations  to  Clierty  limestone.  446. 

relations  to  Eastern  sandstone,  389. 

relations  to  graywacke.  118. 

relations  to  (Treeiislone-eonglomerate, 420, 421. 

relations  to  Iron -bearing  member.  185. 

relations  to  mit  a  schist,  85,  308. 

relations  to  Qnartz-slate,  109, 143. 171. 
(See     Biotite-gncins.      Chlorite-gneiss,     Epidote-gneiss, 
Gmnlfe.  Bonthlenile-gnei's,  f-'ckist,  Sericite-gneiss.) 
Gneissoid  granite.     {See  Gnei'^s,  Granite.) 
Gogebic  lake.  2,  3,  4.  6.  10,  11,  12,14,15,16-17.30,31,32,66.67, 

105, 187,  421,  434.  461,  463,  464,  466. 
Gogogasbngnn  river.     i^eeMontreal  river.  West  branch,) 
Goocli  referred  to,  256. 
Grand  Portage  bay,  262. 

Granite,2.6, 15,19,  20,  67,  US.  SI.  303,  118,  145,146.344,388,394, 
395,  428,  434,  435.  442,  443, 444,  449,  450,  451,  452,  453,  454, 
457,  462,  465.  ^167.  469,  471,  473. 

crystallization  of,  113. 

described.  106-107. 111-113. 122. 

grades  into  gneiss,  107. 123-124, 125. 

of  Keweenaw  series,  80. 

of  Menominee  district.  34-36.  81. 

recomposed,  388,  391,  394,  395.  403.  407,  435. 449,  451,  454.  462. 

relations  to  Cberty  limestone,  127-128,445. 

relations  to  Eastern  sandstone.  389. 

relaiiona  to  Greenstone- conglomerate,  420, 421. 

relations  to  Huronian.  34-36,  37-38. 

relations  to  Iron-bearing  member,  71-72, 185, 187, 189. 

relations  to  Keweenaw  series.  13-14, 104. 

relations  to  Penokee  series,  lii5. 

relations  to  Qnartz-slate,  128, 1 13. 171.  174. 179, 180.  445, 448. 

relations  to  schist.  104-105, 116-117, 124, 442, 445. 
(^ee  GneihSy  Microgranite.) 


Granite,  nnconforniably  below  Animikio  series.  261,262. 

Granite  areas,  relations  of.  10.'). 

(iraiiitoid-gneiss.     (See  GnnvB,  tirnnite.) 

(irapliitic  niaturial  of  iron  carbonate,  I9U. 

Graywacke.  3.  261,  262,  301.  302.  303,304.344,301.433,465,472. 

described,  161,  167-168,  170.  306-3;t7,  3l.''.-316,  317-318,319, 
321.  322,  323,  325,326-327.328.  329-330.  330-331,  332-;)38, 
371,  396-397,  398,  401-402. 512,  514. 

origin  of,  77-78,  334-335. 

relations  to  cluy -slate,  305, 333. 

relations  to  gneiss,  118. 

relations  to  iron  ore.  285. 

relations  to  micit-schist,  307,  308,  309,  341. 

relations  to  mica-slate.  307.  308, 309. 
(See  Graywacke-hlato.) 
Graywacke-slate.  3.  303.  304,  332,  389,  391,  465,  469.  572. 

described,  159-160,  161,  1G5-166,  306-307,  318-310,  321,322, 
324,  325,  326,  329.  331,  397-399.  400.  401. 402-403,  404,  484. 

relations  to  iron  ore,  285. 
(See  Graywacke.) 
Great  Britain,  chert  of.  252. 
Greenstone.    (See  Gabbro,  Eruj)tives,t>r€enstone-conglomerate^ 

Diabase.) 
Greenstone-conglomerate,  429,  430,  431,  432,434,436. 

described,  374-377,  381-382,  383-387,  426, 518. 

grades  into  fragniental  rock,  379-380. 

origin  of,  377-381. 

relations  to  amygdaloids,  380, 418, 419. 

relations  to  day -slate,  374, 375. 

relations  to  diabase,  410. 

relations  to  gneiss  and  granite.  420. 421. 

relations  to  .jasper  conglomerate.  369,  370. 

relations  to  Keweenawan.  419.  420, 435. 

relations  to  porphyrite,  378, 416. 

relations  to  schist,  420, 421. 

relations  to  slate.  419,  420. 

relations  to  Sontbern  Complex,  419,  420,  435. 

thickness  of,  423-425. 
Greenstone  range,  relations  to  Eastern  area  rocks,  422, 
Greenstone-schist,  .52, 71. 
Griinerite.  possible  presence  of,  215. 
Giimbel  on  schalstein,  374. 
Gnnflint  lake,  192, 248, 261, 262, 268, 469,  470,  500, 506. 


Hague  referred  to,  251. 
Hall  referred  to,  7, 27. 
Hematite,  280,281,291. 

described,  368. 

from  iron  carbonate,  199, 204, 205. 

of  concretions,  206. 

of  actinolitic  slate,  195, 196, 197. 198,  210.  211.  215,  262. 

of  diabase,  356. 

of  ferruginons  chert,  202, 203, 209, 254, 265. 

of  ferruginous  slate,  193,  202,  204. 

of  iron  carbonate,  190,  201. 

of  quartzite,  369. 

of  slate,  392. 

pseudomorphous,  265. 
(See  Iron  oxide.) 
Hematite-schist,  391. 
Hillebrand.  analyses  by,  130-131. 191. 
Hinde,  on  origin  of  chert,  140, 251, 252. 
Hobbs  referred  to.  413, 
Hornblende,  alteration  of.  122. 


enlarpment  of,  86, 353, 411, 412, 


528 


INDEX. 


Hornblende,  from  angite,  4^5. 

from  dialUtge,  115. 

from  feldspar,  118, 125,  353. 

from  pyroxene,  115,  353. 

inclusions,  119. 

of  diabase,  353, 359. 

of  gabbro,  115. 

of  gneiss,  108, 109, 110, 116, 118-119, 120, 121. 

of  granite,  112, 123, 123. 

of  greenatone-t'onglomerate,  377. 

of  syenite,  114,115, 122. 

of  syenite-schist,  125. 

pararaorphic.  353, 412. 
Hornblende-biotite-syenite,  described,  478. 
Hornblende-gneiss,  108, 109-111, 116, 120-122,  476. 

origin  of.  111. 
Hoinblende-grauito,  described,  111-113, 478. 
Hornbleude-roclc,  3'i. 
Hornblende-schist,  described,  476. 

origin  of.  111,  121. 
Ilornltlende-ayenite,  114. 
llornstoue,  cliaracter  of,  200. 
Hudson  river,  263. 
Hunt  referred  to,  44,  56,  79. 
Huronian,  1-2, 10,  37, 47,  48,  59,  62-65, 86-87,  468,  472,  473, 474. 

relations  to  Aniniikie.  2,  81. 

relations  to  basement  complex,  81-82,  86. 

relations  to  granite,  34-36, 37-3S. 

relations  to  Keweenawan,  47,  65-66,  67,  81,  82, 8G. 

relations  to  L»irentian,  43-44,  47,  58,  61,  82. 
(See  Peiiokec  scries.) 
Hydro-miea-slate.  position  of,  80. 
Hyperathene  of  diabase,  354. 


Iowa,  473, 474. 
Ireland,  chert  of,  140, 251. 

Iron-bearing  member,  3,  12,15,17,71-72,  146,  182-295,297.361, 
362,  363,  364.  365,  369.  429,  430, 432,  433,  430,  438.  440,  441, 
455-456,  457,  458,  459,  460,  464,  465.  468,  469,  471,  472,  473, 
474, 490,  492.  494,  496,  498,  500. 502,  504. 
content  of  iron  in.  182-184. 
diabase  in,  348,  350.  359. 

dikes  in, 12, 271-274,  347,  348, 355-358,  359,  465-466. 
flexures  of  437.  438. 
jasper  fragments  from,  174. 
magnetic  attractions  of,  186. 
nonfragmental,  245,  246. 
quartzite  fragments  in,  175. 
relations  to  eruptivcs,  346,  361. 
relations  to  gabbro,  185. 
relations  to  gneiss,  185. 
relations  to  granite,  71-72, 185, 187, 189. 
relations  to  Ke-n-ceuawan,  187, 188, 362. 
relations  to  quartzito,  181, 185. 
relations  to  Quartz-slate,  144. 175, 176. 181, 184-185, 188, 189, 

200, 294,  299,  363,  455.  456,  464. 
relations  to  Southern  Complex.  189. 
relations  to  Upper  slate,  200,  294,  296,  297,  298,  299,  300,  456, 

464-465. 
thickness  of,  187, 188, 189-190,  361,  362. 
topography  of,  145, 188-189,  301. 
Iron  Belt  mine,  269. 
Iron  carbonate,  280,  364,  379,  464,  466, 469,  471. 

alteration  of,  199,  201,  204,  205,  254.  283,  284,  286-290,  291, 292, 

294,  295,  393,  434. 
analysis  of,  191, 


Iron  carbonate,  descrilied,  190-192,  200-203,  332,  233.  235,  236, 
237,  238,  239,  262-264,  496,  498,  506. 
grades  into  actinolitic  slate,  246.  257,258,360,362.266.267. 

294, 362.  ., 

grades  into  ferruginous  chert,  199,  202,  246,  256, 259,  266. 

294. 
grades  Into  ferruginous  slate,  193,  201-203, 205, 253, 294. 
of  actinolitic  slate,  258,  259. 
of  Animikie  series.  259,  268. 
of  concretions,  207,  208. 
of  Eastern  area,  365. 
of  ferruginous  chert,  257,  258. 
of  feiTuginous  slate,  193,  203,  204. 
of  other  series,  252,  260. 
origin  of,  246-253,  263. 
position  of,  198. 
pseudomorpbs  of,  201. 
relations  to  iron  ore,  260, 268,  282,  283. 
eiderite  of,  256. 
silica  of,  199,  202,  256. 
(See  SideHte.) 
Iron  King  mine.    (See  Mount  Bope  mine.) 
Iron  mines,  Birkinhineon,  87-92,  92-96. 

Iron  ore,  67.  09-71, 102, 198.  268,  26  l,  281,  283,  291,  471,  508,  510. 
analyses  of,  381 . 
character  of,  280-283. 
concentration  of  254,  283-292,  295. 
depth  of,  274-275,  292-293. 
foot-ivall  of,  270-271,  274,  376,  277. 
from  iron  carbon.ate,  260,  283,  284,  286-292.  295. 
in  Laurentian,  94. 
manganese  in,  280-381,  392. 
of  actinolitic  slate,  258. 
of  Eastern  area,  365-366. 
of  ferrnginons  slate,  203,  204. 
of  other  districts,  293. 
origin  of,  85-86,  281-290. 
position  of,  184, 185, 198.  268-271,  294. 
relations  to  clay-slate,  285.  286. 
relations  to  dikes,  93,  274-275.  276,  277,  287,  289.  290,  291.  292, 

295,  365. 
relations  to  ferruginous  chert,  202,  203,  275,  276,  283.  287. 

295. 
relations  to  gray wacke,  285. 
relations  to  greenstone,  12. 
relations  to  iron  carbonate,  260.  268,  282,  383. 
relations  to  mang.anese.  91-93,  282. 
relations  to  quartzite,  94. 185, 199,  274,  276.  285,  286,  387.  391, 

292,  294,  365. 
relations  to  Quartz-slate,  135, 199,269,  270,  286,  287,  294,  395. 
relations  to  sandstone,  285. 
relations  to  silica,  282,  284. 
relations  to  soapstone,  91, 94,  294. 
sulphnr  in,  281. 
(See  Iron  oxide.) 
Iron  oxide  from  iron  carbonate,  253,  268.  393.  434, 
of  actinolitic  slate,  210,  212,  215,  257,  266. 
of  chert,  133. 

of  concretion,  205,  206, 208. 
of  feiTuginous  chert,  254, 257. 
of  gneiss,  119, 120.  '"• 

of  graywacke,  333. 
of  greenstone-conglomerate,  375. 
of  quartzite.  153. 
of  slate,  305,  370,  376,  434. 
(See  Ferrite,  Hematite,  Iron  ore,  Magnetite.) 
Ironton  mine,  273. 


INDEX. 


529 


Irving,  referred  t«.  xiii.  Xiv,  xv.  3.  8.  P.  33.  34,  3R.  40.  41-43. 
44-r>2.  57,  5S.  «t(-ti7.  77-78.  7»*.  70.  81 .  82.  82,  83. 85.  8((,  87.  OR, 
183, 250,  200,  2(11,  ;J10,  377,  412,  438,  402,  405. 


.Tiuksnn,  roforri'tl  to,  5,  0. 13, 14, 15. 
Jasper,  145.  208,  363.  4«2,  469.  471,  504. 

described,  233,  234, 238.  239,  240. 

.Iiixtf.3fi9.3fln. 

I'ragiueuts  iii  Eastern  area.  423. 

of  couglonu-rate.  174. 180.  305,  448,  4,55. 

of  irun-bearing  member.  189. 19i),  193, 100, 204, 209. 

origiu  uf,  249. 

relations  to  chert,  133. 
(See  C'lieif,  Flivt) 
rraspereongloinerate,  described,  372. 

relations  to  Greenstone  i'ouj;lomerate,  309,  370, 

relations  to  iiorpliyrite,  414. 
Julien.  referred  to,  33,  52. 

K. 
Kakabikka  falls,  192. 
Kaministiquia  river.  192, 202. 
Kaministiqiiia  district,  473, 474. 
Kaolinfrom  feldspar,  115, 118, 151, 155, 179,  333,  334.  353, 413,  414, 

417,418. 
Kaoliuite  of  argillaceous  slate,  155. 

of  augite-porpliyrite,  415. 

of  clay-slate,  305. 

of  diabase,  359. 

of  gneiss,  109. 

of  gray  wacke,  333, 334. 

of  Quartz-slate,  148, 151. 

of  slate,  370. 

of  soapstone,  358. 
Keewatiu,408,473. 

Keweenawan,  2,  19,  20,  30,  36.  37,  38,  47,  48,  59,  00-67.  80,  81,  86, 
261,  349,  358,  377,  424,  430,  431,434,  436,  441,  456, 465,  406, 
407, 473. 

eruptives,  relations  to  P«uokee  eruptives,  349. 358. 

relations  to  Animikie  series,  201, 409. 470. 

relations  to  Basemertti  Cora,x)lex,  470. 

relations  to  dikes,  465. 

relations  to  Eastern  sandstone,  47, 401-463, 406. 

relations  to  granite,  and  syenite,  13-14, 104. 

relations  to  greenstone  conglomerate,  419. 420, 435. 

relations  to  Huronian,  47,  65-06,  07,  81,  82,  80. 

relations'to  Penokee  series.  80,  187,  188,  298,  299,  300,  301. 
347.  348,  358,  302,  443,  450-461,  409-470. 

topograpby  of,  301. 

unconformably  below  Lake  Superior  sandstone,  47. 
{See  Copper -bearing  acricii.) 
Kieselschiefer,  143. 
Kimball,  referred  to.  56. 

L. 

Labradorite,  analysis  of,  352. 

of  augite-])orpliyrite,  415. 

of  diabase,  352, 357, 413. 

of  gabbro,  352. 
Lac  des  Anglais.     {See  English  lake.) 
J.ake  Superior,  470. 

Lake  Superior  region,  N.  H.  Wincbell  on,  78-81, 
Lake  Superior  sandstone,  47, 48. 

(See  Eastsrn  sandstoin'.) 
Lake  Superior  synclinal,  407,  470. 

MON  xi:^ 34 


Laphnm  referml  tn.  7-8, 22-27. 

Laureiilian.  47.  48.  ."ilMil.  94.  139.  468,  473. 
ivlalion  1o  IIuninian.43-44,47,58,0I,82. 
(Sue  lifiNvmvnt  vumpU'X,  Sonthfni  coinplfx.) 
Lava  Hows,  360,  301,302. 
Lava  of  greensloiir conglomerate,  379. 
Lawscm  referred  to.  121. 
Lawton  referred  to,  92. 
Lead  ores  of  LeiidviHo.  293. 
Leadville.  (,'oln?iicbi.  293. 
Leucoxeue  from  menaccauite,  414, 415, 416. 

of  augite-porphyrite,  415. 

of  diabase,  351. 

of  gre.enatoue-conglomeratc,  375.  378. 

of  sehiat.  122. 
Life  in  iron  carbonate,  250. 
Limestone,  52, 139,  205.     . 

al)scnce  of  life  remains  in,  140, 141. 

analyses  of.  130^131. 

brecciation  of.  207. 

described,  130-132, 133, 139. 

concretions  in,  205. 

origin  of,  140-141, 142. 

relations  to  cbert,  127-128. 132, 139, 141. 

relations  to  actiuolitic  slate,  259. 
(See  Chert!/  litnesfone,-  Dolomite.) 
Limonite  of  diabase,  3561 

of  iron  carbonate,  190. 

of  quartzite,  369. 

of  slate,  392. 
Little  Falls,  Minnesota,  85. 
Little  Presque  Isle  river,  128, 188, 199,  360,  361,  368,  369,  389,  414, 

421  428, 433. 
Little  Presque  Isle  river  area,  71. 
Livingston  Manor  House,  New  York,  263. 
Logan  referred  to,  56. 
Longyear,  J.  M.,  indebtedness  to,  xiv,  12. 


M. 


Magnesian  slate,  position  of,  80. 
Magnetic  attraction.  180,  195, 
Magnetite,  alteration  ()f,  350,  358. 

from  iron  carbonate,  205,  302. 

in  concretions,  206. 

in  upper  slate,  297. 

in  aetinolitic  slate,  195.  197,  198,  210,  211.  212.  213.  215.  257, 
258,  206,  267,  279,  294,  302. 

of  chert,  133, 134. 

of  chlorite-rock.  111, 

of  clay-.slatp.,  305,  300. 

of  diabase,  350,  351,  .354,  350,  358. 

of  ferruginous  chert,  202,  203.  209,  254.  205,  266, 

of  ferruginous  slate.  193,  202,  204,  204,  260. 

of  gabbro,  115,351. 

ofgnei.=is,  108, 109,  111, 

of  iron  carbonate,  190,  200,  201,  266. 

of  quartzite,  309. 

of  .schist,  121. 

of  slate,  303,  393. 

origin  of,  258.  '^ 

pseudomorpha.  201, 258, 265, 200. 

relations  to  actinolite,  258. 

relations  to  biotite,  354. 

relatitms  to  siderite,  433. 
{See  Iron  oxide). 
Magnetite -schist,  described,  24;3. 


530 


INDEX. 


Magnetite-slate,  52, 190, 194-198,  242,  294. 

(See  Actiiwlittc  date.) 
Malacolitb  of  mica-hornbleiKle-syenite,  115. 
Maiigauese,  relatiou.s  to  iron  ore.  91-92, 282. 
Marca,site  or  graywacke,  mIJd,  337. 
Marengo  river,  107, 128-129, 138, 175,  476. 
Marengo  river  iron  range,  217. 
Mareuisran,  473. 

Marquette  district,  sill,  2, 10,  37,  .56-58,  8.n,  26U,  261,  293.  356,  460. 
468, 471-473. 
unconformities  in,  471, 472.  " 

Marquette  series,  relations  It)  Penokee  series.  56. 470-472. 
Melaphyre  of  Keweenawan.  349. 
Mellon  ,)iinction,  30.'),  318. 
Menaccanite.  alteration  of.  414,  415, 416. 
of  angite-porpliyrite,  415. 
of  dialiase,  351, 414. 
of  gabbro,  351. 

of  greenstone-conglomerate,  377. 
of  horubloniie-gneiss,  120. 
of  porphyrite,  416,  417, 
of  schist,  122. 
Menominee  district,  1, 2, 10,  34-36,  37, 260,  261,  293,  356. 472,  473. 
Menominee  river,  81. 
Mesabi,  Huronian,  2. 
Mesozoic,  87. 

Metamorphism,  467-468.  ^ 

Mica  from  feldspar,  179,  335-340,  343. 
of  biotite-sdiiat,  339,  340,  341. 
of  biotite-slate,  338. 
of  granite,  107. 

of  gray  w.acke,  304, 307,  334,  336,  337,  338. 
of  mica-pyroxene-syenite.  115. 
of  mica-schist,  .'108,  309,  345. 
of  mica-slate,  308, 309. 
of  Quartz-Slate,  148, 149, 151, 179-180,  468. 
of  Upper  slate,  303. 
Mica-pyroxeue-syenite,  114-115. 

Mica-schist,  3,  4,  34,  52,  67.  84-85,  152,  302,  303,  332,  344.  345,  388, 
465,  466,  409,  472. 
described, 307-309. 
foliation  of  308. 

origin  of  77-78,  SO,  81.  107, 108,  335,  339-341. 
relations  to  gneiss,  85.  308. 
relations  to  gray  wacke.  307.  308,  309.  341. 
(See  jlffca-bia/f,  BiitUc-datc,  Biotite-sehist.) 
Mica-slate,  3,  52,  84-85,  302,  303,  332,  344,  465,  472. 
described,  307-309. 
origin  of,  335,  341-343. 
relations  to  gray  wacke,  307,  308,  309. 

(See  Mica-tichisf,  Biotite-slate,  Biotite-schittt.} 
Microeline,  alteration  of  335. 
of  conglomerate,  448. 
of  granite,  106, 112, 123. 
of  granitoid  gneiss,  123. 
of  gray  wacke,  333,  335,  336. 
of  Quartz-slate,  150. 
of  schist,  118. 
of  syenite-schist,  114. 
Microgranite.  crystallization  of  112-113. 
Milwaukee  Iron  Compau,v  referred  to,  8. 
Miner  t  Wells  option,  191,  286. 
Minewawa  mine,  51U. 
Mining,  rules,  for,  276-279. 
Minnesota,  36,  260,  261 ,  411, 470, 474. 
Mississippi  iron  carbonate,  247. 
Montreal  mine,  269, 270. 


Montreal  river,  127, 299,  308,  456,  467,  482. 502,  508. 

west  branch,  147, 161-162, 173, 173, 178,  227,  369,  418. 
Montreal  river  area  6,  7, 10, 11, 12, 15, 17-19,  31,  32. 43, 48,  67,  71, 
84,  lll-llo  115-116, 145,  30(. 

west  brancii   84,  lOi,  109-111, 121, 129, 144, 146,304,  328,329. 
Mount  Hope  mine,  16i,  231,  209,  270,  272-273,  281. 
Mount  'Whittle.se.v  ,  144, 145, 146, 176, 189, 454. 
Murray  referreu  to,  56. 
i   Muscovite  from  feldspar,  305,  333,  334,  335,  336,  337,  338,  339. 

of  biotite-sohist,  339,  340,  341 . 

of  biotite-slate,  339. 

of  granite,  106. 

of  gray  wacke,  307,  333,  334,  335, 336. 

of  mica-schist,  307,  308- 

ofmica-slate,  307,  308. 

of  Qmti'tz-slate,  148, 151- 


Nipigon,  473. 

Nonfragmental  and  fragmental  rocks,  gradations  between, 

246. 
Norrie  mine,  84,  92,  273,  277,  281. 
North  lake,  261. 
Novaculite,  147, 148, 178. 

described, 154-155, 164. 
Numakagon  lake,  2. 186,  456,  460,  464, 468. 
Nnmakagon  lake  area.  9, 11.  66. 
Numakagon  river,  104, 457, 469, 470. 

O. 

Ogishki  conglomerate,  473. 
Ohio  iron  carbonates,  247,  263,  490. 
Oley,  indebtedness  to,  xiv. 
Oligoclase.  alteration  of,  335. 

of  gray  wacke,  355. 
-  of  Quartz-slate,  150. 
Olivine  of  diabase,  350,  351,  352. 
Ontario,  260,  469,  470,  473. 
Ontonagon  river  area.  6.  8, 14,  30. 
Ophitic  structure  of  diabase,  330,  351,  410. 
Ores.     (See  Iron  ore.) 
Ore  veins,  number  of,  94. 
Organic  matter  in  iron  carbonate.  250. 

in  .slate,  250. 
Orthoclase,  alteration  of  335,  338. 

ofbiotite-schi.st,340. 

of  biotite-slate,  338- 
■      of  gneiss,  109, 123. 

of  granite,  106, 112, 123. 

of  gray  wacke,  333,  335,  336. 

of  Quart  z-.slate,  1.50. 

of  schist,  118. 

of  syenite-schist,  114. 
Owen  referred  to,  7, 17, 19, 27. 
Oxide  of  iron.    (See  Iron  oxide.) 

P. 

Paint  rock,  270. 

.     (See  Soapstone.) 
Paleozoic.  87,  252. 

Palms  mine,  164-165. 174, 179, 191,  233,  448, 449,  451, 454, 471. 
Paramorphic  amphibole,  435. 

hornblende,  353. 
Prtrker,  referred  to,  94. 
Peale.  referred  to,  256. 
Pegmatitic  structure,  106, 123. 
Pence  mine,  273»508. 


INDl'.X. 


53i 


Peninsular  Mining  Onnipnny.  Hifl. 
PtjnliHylviiniii  Iran  rnrlioimio,  217. 
Pt'UokiM'crnptivert,  rchitiouH  fo  Kewfrnawaii  (-'riiptivrs.  :m;). 

Penokeu  Unp,  xm,  22-27.  30,  40,  77,  104-10.".,  127,  i:;o,  i:i2.  13:), 
urn -139,  144,  irM-l.W,  171.  170,  1S3.  IH4,  I.S.-.,  1117,200.213, 
•.>l«-22;),  298,  300,  303,  344,  430,  410.  152,  ir.l,  4C5. 
Penokeo  Ciogubic,  imiiio  (•iiiiMi(UM'f<l,3. 
Piilioki'n  riui'icf.  11-12,  22-27.  30,  42,  43,  iVJ.  188,  220.  301. 
I'enoken  .s'-ries,  umMnil'uniiahly  ahitvo  rliurly  liim-.slcnu'.  4.'»4- 

455. 
^    ii'latiiiiKS  to  Animikie  .series,  BG,  261,  202, 4G8-470. 

rcliitiuns  to  Eastprii  aaiidstouc'.  12,  301.  401-403,  460. 

rolution.s  to  Kowi^eliawail.  SO,  187,  188,  208,  299,  300,  301.  347. 
348.  358,  302,  443.  450-101.  400, 470. 

rt)latiou3  to  Marquette  series.  56,470—172. 

relations  to  Southern  Complex.  105, 109,  343, 444-1.54, 457. 
Penokie,  first  used  by  Wliittlesey,  22. 
Permian.  140,252. 

Pewabic,  proposal  of,  by  AYiiittlesey,  22. 
Pbyllite,  302,  304,  305-306. 

(See  Clay-slate.) 
Pigeon  point,  261, 
Pigeon  river,  2ol. 
I'itebof  (likes,  273,276. 
Plagioelase,  alteration  of,  115,  353, 413, 417. 

of  augite-porpbyrite,  415. 

of  biotite-scbist.  340. 

of  diabase,  350,  351, 353,  354, 410, 413. 

of  gabbro,  115. 

of  gneiss,  109,121,123. 

of  granite,  106,123. 

of  graywacke,  333, 336. 

of  greenstone-conglomerate,  375,  376, 377. 

of  jasper-conglomerate,  370. 

of  niierogranite,  112. 

of  porpbyrite,  416-417,  ^S. 

of  Quartz-slate,  150, 

of  .schist,  118. 

of  syenite-scbist,  114. 

relations  to  biotite,  354. 
(See  Feldspar.) 
Porpbyrite,  109, 122,  349,  381,  388,  429,  432,  435. 

described, 417-418. 

relations  to  greenstone-conglomerate,  378, 416. 

relations  to  jasper-conglomerate,  414. 
Porpbyritic  structure  of  inicrogranite,  112. 
Porpiiyry,  gr.anitic,  36, 37. 
Poi'tage  lake  area,  14, 15. 
Port  Arthur,  282, 469, 498. 
Potato  river,  109-111, 121 ,  129, 159-160, 173, 178, 198, 199, 223,  303, 

307,  308,  309,  324-325,  369,  440, 446, 448,  454,  471.  , 
Potato  river  area,  10, 43-14, 104, 108-109, 144, 140, 147,  304. 
Potsdam  .sandstone,  19. 
Presque  Isle  river,  179, 191,  279,  368,  369. 
Presque  Isle  river  area,  71. 
Prospecting,  rules  for,  276-279. 
Paeudomorphs  after  iron  carbonate.  201. 

of  .actinolite,  258. 

of  hematite,  265. 

of  magnetite,  258, 265, 266. 
Pumpelly,  referred  to,  7,  8, 15.  ^0-31,  32,  33,  66,  66,  78,  463. 
Puritan  mine,  231. 
Pyrite  of  biotite-scbist,  340, 341. 

of  gneiss,  119. 

of  gray\vack6f333,  335,  S37. 

of  iron  carbonate,  190. 


I'yrile  of  luica-flchist,  308. 

i.lslate.  ;ilW,  342.  313.  370. 
I'vi-iili^ile  ill  il-nn  ore,  2S1. 
L\vro\t-ni-.  alteration  of.  353.  354. 

of  diaba.si'.  3.50.  :i.')3.3.')9,  413. 

ofirabhrii,  11,5,410. 

of  u'reenstone-con;;tonierate.  377. 

of  syenite,  114,  11.5. 

(StMi  ViidUi'je,  Amjite.  Malacolite.) 


(.;. 


(Juariz  IVi>iu  feldspar,  118.  180.305.333-340.342,343.413.414. 

iueliided  in  hiM'nlih'ndi',  118. 

of  aetiiuditie  slate,  195,  210.  211,213,  215.  258,  2,50,  206,  3(j2, 
3(i4, 

of  biotitesehist,  339,  340,  341. 

of  biotite-slate,  338,  339. 

of  chert.  123. 

of  chert-conglomerate,  451: 

of  cherty  limestone,  127. 

of  chlorite  rock.  111. 

of  clay-slate,  305. 

of  concretions,  207. 

of  conglomerate,  174, 365,  395, 448. 

of  diabase.  356. 

of  ferruginous  chert,  205.  254,  255,  257,  362. 

of  ferruginous  slate,  193,  204. 

of  gneiss.  108, 109. 110,  116,  110, 120, 121,  123, 125. 

of  granite,  106. 107,  HI,  112, 113, 122, 123,  344. 

of  graywacke,  306,  307,  332,  333,  334,  335,  336,  337. 

of  greenstone-conglomerate.  375,  376,  377,  379. 

of  iron-bearing  member,  267,  268. 

of  iron  carbonate,  190. 200. 

of  limestone.  132. 

of  mica-schist,  345. 

of  mica-slate,  342. 

of  niierogranite,  112,  Tl3. 

of  porpliyrite,  417, 418. 

of  quartzite,  153, 154,  369,  391. 

of  quartz-rock,  133. 

of  (Juartz-slate,  134, 143,  147.  148,  149,150,151,152.179-1811. 
443, 408. 

of.sebist,  117,118, 121. 125. 

of  slate,  370,  376,  380. 

of  syenite,  113, 114, 122.    . 

ol'  Upper  slate,  303,  343,  344,  345, 465. 

replaces  piagioclase,  115. 

solubility  of,  256. 
(See  Chert,  Silica.) 
Quartz  amygdules,  377,  418. 
Quartz  enlargements,  112,  132,  150, 152,  153, 180,  185,  209,  267, 

288,  305,  334,  335,  339,  343,  345,  377,  395,  396, 444, 462,  468. 
Quartz  veins,  418. 

Quartzito,  34,  30, 147, 148, 149, 176, 177, 178, 179, 180,  261,  301,  303, 

304-305,  347,  363,  368,  369,  391,  303,  394.  395-'106.  438.  440, 

.  441 .  444,  440-451,  455, 457,  460,  464, 468,  471.  472,  473.  474. 

described,  153-154.  1.55-156,  157,  158,  159-160.  161.  162,  103, 
164,105.166.  167,  160,170,171,  320.371,372.373,390,400, 
404-105,408-409,498,518. 

dip  of,  270. 

fragments  of,  in  iron-bearing  member,  84, 175. 

origin  of,  77-78. 

relations  to  dikes,  272,  274. 

relations  to  Iron-bearing  member,  181, 185. 

relations  to  Iron  ore,  94, 185. 199,  274,  276,  285,  286,  287,  291, 
292.  294. 365. 

relations  to  C|nartz-rock,  245. 


632 


INDEX. 


Quartzite  relations  to  shafts,  277.,279. 

Qtiartz-porpliyry,  462. 

Qnartz-rock,  290.  * 

aesurilied,  133, 136, 130. 

recomposed,  in  quartz-slate,  172. 

relations  to  quartzite,  245. 
Quartzsehist,  de.scriljed,  216,  217,  21R-219,  220-223,  323-224,  239. 
Quartz,slate,  3,  4,  93,  143-181,  18C,  209,  38.5,  397,  363,  368,  369,  428- 
430.  432,  433, 435, 436,  438-440,  443-444,  459-460,  4C7,  468, 
409,  471,  472,  474,  482,  484,  4,S6,  488. 

basal  conglomerate  in,  139, 172, 173, 174, 180, 181. 

tle.'Cure3  0i;437,43S. 

of  Eastern  area,  366,  308-371 . 

recomposed  quartz-rock  in,  172. 

relations  to  clierty  limestone,  130, 134, 139,  141, 142. 143, 144, 
147, 171,  180, 1 81,  443,  454,  464. 

relations  to  gneiss,  143, 171. 

relations  to  granite,  128,  143,  171,  174,  179, 180,  445,  448. 

relations  to  iron-bearing  member,  144, 175, 176, 181. 184-185, 
188,  189,  300,  394.  299.  363,  455.  456,  464. 

relations  to  iron  ore,  185,  19,1,  36S,  270.  286,  287.  294,  295. 

relations  to  schist,  129, 143, 171,  173, 178,445,  446-449. 

relations  to  Southern  Complex,  143, 171, 172-174, 179-180, 
181,  444-454. 

relations  to  Upper  slate,  296,  297. 

tbiclaieas  of,  143, 144, 146, 180,  444. 
•  topography  of,  145. 188-189,  301. 
Quaternary,  48. 

E. 
Randall,  referred  to,  7, 17. 
Eigga  relerred  to,  191, 192. 
Rohrbach  referred  to,  413. 
Rominger  referred  to,  5,  ia,.67-73,  78. 
Roscoe  referred  to,  259, 289, 
Roth  referred  to.  355. 

S. 
St.  Louis  district,  1, 472. 
Sandstone,  147, 148, 149,  256,  456-457,  462,  463,  464. 

described,  154-155, 103, 165,  167,  486. 

relations  to  iron  ore,  285. 
Schalstein.  Giimbel  on,  374. 

Schist,  2,  20,  80,  81.  94, 103, 104-105,  107-111, 116-122, 125,  438,  435, 
438,  440,  442,  443,  450,  451 ,  452-453,  454,  462,  469, 471,  473. 

of  Iron-bearing  niembei',  described,  217,  244-245. 

of  Qu.artz-slate.  described,  160. 

of  Southern  complex,  described,  107-111, 116-122. 

of  Upper  sl.ate,  described,  316-317,  397,  404. 

foliation  of,  103, 114,  117, 125.  308,  442. 

origin  of,  124, 125-126. 

relations  to  cherty^  limestone,  137-128, 446. 

relations  to  granite,  104-105, 116-117, 123-124,  442, 445. 

relations  to  greenstfuie-conglmnerate,  420,  421. 

relatiou.s  to  quartz-slate,  129, 143, 171. 173, 178.  445,  440-449. 

rehations  to  syenite,  445. 

uuconforniably  below  Auiniikie  series,  261,  262. 

{^ee  J^ctlnolitic-schist,  Sioiitr-schist^  Gneiss.  Greenstone- 
schist,  Hornblende  schist,  Mica-schist,  Syenite-schist.) 
Schistose  rocks,  Whittlesey  on,  20. 
Schorlcmnier  referred  to,  259,  289. 
'  Sericite  from  feldspar,  118,  333,  334. 

of  chert,  133. 

of  cla.v-slate,  305. 

of  gneiss,  109,  111,  110, 118. 

of  gray  wacke,  307,  333, 334. 

of  mica-.schist,  307,  308. 

of  mica-slate,  307,  308. 


Sericite  of  Quartz-slate,  148, 151, 370. 

Sericite-gneiss,  110. 

Serieite-sohist,  described,  324,400-407. 

foliation  of,  428. 
Sericite-slate,  391. 

described,  404, 405^00. 
Serpentine  of  diabase,  356. 

Shafts  in  Peuokee-tTOgebic  r.ange,  relations  of,  377,  279. 
Shale,  148, 177, 178. 

described,  167,  480. 
(See  Clay  shale,  Clay  slate,  Slate.) 
Siderite,  alteration  of,  201, 303,  253, 283, 362. 

described,  337,  338-330,  234,  340-242.  367-368. 

grades  into  ferruginous  slate,  201-202. 

of  actinolitic  .slate,  258,  266,  267,  362,  364. 

of  clay-slate,  434. 

of  ferruginous  chert,  258,  265. 

of  gray  wacke,  333. 

of  iron  carbonate,  200,  201,  258,  263,  204,  268, 294. 

of  slate,  392.       • 

of  veins,  202. 

position  of,  199. 

relations  to  actinolite,  433. 

relations  to  iron  ore,  282. 

relations  to  magnetite,  433. 
(See  Iron  carbonate.) 
Sideritic  chert,  described,  490, 498,  500. 

slate,  described,  490. 
Silica,  concentration  of,  255. 

deposition  of,  384,  28S-290,  292,  293. 

of  actinolitic  slate,  198,  211,  212,  257,  258,  259,  266,  267,  294. 

of  chert  breccia,  395-. 

of  concretions,  207, 208. 
.  of  ferruginous  chert,  202,  203,  205,  212,  254,  255,  256,  257. 

of  ferruginous  slate.  192, 193, 194,  202,  203,  204.  253. 

of  geyserite,  251. 

of  iron  carbonate,  190, 1_99, 200,  202,  256,  263,  264, 294. 

of  jasper  conglomerate,  370. 

of  quartzite,  396. 

of  slate.  392. 

relations  to  iron  ore,  282, 284. 

solubility  of,  256,  288,  289,  290,  292,  293. 
Silurian,  Irving  on,  86. 
Silver  creek  area,  43. 
Sioux  district,  2,  472. 

Slate,  7,  15,  52,  67,  80, 143, 145, 147, 148,  156, 158,  245,  250,  261,  262, 
363,  364,  368,  309,  370.  376,  431,  438-440, 449. 

described,  154-155,  159,  161,  164,  165,  107,  168,  170,  171,  299, 
313-315,  360.  373,  381,  382,  401-402,  402^04, 405,  406,  484. 

dip  of,  390. 

relations  to  cherty  limestone,  128, 134, 139,445. 

relations  to  gabbro,  36. 

relations  to  granite  and  scliist,  128-129. 

relations  to  greenstone-conglomerate,  419,  420. 

relations  to  ii-on  oi-e,  282,  283. 

relations  to  shafts,  279. 

(See  Actinolitie  slate,  Biotite-slate,  Chloiitc-slate,  Clay- 
slate,  Clay-shale.  Ferruginous  flate,  Grayuacke-slute,. 
Magnetite-slate,  Mica-slate,  Quartz-slate,  Shale,  Upper 
slate.) 
Bmaragdite  from  augite,  415.  417. 

from  diallage,  353.    ^ 

from  feldspar,  353. 413,  414,  415, 417,  418. 

of  diabase,  359, 413. 

of  porphyrite,  415,  417. 

relations  to  augite.  413.  435. 
Soapstone  from  diabase,  294,  355, 356, 357, 359. 


INDEX. 


)3;5 


Soapatoiio  of  Irnn-ltnarin^  nu'inln'r,  12.  271,  X>t\  '.i^>7, 

rcliitioiis  t»»  iron  (>ii\m,U4.2ii-*. 
SuUilH  ri'lVlTi'il  tn,  2">l,  25li. 
^■llrhy  rol'i'rrcd  to.  427. 

Smitiii-ni  (!iim|il>'S,  10:1-120.  115,  429,4:11.  1411,  141  4411,  1.18.  4.'.9, 
41111,  4i;i ,  4li7,  47:1,  474,  47li,  478. 

iliiiiia-io  ui;  349,  :ir)8. 

ilikrs  or,  :i49. 

mi'.'ili  ol'.  124-120. 

rolatinii.s  f(i  bayal  roiiLjlomprato.  422. 

rclalitma  Ut  Kastorn  sainlstimo.  4Rl-4t]:t, 

rrlation.s  ti)  gr»jii8linu>-)-(Hi;iloiiusj'uUi,  419-420,  4:15.  . 

rehitioiis  to  Froli-bo^^rini;  nii'liiber.  180. 

velation.-i  to  Pfiiokeo  aerius.  lO.'i,  109.  :!4:l.  444-l.'i4, 457. 

relations  to  Quartz-slate,  109, 14;i,  171, 172-174, 179-18Q,  181, 
444-4.-)4. 

relatiinis  to  Upper  slate,  34:i-344,  :J45. 

uucoiitbriuably  lielow  chorty  linie.stoue,  444-454. 

topography  of,  145, 189,  301. 

(See  Basenii'itt  Complex,  Laui-entian.)' 
Specular  ore  of  aetinolitic  .slate,  19G. 
Spitzliergeu,  chert  of,  140.  251,  253. 
Sponge  spicules,  chert  formed  from,  140.  251. 
Stanrolite  of  niica-scbist,  119. 

Sunday  late,  129, 130-131,  139-140,  144. 146,  105-166, 179, 188,  187, 
191,  25.5,  209,  271,  290,  291,  292,  302,  437,  438,  444,  449,  454, 
456,  459,  400. ' 
Sunday  lake  area,  145, 154,  279. 
Sunday  lake  mine,  273. 
Sunday  lake  outlet,  283. 
Sweet  referred  to,  9, 39-40, 183. 
Swineford  referred  to,  59. 
S.yenite,  36,  80.  103,  105,  111,   110,120,122,123,344,443. 

described,  113-115. 

grades  into  syenite-schist,  125. 

relations  to  ehert^\'  liniestoue,  445 

relations  to  Kcweenawan,  13-14. 

relations  to  schist,  445. 
Syenite-schist,  103, 114, 116, 117, 120, 125. 

described,  125. 
(See  Kornbhnde-syeuite.  2Uca-kornblende-sye7iite,  Mica- 
pyroxene-syejiite.) 


Teall  referred  to,  413. 

Tertiary  iron  carbonate,  247, 

Thunder  bay,  261, 470. 

Tilden  mine,  164,  2:!3. 

Titanite  of  hornblende-gneisa,  120. 

of  greenstone-conglomerate,  375,  378. 
Topography  of  Iron-bearing  member,  145, 188-189, 301, 

of  Keweenawan,  301. 

of  Quartz-slate,  145, 188-189,  301. 

of  Southern  Complex,  145, 189,  301. 

of  Upper  slate,  301-303. 
Tourmaline  of  gneiss,  119. 
Town.  43  N.,  E.  7  W.,  Wisconsin,  104, 186, 451, 457. 

43  N.,  ,R.  6  W.,  Wisconsin,  186. 

44  N .,  E.  6  W.,  Wisconsin,  186, 215, 216, 458. 

44  N.,  E.  5  W.,  Wi-sconsin,  104,107,128,129,  130,131,  135, 

138,  144, 175, 186, 188,  316,  217,  346,  443,  458,  476,  496. 
44  N.,  E.  4  W.,   WisConsin,129,  l;i5, 138,  144, 146,  155,  150, 

175, 170, 185, 188.  217,  298,  351,  458. 
U  N.,  E.  3  W.,   Wisconsin,  104,  129, 135. 136, 138, 139, 140, 

156,  157,  158,  171,  176,  185, 197,  217,  218,  219,  220,  297,  29S. 

303,  309,  310,  311,  312,  315,  459, 476,  482, 480,  494,  490,  504. 

512, 514,  516. 


4,'-,  N . 

K. 

1  W 

45  N. 

K. 

-.1  \V. 

45  N. 

li. 

2  W. 

45  N. 

li. 

1  W 

Town.  44  N.,  1{.  2  \V.,  Wisconsin,  10. 129,  i:i0, 14  1,  II.'.,  140  1.19, 
171,  170,  177,  197,  220,  :ilK,  319,  :121,:147,  510. 
1  I  N.,  1;.  1  W.,  Wisi'onsiji,  108,  19S. 
Wisconsin,  180. 
Wisconsin.  42. 
Wisconsin,  10,  291).  :i2l. 

,  Wisconsin,  129.  149,  1.14.  159,  184,  185,220, 

:i,208,  209,  :!U4,:i05,  321.  322,  32;i,  :I24,  :i25,;i20, 

:'.51, 459,  494, 504, 612,510. 

45  N.,  K.  1  !•;.,  Wisconsin.  129, 144, 140,  159, 100, 178,  200,  223, 

324,  S25,  209,  302,  303, 305,  306,  325,  327,  440,  440,  462, 488. 

45  N.,  R.  2  E  ,  Wisconsin,  42, 161, 191, 220, 227, 297, 305, 306, 

327,  490. 

46  N.,  R.  1  K.,  Wisconsin,  328. 

46  N.,  E.  2  E.,  Wisconsin,  104,  105, 139,  144, 140,  101,  102,  227, 
228,  269,  274,  302,  :i08,  309,  328,  329,  33U,  448,  476,  488,492. 

46  N,,  K.  44  W.,  Michigan,  105. 

47  N.,E.  42  W.,  Michigan,  :i87,  388,  389,  393,  394,  405,406, 

407,  409, 422.  424,  425,  431,  450,  459,  461. 

47N.,  E.  43  W.,  Michigan,  6,  105,  144.191,301,362.303,364, 
365,  367,  308,  309,  370,  371,  372,  373,  374,  379,  380,  381,386, 
387,  388,  389,  392,  393,  394,  396,  397,  398,  399,  400,  401,  402, 
403, 404,  414,  422,  433, 424,  425,  426,  429,  430,  431,  449,  450, 
459,461. 

47  N.,  E.  44  W.,  Michigan,  4, 128,  139, 130-131, 137,  138, 139, 
140, 170, 171, 179,  185,  187, 188, 199,  340,  241,  242,  343,  244^  ■ 
245,  360,  361,  363,  366,  367,  368,  369,  374,  380,  381,  382,  383, 
384,  387,  388,  389,  391,  392,  396,  410,  415,  416,  422,  423, 424, 
420,  429,  430,  431,  433,  441 ,  443,  459,  461,  480,  518. 

47  N.,  R.  45  W.,  Michigan,  129,  136,  137,  139,  144,  146, 154, 
165, 166, 167, 168. 169, 171, 179, 187,  191,  198,  199,  237,  238, 
239,  240,  268,  269,  290,  298,  346,  355,  437,  449,  452,  454, 480, 
482,  484,  486,  504. 

47  N.,  E.  46  W.,  Michigan,  105,  144,  146,  164,  165,  174, 179, 
187, 189, 191, 199,  231,  232, 233,  234,  235,  236,  237,  255,  269, 
272, 280,  283, 290,  297,  305,  330,  331,  346,  351,  357,  448, 449, 
456,  484,  490,  502. 

47  N.,  E.  47  W.,  Michigan,  112,  144,  146,  154,  162,  163, 104, 

179, 199,  228,  229,  230,  231 ,  269,  305,  329,  357, 486. 

48  N".,  E.  46  W.,  Michigan,  6. 
49N.,E.  41  W.,  Michigan,  6- 
65  N.,  E.  2  W.,  Minnesota,  262. 

65  N.,  R.  3  W.,  Minnesota,  262, 498. 

65  N.,  E.  4  W.,  Minnesota,  506. 
Trap  range,  relations  to  Upper  slate,  301, 302. 
Tremolite  of  limestone,  130, 131-133, 138, 141,  259,  260,  207. 
Trimble  mine,  274,  610. 
Trimmingham  chalks,  251, 252, 250. 
Tuff,  volcanic,  360,  361,  379,  380. 

Tylers  Fork,  154, 159, 177, 178, 184, 185, 188, 189,  198,  206,  220-223, 
279,  298.  299.  300,  302,  303,  304.  307,  309,  321,  324,  369,  428, 
456,  459,  460. 
Tylers  Fork  area,  129, 145. 

U. 
Unconformities.  470. 

in  Marquette  district,  471 ,  472. 
Unconformity  below  Auiraikie  series,  261,  362. 

below  Eastern  sandstone,  388, 461-463. 

between  Animikie  series  and  Keweenawan,  470. 

between  Cherty  limestone  and  Penokce  sei'ies,  454-455. 

between  Cherty  limestone  and  Southern  Complex,  444-454. 

between  Huronian  and  Basement  Complex,  81. 

between  Huronian  and  Laurentiau,  47,  82. 

between  Keweenawan  and  Fluronian,  47,  81,  82. 

between  Lake  Superior  sandstone  and  Keweenawan,  47. 

between  Penokee  series  and  Keweenawan,  455-461. 


534 


INDEX. 


Unconformity  between  Penokee  series  and  Southern  Complex, 

56,  77,  81,  82,  96,  107-108, 109,  129, 173,  343,  444-454, 469. 
between  Quartz-slate  and  Southern  Complex  1,  29, 173, 444, 

454. 
Upper  slate,  285,  296-345.  431,  434, 436, 438, 457,  459,  460,  468,  471, 

472,474,512,514,516. 
bedding  of,  296. 
cleavage  of,  296. 
diabase  of.  348,  359. 
dikes  of,  358. 
greenstouea  of,  285. 
oris'in  of,  332-345. 
relations  to  uctiiiolitic  slate.  214. 
relations  to  cruptives.  347,  348,  358. 
relations  to  greenstone-conglomerate.  420. 
relations  to  Iron-bearing  member,  200,  294,  296,  297,  298, 

299,  300, 456,  464^65. 
relations  to Keweenawan.  297-299,  300, 301, 302, 347,  348,358. 
relations  to  Quartz-slate,  296,  297. 
relations  to  Southern  Complex,  343-344,  345. 
thickness  of  296,  298-299,  456,  459-461. 
topography  of,  301,  302. 
(See  Slate.) 


Van  Hise  referred  to,  9. 10-11, 77-78, 84-85, 86, 102. 
Veins  of  chlorite,  418. 

of  concretions,  206-207,  208. 

of  epidote,  418. 


Veins  of  ferruginous  cliert,  254. 

t)f  iron  carbonate.  202. 

of  quartz,  418. 
Vermillion  district,  2,  260,  201,  293,  356,  468,  472.  473.  474,  496. 
Vermillion  lake,  496. 
Viridite  of  iron  carbonate,  190. 
Volcanic  tuft'.    (See  Tuf.) 

W.    - 

Wadsworth  referred  to,  56-58, 71, 73-75, 75-77, 249, 412. 

Wales,  chert  of,  140,  251,  252. 

"White,  ackuowledguieuts  tn,  xiv. 

WhitUeld  referred  to,  256. 

Whitney  refcn-ed  to,  6, 10, 13-14, 15. 16-17,  56,  71,  73-77. 

Whittlesey  referred  to,  7, 17-22,  27-30,  34,  36. 

Wight  referred  to,  9. 

Williams  referred  to,  412. 

Winchell,  Alexander,  referred  to,  99-102,  473. 

Wiuchell,  N.  H.,  referred  to.  78-81,  96-99, 101-102. 

Wisconsin,  1,41,  474. 

Wisconsin  Central  Kailroad,  XIU,  347. 

Wisconsin  geological  survey  referred  to,  3,  5, 7,  8,  9, 10, 11,  20, 

22,  27,  39,  40.  45,  58.  66.  84, 127, 128, 138,  143.  183-184,  186,  195, 

196. 
Wright  referred  to',  3,9,  40-41 ,  52-56,  58-59,  78,  83-84, 104, 186. 

T. 

Yellowstone  National  Park,  251, 256. 
Yorkshire,  chert  of,  140, 251. 


